Microprocessors8085

Microprocessors and Microcontrollers/Architecture of Microprocessors Lecture Notes

Module 1 learning unit 1

• A Computer is a programmable machine.
• The two principal characteristics of a computer are:
• It responds to a specific set of instructions in a well-defined manner.
• It can execute a prerecorded list of instructions (a program ).
• Modern computers are electronic and digital.
• The actual machinery wires, transistors, and circuits is called hardware. the
instructions and data are called software.
•

• All general-purpose computers require the following hardware components:
• Memory: Enables a computer to store, at least temporarily, data and programs.
• Mass storage device: Allows a computer to permanently retain large amounts of
data. Common mass storage devices include disk drives and tape drives.
• Input device: Usually a keyboard and mouse are the input device through which
data and instructions enter a computer.
• Output device: A display screen, printer, or other device that lets you see what
the computer has accomplished.
• Central processing unit (CPU): The heart of the computer, this is the component
that actually executes instructions.
• In addition to these components, many others make it possible for the basic
components to work together efficiently.
• For example, every computer requires a bus that transmits data from one part of
the computer to another.

M. Krishna Kumar/IISc. Bangalore M1/V1/June 04/1


• Computers can be generally classified by size and power as follows, though there
is considerable overlap:
• Personal computer: A small, single-user computer based on a microprocessor.
• In addition to the microprocessor, a personal computer has a keyboard for
entering data, a monitor for displaying information, and a storage device for
saving data.
• Working station: A powerful, single-user computer. A workstation is like a
personal computer, but it has a more powerful microprocessor and a higher-
quality monitor.
• Minicomputer: A multi-user computer capable of supporting from 10 to
hundreds of users simultaneously.
• Mainframe: A powerful multi-user computer capable of supporting many
hundreds or thousands of users simultaneously.
• Supercomputer: An extremely fast computer that can perform hundreds of
millions of instructions per second.
Minicomputer:
• A midsized computer. In size and power, minicomputers lie between workstations
and mainframes.
• A minicomputer, a term no longer much used, is a computer of a size intermediate
between a microcomputer and a mainframe.
• Typically, minicomputers have been stand-alone computers (computer systems
with attached terminals and other devices) sold to small and mid-size businesses
for general business applications and to large enterprises for department-level
operations.
• In recent years, the minicomputer has evolved into the "mid-range server" and is
part of a network. IBM's AS/400e is a good example.
• The AS/400 - formally renamed the "IBM iSeries," but still commonly known as
AS/400 - is a midrange server designed for small businesses and departments in
large enterprises and now redesigned so that it will work well in distributed
networks with Web applications.
• The AS/400 uses the PowerPC microprocessor with its reduced instruction set
computer technology. Its operating system is called the OS/400.
• With multi-terabytes of disk storage and a Java virtual memory closely tied into
the operating system, IBM hopes to make the AS/400 a kind of versatile all-
purpose server that can replace PC servers and Web servers in the world's
businesses, competing with both Wintel and Unix servers, while giving its present
enormous customer base an immediate leap into the Internet.
Workstation:
1) A type of computer used for engineering applications (CAD/CAM), desktop
publishing, software development, and other types of applications that require a
moderate amount of computing power and relatively high quality graphics
capabilities.
• Workstations generally come with a large, high- resolution graphics screen, at
least 64 MB (mega bytes) of RAM, built-in network support, and a graphical user
interface.



• Most workstations also have a mass storage device such as a disk drive, but a
special type of workstation, called a diskless workstation, comes without a disk
drive.
• The most common operating systems for workstations are UNIX and Windows
NT.
• In terms of computing power, workstations lie between personal computers and
minicomputers, although the line is fuzzy on both ends.
• High-end personal computers are equivalent to low-end workstations. And high-
end workstations are equivalent to minicomputers.
• Like personal computers, most workstations are single-user computers. However,
workstations are typically linked together to form a local-area network, although
they can also be used as stand-alone systems.
2) In networking, workstation refers to any computer connected to a local-area
network. It could be a workstation or a personal computer.
• Mainframe: A very large and expensive computer capable of supporting
hundreds, or even thousands, of users simultaneously. In the hierarchy that starts
with a simple microprocessors (in watches, for example) at the bottom and moves
to supercomputer at the top, mainframes are just below supercomputers.
• In some ways, mainframes are more powerful than supercomputers because they
support more simultaneous programs.
• But supercomputers can execute a single program faster than a mainframe. The
distinction between small mainframes and minicomputers is vague, depending
really on how the manufacturer wants to market its machines.
• Microcomputer: The term microcomputer is generally synonymous with
personal computer, or a computer that depends on a microprocessor.
• Microcomputers are designed to be used by individuals, whether in the form of
PCs, workstations or notebook computers.
• A microcomputer contains a CPU on a microchip (the microprocessor), a memory
system (typically ROM and RAM), a bus system and I/O ports, typically housed
in a motherboard.
• Microprocessor: A silicon chip that contains a CPU. In the world of personal
computers, the terms microprocessor and CPU are used interchangeably.
• A microprocessor (sometimes abbreviated µP) is a digital electronic component
with miniaturized transistors on a single semiconductor integrated circuit (IC).
• One or more microprocessors typically serve as a central processing unit (CPU) in
a computer system or handheld device.
• Microprocessors made possible the advent of the microcomputer.
• At the heart of all personal computers and most working stations sits a
microprocessor.
• Microprocessors also control the logic of almost all digital devices, from clock
radios to fuel-injection systems for automobiles.
• Three basic characteristics differentiate microprocessors:
• Instruction set: The set of instructions that the microprocessor can execute.
• Bandwidth: The number of bits processed in a single instruction.
• Clock speed: Given in megahertz (MHz), the clock speed determines how many
instructions per second the processor can execute.



• In both cases, the higher the value, the more powerful the CPU. For example, a 32
bit microprocessor that runs at 50MHz is more powerful than a 16-bit
microprocessor that runs at 25MHz.
• In addition to bandwidth and clock speed, microprocessors are classified as being
either RISC (reduced instruction set computer) or CISC (complex instruction set
computer).
• Supercomputer: A supercomputer is a computer that performs at or near the
currently highest operational rate for computers.
• A supercomputer is typically used for scientific and engineering applications that
must handle very large databases or do a great amount of computation (or both).
• At any given time, there are usually a few well-publicized supercomputers that
operate at the very latest and always incredible speeds.
• The term is also sometimes applied to far slower (but still impressively fast)
computers.
• Most supercomputers are really multiple computers that perform parallel
processing.
• In general, there are two parallel processing approaches: symmetric
multiprocessing (SMP) and massively parallel processing (MPP).
• Microcontroller: A highly integrated chip that contains all the components
comprising a controller.
• Typically this includes a CPU, RAM, some form of ROM, I/O ports, and timers.
• Unlike a general-purpose computer, which also includes all of these components,
a microcontroller is designed for a very specific task - to control a particular
system.
• A microcontroller differs from a microprocessor, which is a general-purpose chip
that is used to create a multi-function computer or device and requires multiple
chips to handle various tasks.
• A microcontroller is meant to be more self-contained and independent, and
functions as a tiny, dedicated computer.
• The great advantage of microcontrollers, as opposed to using larger
microprocessors, is that the parts-count and design costs of the item being
controlled can be kept to a minimum.
• They are typically designed using CMOS (complementary metal oxide
semiconductor) technology, an efficient fabrication technique that uses less power
and is more immune to power spikes than other techniques.
• Microcontrollers are sometimes called embedded microcontrollers, which just
means that they are part of an embedded system that is, one part of a larger device
or system.
• Controller: A device that controls the transfer of data from a computer to a
peripheral device and vice versa.
• For example, disk drives, display screens, keyboards and printers all require
controllers.
• In personal computers, the controllers are often single chips.
• When you purchase a computer, it comes with all the necessary controllers for
standard components, such as the display screen, keyboard, and disk drives.



• If you attach additional devices, however, you may need to insert new controllers
that come on expansion boards.
• Controllers must be designed to communicate with the computer's expansion bus.
• There are three standard bus architectures for PCs - the AT bus, PCI (Peripheral
Component Interconnect ) and SCSI.
• When you purchase a controller, therefore, you must ensure that it conforms to
the bus architecture that your computer uses.
• Short for Peripheral Component Interconnect, a local bus standard developed by
Intel Corporation.
• Most modern PCs include a PCI bus in addition to a more general IAS expansion
bus.
• PCI is also used on newer versions of the Macintosh computer.
• PCI is a 64-bit bus, though it is usually implemented as a 32 bit bus. It can run at
clock speeds of 33 or 66 MHz.
• At 32 bits and 33 MHz, it yields a throughput rate of 133 MBps.
• Short for small computer system interface, a parallel interface standard used by
Apple Macintosh computers, PCs, and many UNIX systems for attaching
peripheral devices to computers.
• Nearly all Apple Macintosh computers, excluding only the earliest Macs and the
recent iMac, come with a SCSI port for attaching devices such as disk drives and
printers.
• SCSI interfaces provide for faster data transmission rates (up to 80 megabytes per
second) than standard serial and parallel ports. In addition, you can attach many
devices to a single SCSI port, so that SCSI is really an I/O bus rather than simply
an interface
• Although SCSI is an ANSI standard, there are many variations of it, so two SCSI
interfaces may be incompatible.
• For example, SCSI supports several types of connectors.
• While SCSI has been the standard interface for Macintoshes, the iMac comes with
IDE, a less expensive interface, in which the controller is integrated into the disk
or CD-ROM drive.
• The following varieties of SCSI are currently implemented:
• SCSI-1: Uses an 8-bit bus, and supports data rates of 4 MBps.
• SCSI-2: Same as SCSI-1, but uses a 50-pin connector instead of a 25-pin
connector, and supports multiple devices. This is what most people mean when
they refer to plain SCSI.
• Wide SCSI: Uses a wider cable (168 cable lines to 68 pins) to support 16-bit
transfers.
• Fast SCSI: Uses an 8-bit bus, but doubles the clock rate to support data rates of 10
MBps.
• Fast Wide SCSI: Uses a 16-bit bus and supports data rates of 20 MBps.
• Ultra SCSI: Uses an 8-bit bus, and supports data rates of 20 MBps.
• Wide Ultra2 SCSI: Uses a 16-bit bus and supports data rates of 80 MBps.
• SCSI-3: Uses a 16-bit bus and supports data rates of 40 MBps. Also called Ultra
Wide SCSI.
• Ultra2 SCSI: Uses an 8-bit bus and supports data rates of 40 MBps.



• Embedded system: A specialized computer system that is part of a larger system
or machine.
• Typically, an embedded system is housed on a single microprocessor board with
the programs stored in ROM.
• Virtually all appliances that have a digital Interface- watches, microwaves, VCRs,
cars -utilize embedded systems.
• Some embedded systems include an operating system, but many are so
specialized that the entire logic can be implemented as a single program.
MICRO CONTROLLER MICRO PROCESSER
• It is a single chip • It is a CPU
• Consists Memory, • Memory, I/O Ports to be
I/o ports connected externally

CP
CPU MEMORY
MEMORY
I/O PORTS
I/O PORTS
Definitions:
• A Digital Signal Processor is a special-purpose CPU (Central Processing Unit)
that provides ultra-fast instruction sequences, such as shift and add, and multiply
and add, which are commonly used in math-intensive signal processing
applications.
• A digital signal processor (DSP) is a specialized microprocessor designed
specifically for digital signal processing, generally in real time.
Digital
– operating by the use of discrete signals to represent data in the form of
numbers.
Signal
– a variable parameter by which information is conveyed through an
electronic circuit.
Processing
– to perform operations on data according to programmed instructions.
Digital Signal processing
– changing or analysing information which is measured as discrete
sequences of numbers.
• Digital signal processing (DSP) is the study of signals in a digital representation
and the processing methods of these signals.
• DSP and analog signal processing are subfields of signal processing.



DSP has three major subfields:
• Audio signal processing, Digital image processing and Speech processing.
• Since the goal of DSP is usually to measure or filter continuous real-world analog
signals, the first step is usually to convert the signal from an analog to a digital
form, by using an analog to digital converter.
• Often, the required output signal is another analog output signal, which requires a
digital to analog converter.
Characteristics of Digital Signal Processors:
• Separate program and data memories (Harvard architecture).
• Special Instructions for SIMD (Single Instruction, Multiple Data) operations.
• Only parallel processing, no multitasking.
• The ability to act as a direct memory access device if in a host environment.
• Takes digital data from ADC (Analog-Digital Converter) and passes out data
which is finally output by converting into analog by DAC (Digital-Analog
Converter).
• analog input-->ADC-->DSP-->DAC--> analog output.
Analog front end Analog back end
Analog Antialiasing DSP D/A converter, Analog
signal in filter, S/H, A/D Processor reconstruction signal
converter filter output

DAP System
Multiply-accumulate hardware:
• Multiply accumulate is the most frequently used operation in digital signal
processing.
• In order to implement this efficiently, the DSP has an hardware multiplier, an
accumulator with an adequate number of bits to hold the sum of products and at
explicit multiply-accumulate instructions.
• Harvard architecture: in this memory architecture, there are two memory spaces.
Program memory and data memory.



X Y
n n

Multiplier

Product register

2n

ADD / SUB

Accumulator
2n

A MAC



X Y
16 16

Multiplier

32

40

ADD / SUB

40
Guard bits
8 32

A MAC unit with accumulator guard bits
• The processor core connects to these memory spaces by two separate bus sets,
allowing two simultaneous access to memory. This arrangement doubles the
processor memory bandwidth.
•
• Zero-overhead looping: one common characteristics of DSP algorithms is that
most of the processing time is split on executing instructions contained with
relatively small loops.
• The term zero overhead looping means that the processor can execute loops
without consuming cycles to test the value of the loop counter, perform a
conditional branch to the top of the loop, and decrement the loop counter.



Processing Result
unit Data bus

Operands

Status Opcode
Instructions Data / Instructions

Control unit Data program
memory

Von Neuman Architecture

Processing Result / operands
unit Data
memory
Address

Status Opcode

Control unit
Program memory
Instructions

Address

Harvard Architecture



Processing Result / operands
unit Data
memory
Address

Status Opcode

Control unit
program memory
Instructions

Address

Modified Harvard Architecture
Data bus

MEMORY Peripheral
Program Data / Data Serial
ROM program DARAM port 1
SARAM Data /
program Serial
DARAM port 2
TDM
Serial port
Program bus Buffered
serial port
Memory Program controller Memory CALU CPU
control Multiplier Timer
Program counter mapped
Multiproc Status/control registers Accumulator Parallel Host
essing registers ACC buffer logic unit port
Auxiliary shifters (PAL) interface
Interrupt Hardware stack Resisters arithmetic
Generation logic Arithmetic
Initialisation logic unit Test /
Oscillator/ Instruction register Unit (ARAU) (ALU) emulation
timer
Data bus
Internal Architecture of the TMS320C5X DSP
• The advantages of DSP are:
Versatility:
• digital systems can be reprogrammed for other applications (at least where
programmable DSP chips are used)
• digital systems can be ported to different hardware (for example a different DSP
chip or board level product)
Repeatability:
• digital systems can be easily duplicated
• digital system responses do not drift with temperature



• digital systems do not depend on strict component tolerances.
•
Simplicity:
• some things can be done more easily digitally than with analogue systems
• DSP is used in a very wide variety of applications but most share some common
features:
• they use a lot of multiplying and adding signals.
• they deal with signals that come from the real world.
• they require a response in a certain time.
•

Figure: A block diagram (or dataflow graph)
• What is the difference between a DSP and a microprocessor ?
• The essential difference between a DSP and a microprocessor is that a DSP
processor has features designed to support high-performance, repetitive,
numerically intensive tasks.
• In contrast, general-purpose processors or microcontrollers (GPPs / MCUs for
short) are either not specialized for a specific kind of applications (in the case of
general-purpose processors), or they are designed for control-oriented
applications (in the case of microcontrollers).
• Features that accelerate performance in DSP applications include:
• Single-cycle multiply-accumulate capability; high-performance DSPs often have
two multipliers that enable two multiply-accumulate operations per instruction
cycle; some DSP have four or more multipliers.
•
• Specialized addressing modes, for example, pre- and post-modification of address
pointers, circular addressing, and bit-reversed addressing.
• Most DSPs provide various configurations of on-chip memory and peripherals
tailored for DSP applications. DSPs generally feature multiple-access memory
architectures that enable DSPs to complete several accesses to memory in a single
instruction cycle.



• Specialized execution control. Usually, DSP processors provide a loop instruction
that allows tight loops to be repeated without spending any instruction cycles for
updating and testing the loop counter or for jumping back to the top of the loop
• DSP processors are known for their irregular instruction sets, which generally
allow several operations to be encoded in a single instruction.
• For example, a processor that uses 32-bit instructions may encode two additions,
two multiplications, and four 16-bit data moves into a single instruction.
• In general, DSP processor instruction sets allow a data move to be performed in
parallel with an arithmetic operation. GPPs / MCUs, in contrast, usually specify a
single operation per instruction.
• What is really important is to choose the processor that is best suited for your
application.
• If a GPP/MCU is better suited for your DSP application than a DSP processor, the
processor of choice is the GPP/MCU.
• It is also worth noting that the difference between DSPs and GPPs/MCUs is
fading: many GPPs/MCUs now include DSP features, and DSPs are increasingly
adding microcontroller features.
Module 1: learning unit 2
8085 Microprocessor
ContentsGeneral definitions
• Overview of 8085 microprocessor
• Overview of 8086 microprocessor
• Signals and pins of 8086 microprocessor
The salient features of 8085 µp are:
• It is a 8 bit microprocessor.
• It is manufactured with N-MOS technology.
• It has 16-bit address bus and hence can address up to 216 = 65536 bytes (64KB)
memory locations through A0-A15.
• The first 8 lines of address bus and 8 lines of data bus are multiplexed AD0 – AD7.
• Data bus is a group of 8 lines D0 – D7.
• It supports external interrupt request.
• A 16 bit program counter (PC)
• A 16 bit stack pointer (SP)
• Six 8-bit general purpose register arranged in pairs: BC, DE, HL.
• It requires a signal +5V power supply and operates at 3.2 MHZ single phase
clock.
• It is enclosed with 40 pins DIP (Dual in line package).
Overview of 8085 microprocessor
8085 Architecture
• Pin Diagram
• Functional Block Diagram



X1 1 40 Vcc
X2 39 HOLD
2 DMA
RESE OUT 3 38 HLDA
SOD 4 37 CLK ( OUT)
Serial i/p, o/p signals RESET IN
SID 5 36
TRAP 6 35 READY
7
IO / M
RST 7.5 34
RST 6.5 8 33 S1
RST 5.5
9
8085 A 32 RD
INTR 10 31 WR
IN T A 11
30 ALE
AD0 12
29 S0
AD1 13
28 A15
AD2 A14
14 27
AD3 15 26 A13
AD4 25 A12
16
AD5 24 A11
17
AD6 18 23 A10
AD7 19 22 A9
VSS 20 21 A8

Pin Diagram of 8085
Signal Groups of 8085
+5V GND
XTAL
X1 X2 Vcc Vss
A15
SID 5 High order Address bus
A8
SOD 4
TRAP AD7
RESET 7.5
RESET 6.5 AD0
RESET 5.5 ALE
INTR S1
READY S0
HOLD
IO / M
____
RESET IN
HLDA RD
WR
INTA

REST OUT CLK OUT



RES RES RES TRAP SID
INTA SIO
5. 5 6. 5 7. 5

INT
SERIAL I / O CONTROL
INTERRUPT CONTROL

8 BIT INTERNAL
DATA BUS

INSTRUCTION
ACCUMU- (8) MULTIPLXER
REGISTER( 8 )
LATOR TEMP REG
(8) R W(8)
E TEMP. REG.
G C REG ( 8 )
B REG ( 8 )
FLAG ( 5) .
S D REG ( 8 )
FLIP FLOPS E REG ( 8 )
E
INSTRUCTION H REG ( 8 ) L REG ( 8 )
ARITHEMETIC L
DECODER AND E
LOGIC UNIT ( ALU) STACK POINTER ( 16 )
MACHINE C
ENCODING PROGRAM COUNTER ( 16 )
(8) T
+5V INCREAMENT / DECREAMENT
ADDRESS LATCH ( 16 )
GND

X1 TIMING AND CONTROL
CLK
ADDRESS BUFFER ( DATA / ADDRESS
GEN 8)
X2 BUFFER ( 8 )
CONTROL STATUS DMA

CLK A 15 – A8
RESET IN
OUT READY RD WR ALE S0 S1 IO / M HOLD HLDA RESET OUT ADDRESS BUS
AD7 – AD0 ADDRESS /
BUFFER BUS

Block Diagram

Flag Registers
D7 D6 D5 D4 D3 D2 D1 D0

S Z AC P CY

General Purpose Registers
INDIVIDUAL B, C, D, E, H, L

COMBININATON
B & C, D & E, H&L



Memory
• Program, data and stack memories occupy the same memory space. The total
addressable memory size is 64 KB.
• Program memory - program can be located anywhere in memory. Jump, branch
and call instructions use 16-bit addresses, i.e. they can be used to jump/branch
anywhere within 64 KB. All jump/branch instructions use absolute addressing.
• Data memory - the processor always uses 16-bit addresses so that data can be
placed anywhere.
• Stack memory is limited only by the size of memory. Stack grows downward.
• First 64 bytes in a zero memory page should be reserved for vectors used by RST
instructions.
Interrupts
• The processor has 5 interrupts. They are presented below in the order of their
priority (from lowest to highest):
•
• INTR is maskable 8080A compatible interrupt. When the interrupt occurs the
processor fetches from the bus one instruction, usually one of these instructions:
• One of the 8 RST instructions (RST0 - RST7). The processor saves current
program counter into stack and branches to memory location N * 8 (where N is a
3-bit number from 0 to 7 supplied with the RST instruction).
• CALL instruction (3 byte instruction). The processor calls the subroutine, address
of which is specified in the second and third bytes of the instruction.
• RST5.5 is a maskable interrupt. When this interrupt is received the processor
saves the contents of the PC register into stack and branches to 2CH
(hexadecimal) address.
saves the contents of the PC register into stack and branches to 34H
saves the contents of the PC register into stack and branches to 3CH
• TRAP is a non-maskable interrupt. When this interrupt is received the processor
saves the contents of the PC register into stack and branches to 24H
• All maskable interrupts can be enabled or disabled using EI and DI instructions.
RST 5.5, RST6.5 and RST7.5 interrupts can be enabled or disabled individually
using SIM instruction.
Reset Signals
• RESET IN: When this signal goes low, the program counter (PC) is set to Zero,
µp is reset and resets the interrupt enable and HLDA flip-flops.
• The data and address buses and the control lines are 3-stated during RESET and
because of asynchronous nature of RESET, the processor internal registers and
flags may be altered by RESET with unpredictable results.
• RESET IN is a Schmitt-triggered input, allowing connection to an R-C network
for power-on RESET delay.



• Upon power-up, RESET IN must remain low for at least 10 ms after minimum
Vcc has been reached.
• For proper reset operation after the power – up duration, RESET IN should be
kept low a minimum of three clock periods.
• The CPU is held in the reset condition as long as RESET IN is applied. Typical
Power-on RESET RC values R1 = 75KΩ, C1 = 1µF.
• RESET OUT: This signal indicates that µp is being reset. This signal can be used
to reset other devices. The signal is synchronized to the processor clock and lasts
an integral number of clock periods.
Serial communication Signal
• SID - Serial Input Data Line: The data on this line is loaded into accumulator bit
7 whenever a RIM instruction is executed.
• SOD – Serial Output Data Line: The SIM instruction loads the value of bit 7 of
the accumulator into SOD latch if bit 6 (SOE) of the accumulator is 1.
DMA Signals
• HOLD: Indicates that another master is requesting the use of the address and data
buses. The CPU, upon receiving the hold request, will relinquish the use of the
bus as soon as the completion of the current bus transfer.
• Internal processing can continue. The processor can regain the bus only after the
HOLD is removed.
• When the HOLD is acknowledged, the Address, Data RD, WR and IO/M lines are
3-stated.
• HLDA: Hold Acknowledge: Indicates that the CPU has received the HOLD
request and that it will relinquish the bus in the next clock cycle.
• HLDA goes low after the Hold request is removed. The CPU takes the bus one
half-clock cycle after HLDA goes low.
• READY: This signal Synchronizes the fast CPU and the slow memory,
peripherals.
• If READY is high during a read or write cycle, it indicates that the memory or
peripheral is ready to send or receive data.
• If READY is low, the CPU will wait an integral number of clock cycle for
READY to go high before completing the read or write cycle.
• READY must conform to specified setup and hold times.
Registers
• Accumulator or A register is an 8-bit register used for arithmetic, logic, I/O and
load/store operations.
• Flag Register has five 1-bit flags.
• Sign - set if the most significant bit of the result is set.
• Zero - set if the result is zero.
• Auxiliary carry - set if there was a carry out from bit 3 to bit 4 of the result.
• Parity - set if the parity (the number of set bits in the result) is even.
• Carry - set if there was a carry during addition, or borrow during
subtraction/comparison/rotation.



General Registers
• 8-bit B and 8-bit C registers can be used as one 16-bit BC register pair. When
used as a pair the C register contains low-order byte. Some instructions may use
BC register as a data pointer.
• 8-bit D and 8-bit E registers can be used as one 16-bit DE register pair. When
used as a pair the E register contains low-order byte. Some instructions may use
DE register as a data pointer.
• 8-bit H and 8-bit L registers can be used as one 16-bit HL register pair. When
used as a pair the L register contains low-order byte. HL register usually contains
a data pointer used to reference memory addresses.
• Stack pointer is a 16 bit register. This register is always
decremented/incremented by 2 during push and pop.
• Program counter is a 16-bit register.
Instruction Set
• 8085 instruction set consists of the following instructions:
• Data moving instructions.
• Arithmetic - add, subtract, increment and decrement.
• Logic - AND, OR, XOR and rotate.
• Control transfer - conditional, unconditional, call subroutine, return from
subroutine and restarts.
• Input/Output instructions.
• Other - setting/clearing flag bits, enabling/disabling interrupts, stack operations,
etc.
Addressing mode
• Register - references the data in a register or in a register pair.
Register indirect - instruction specifies register pair containing address, where
the data is located.
Direct, Immediate - 8 or 16-bit data.
Module 1: learning unit 3
8086 Microprocessor
•It is a 16-bit µp.
•8086 has a 20 bit address bus can access up to 220 memory locations (1 MB).
•It can support up to 64K I/O ports.
•It provides 14, 16 -bit registers.
•It has multiplexed address and data bus AD0- AD15 and A16 – A19.
•It requires single phase clock with 33% duty cycle to provide internal timing.
•8086 is designed to operate in two modes, Minimum and Maximum.
•It can prefetches upto 6 instruction bytes from memory and queues them in order to
speed up instruction execution.
•It requires +5V power supply.
•A 40 pin dual in line package
Minimum and Maximum Modes:
•The minimum mode is selected by applying logic 1 to the MN / MX input pin. This is a
single microprocessor configuration.
•The maximum mode is selected by applying logic 0 to the MN / MX input pin. This is a
multi micro processors configuration.



GND 1 40 VCC
AD14 39 AD15
2
AD13 3 38 A16 / S3
AD12 4 37 A17 / S4
AD11 5 36 A18 / S5
AD10 6 35 A19/S6
AD9 7 34 BHE / S7
____
AD8
AD7
8 8086 33 MN/ MX

AD6
9
10
CPU 32 RD _____ _____
RQ / GT0 ( HOLD)
31 ___ _____
AD5 11 RQ / GT1
AD4
30 ( HLDA)
12 _______ ___
29 LOCK (WR) ____
AD3 13
28 ___ S2 (M / IO )
___
AD2
14 27 S1 (DT / R )
_____
AD1 15 26
AD0 S0 ( DEN )
16 25 ________ QS0 (ALE)
NMI 24 QS1 ( INTA )
17
INTR 18 23 TEST
CLK 19 22
READY
GND 20 21
RESET
Pin Diagram of 8086



VCC GND

A0 - A15, A16 / S3 – A19/S6

INTR

INTA ADDRESS / DATA BUS
INTERRUPT
INTERFACE
TEST D0 - D15

NMI 8086 ALE
MPU ___
RESET BHE / S7

M / IO
MEMORY I
/O
HOLD DMA ____ DT / R
CONTROLS
INTERFACE RD
_____
HLDA
WR
VCC
DEN
MODE
____
SELECT READY
MN / MX

CLK

Signal Groups of 8086



AH AL
ADDRESS BUS
BH BL ∑
CH CL ( 20 )
GENERAL DH DL BITS
REGISTERS
SP DATA BUS
BP
( 16 )
SI BITS
DI
ES
CS
SS
DS
ALU DATA IP
8
16 BITS 0
BUS 8
6
TEMPORARY REGISTERS CONTR B
OL U
LOGIC S

EU INSTRUCTION QUEUE
ALU CONTROQ BUS
L 1 2 3 4 5 6
SYSTEM
8 BIT

FLAGS BUS INTERFACE UNIT ( BIU)
EXECUTION UNIT ( EU )

Block Diagram of 8086
Internal Architecture of 8086
•8086 has two blocks BIU and EU.
•The BIU performs all bus operations such as instruction fetching, reading and writing
operands for memory and calculating the addresses of the memory operands. The
instruction bytes are transferred to the instruction queue.
•EU executes instructions from the instruction system byte queue.
•Both units operate asynchronously to give the 8086 an overlapping instruction fetch and
execution mechanism which is called as Pipelining. This results in efficient use of the
system bus and system performance.
•BIU contains Instruction queue, Segment registers, Instruction pointer, Address adder.
•EU contains Control circuitry, Instruction decoder, ALU, Pointer and Index register,
Flag register.
BUS INTERFACR UNIT:
•It provides a full 16 bit bidirectional data bus and 20 bit address bus.
•The bus interface unit is responsible for performing all external bus operations.
Specifically it has the following functions:
•Instruction fetch, Instruction queuing, Operand fetch and storage, Address relocation and
Bus control.
•The BIU uses a mechanism known as an instruction stream queue to implement a
pipeline architecture.
•This queue permits prefetch of up to six bytes of instruction code. When ever the queue
of the BIU is not full, it has room for at least two more bytes and at the same time the EU



is not requesting it to read or write operands from memory, the BIU is free to look ahead
in the program by prefetching the next sequential instruction.
•These prefetching instructions are held in its FIFO queue. With its 16 bit data bus, the
BIU fetches two instruction bytes in a single memory cycle.
•After a byte is loaded at the input end of the queue, it automatically shifts up through the
FIFO to the empty location nearest the output.
•The EU accesses the queue from the output end. It reads one instruction byte after the
other from the output of the queue. If the queue is full and the EU is not requesting access
to operand in memory.
•These intervals of no bus activity, which may occur between bus cycles are known as
Idle state.
•If the BIU is already in the process of fetching an instruction when the EU request it to
read or write operands from memory or I/O, the BIU first completes the instruction fetch
bus cycle before initiating the operand read / write cycle.
•The BIU also contains a dedicated adder which is used to generate the 20bit physical
address that is output on the address bus. This address is formed by adding an appended
16 bit segment address and a 16 bit offset address.
•For example: The physical address of the next instruction to be fetched is formed by
combining the current contents of the code segment CS register and the current contents
of the instruction pointer IP register.
•The BIU is also responsible for generating bus control signals such as those for memory
read or write and I/O read or write.
EXECUTION UNIT
The Execution unit is responsible for decoding and executing all instructions.
•The EU extracts instructions from the top of the queue in the BIU, decodes them,
generates operands if necessary, passes them to the BIU and requests it to perform the
read or write bys cycles to memory or I/O and perform the operation specified by the
instruction on the operands.
•During the execution of the instruction, the EU tests the status and control flags and
updates them based on the results of executing the instruction.
•If the queue is empty, the EU waits for the next instruction byte to be fetched and shifted
to top of the queue.
•When the EU executes a branch or jump instruction, it transfers control to a location
corresponding to another set of sequential instructions.
•Whenever this happens, the BIU automatically resets the queue and then begins to fetch
instructions from this new location to refill the queue.
Module 1 and learning unit 4:
Signal Description of 8086•The Microprocessor 8086 is a 16-bit CPU available in
different clock rates and packaged in a 40 pin CERDIP or plastic package.
•The 8086 operates in single processor or multiprocessor configuration to achieve high
performance. The pins serve a particular function in minimum mode (single processor
mode) and other function in maximum mode configuration (multiprocessor mode ).
•The 8086 signals can be categorised in three groups. The first are the signal having
common functions in minimum as well as maximum mode.
•The second are the signals which have special functions for minimum mode and third
are the signals having special functions for maximum mode.



•The following signal descriptions are common for both modes.
•AD15-AD0: These are the time multiplexed memory I/O address and data lines.
• Address remains on the lines during T1 state, while the data is available on the data bus
during T2, T3, Tw and T4.
•These lines are active high and float to a tristate during interrupt acknowledge and local
bus hold acknowledge cycles.
•A19/S6,A18/S5,A17/S4,A16/S3: These are the time multiplexed address and status
lines.
•During T1 these are the most significant address lines for memory operations.
•During I/O operations, these lines are low. During memory or I/O operations, status
information is available on those lines for T2,T3,Tw and T4.
•The status of the interrupt enable flag bit is updated at the beginning of each clock cycle.
•The S4 and S3 combinedly indicate which segment register is presently being used for
memory accesses as in below fig.
•These lines float to tri-state off during the local bus hold acknowledge. The status line
S6 is always low.
•The address bit are separated from the status bit using latches controlled by the ALE
signal.
S4 S3 Indication
0 0 Alternate Data
0 1 Stack
1 0 Code or none
1 1 Data

• BHE /S7: The bus high enable is used to indicate the transfer of data over the higher
order ( D15-D8 ) data bus as shown in table. It goes low for the data transfer over D15-
D8 and is used to derive chip selects of odd address memory bank or peripherals. BHE is
low during T1 for read, write and interrupt acknowledge cycles, whenever a byte is to be
transferred on higher byte of data bus. The status information is available during T2, T3
and T4. The signal is active low and tristated during hold. It is low during T1 for the first
pulse of the interrupt acknowledges cycle.
BHE A0 Indication
0 0 Whole word
0 1 Upper byte from or to odd address
even address
1 0 Lower byte from or to even address
1 1 None

• RD Read: This signal on low indicates the peripheral that the processor is performing s
memory or I/O read operation. RD is active low and shows the state for T2, T3, Tw of
any read cycle. The signal remains tristated during the hold acknowledge.



•READY: This is the acknowledgement from the slow device or memory that they have
completed the data transfer. The signal made available by the devices is synchronized by
the 8284A clock generator to provide ready input to the 8086. the signal is active high.
•INTR-Interrupt Request: This is a triggered input. This is sampled during the last
clock cycles of each instruction to determine the availability of the request. If any
interrupt request is pending, the processor enters the interrupt acknowledge cycle.
•This can be internally masked by resulting the interrupt enable flag. This signal is active
high and internally synchronized.
• TEST This input is examined by a ‘WAIT’ instruction. If the TEST pin goes low,
execution will continue, else the processor remains in an idle state. The input is
synchronized internally during each clock cycle on leading edge of clock.
•CLK- Clock Input: The clock input provides the basic timing for processor operation
and bus control activity. Its an asymmetric square wave with 33% duty cycle.
•MN/ MX : The logic level at this pin decides whether the processor is to operate in either
minimum or maximum mode.
•The following pin functions are for the minimum mode operation of 8086.
•M/ IO – Memory/IO: This is a status line logically equivalent to S2 in maximum mode.
When it is low, it indicates the CPU is having an I/O operation, and when it is high, it
indicates that the CPU is having a memory operation. This line becomes active high in
the previous T4 and remains active till final T4 of the current cycle. It is tristated during
local bus “hold acknowledge “.
• INTA Interrupt Acknowledge: This signal is used as a read strobe for interrupt
acknowledge cycles. i.e. when it goes low, the processor has accepted the interrupt.
•ALE – Address Latch Enable: This output signal indicates the availability of the valid
address on the address/data lines, and is connected to latch enable input of latches. This
signal is active high and is never tristated.
•DT/ R – Data Transmit/Receive: This output is used to decide the direction of data
flow through the transreceivers (bidirectional buffers). When the processor sends out
data, this signal is high and when the processor is receiving data, this signal is low.
•DEN – Data Enable: This signal indicates the availability of valid data over the
address/data lines. It is used to enable the transreceivers ( bidirectional buffers ) to
separate the data from the multiplexed address/data signal. It is active from the middle of
T2 until the middle of T4. This is tristated during ‘ hold acknowledge’ cycle.
•HOLD, HLDA- Acknowledge: When the HOLD line goes high, it indicates to the
processor that another master is requesting the bus access.
•The processor, after receiving the HOLD request, issues the hold acknowledge signal on
HLDA pin, in the middle of the next clock cycle after completing the current bus
cycle.•At the same time, the processor floats the local bus and control lines. When the
processor detects the HOLD line low, it lowers the HLDA signal. HOLD is an
asynchronous input, and is should be externally synchronized.
•If the DMA request is made while the CPU is performing a memory or I/O cycle, it will
release the local bus during T4 provided:
1.The request occurs on or before T2 state of the current cycle.
2.The current cycle is not operating over the lower byte of a word.
3.The current cycle is not the first acknowledge of an interrupt acknowledge sequence.



4. A Lock instruction is not being executed.
•The following pin function are applicable for maximum mode operation of 8086.
•S2, S1, S0 – Status Lines: These are the status lines which reflect the type of operation,
being carried out by the processor. These become activity during T4 of the previous cycle
and active during T1 and T2 of the current bus cycles.
S2 S1 S0 Indication
0 0 0 Interrupt Acknowledge
0 0 1 Read I/O port
0 1 0 Write I/O port
0 1 1 Halt
1 0 0 Code Access
1 0 1 Read memory
1 1 0 Write memory
1 1 1 Passive
• LOCK This output pin indicates that other system bus master will be prevented from
gaining the system bus, while the LOCK signal is low.
•The LOCK signal is activated by the ‘LOCK’ prefix instruction and remains active until
the completion of the next instruction. When the CPU is executing a critical instruction
which requires the system bus, the LOCK prefix instruction ensures that other processors
connected in the system will not gain the control of the bus.
•The 8086, while executing the prefixed instruction, asserts the bus lock signal output,
which may be connected to an external bus controller.
•QS1, QS0 – Queue Status: These lines give information about the status of the code-
prefetch queue. These are active during the CLK cycle after while the queue operation is
performed.
•This modification in a simple fetch and execute architecture of a conventional
microprocessor offers an added advantage of pipelined processing of the instructions.
•The 8086 architecture has 6-byte instruction prefetch queue. Thus even the largest (6-
bytes) instruction can be prefetched from the memory and stored in the prefetch. This
results in a faster execution of the instructions.
•In 8085 an instruction is fetched, decoded and executed and only after the execution of
this instruction, the next one is fetched.
•By prefetching the instruction, there is a considerable speeding up in instruction
execution in 8086. This is known as instruction pipelining.
•At the starting the CS:IP is loaded with the required address from which the execution is
to be started. Initially, the queue will be empty an the microprocessor starts a fetch
operation to bring one byte (the first byte) of instruction code, if the CS:IP address is odd
or two bytes at a time, if the CS:IP address is even.
•The first byte is a complete opcode in case of some instruction (one byte opcode
instruction) and is a part of opcode, in case of some instructions ( two byte opcode
instructions), the remaining part of code lie in second byte.
•The second byte is then decoded in continuation with the first byte to decide the
instruction length and the number of subsequent bytes to be treated as instruction data.



•The queue is updated after every byte is read from the queue but the fetch cycle is
initiated by BIU only if at least two bytes of the queue are empty and the EU may be
concurrently executing the fetched instructions.
•The next byte after the instruction is completed is again the first opcode byte of the next
instruction. A similar procedure is repeated till the complete execution of the
program.•The fetch operation of the next instruction is overlapped with the execution of
the current instruction. As in the architecture, there are two separate units, namely
Execution unit and Bus interface unit.
•While the execution unit is busy in executing an instruction, after it is completely
decoded, the bus interface unit may be fetching the bytes of the next instruction from
memory, depending upon the queue status.
QS1 QS0 Indication
0 0 No operation
0 1 First byte of the opcode from the queue
1 0 Empty queue
1 1 Subsequent byte from the queue

• RQ / GT0 , RQ / GT1 – Request/Grant: These pins are used by the other local bus master
in maximum mode, to force the processor to release the local bus at the end of the
processor current bus cycle.
•Each of the pin is bidirectional with RQ/GT0 having higher priority than RQ/GT1.
•RQ/GT pins have internal pull-up resistors and may be left unconnected.
•Request/Grant sequence is as follows:
1.A pulse of one clock wide from another bus master requests the bus access to 8086.
2.During T4(current) or T1(next) clock cycle, a pulse one clock wide from 8086 to the
requesting master, indicates that the 8086 has allowed the local bus to float and that it
will enter the ‘hold acknowledge’ state at next cycle. The CPU bus interface unit is likely
to be disconnected from the local bus of the system.
3.A one clock wide pulse from the another master indicates to the 8086 that the hold
request is about to end and the 8086 may regain control of the local bus at the next clock
cycle. Thus each master to master exchange of the local bus is a sequence of 3 pulses.
There must be at least one dead clock cycle after each bus exchange.
•The request and grant pulses are active low.
•For the bus request those are received while 8086 is performing memory or I/O cycle,
the granting of the bus is governed by the rules as in case of HOLD and HLDA in
minimum mode.
General Bus Operation:
•The 8086 has a combined address and data bus commonly referred as a time multiplexed
address and data bus.
•The main reason behind multiplexing address and data over the same pins is the
maximum utilisation of processor pins and it facilitates the use of 40 pin standard DIP
package.



•The bus can be demultiplexed using a few latches and transreceivers, when ever
required.
•Basically, all the processor bus cycles consist of at least four clock cycles. These are
referred to as T1, T2, T3, T4. The address is transmitted by the processor during T1. It is
present on the bus only for one cycle.
•The negative edge of this ALE pulse is used to separate the address and the data or status
information. In maximum mode, the status lines S0, S1 and S2 are used to indicate the
type of operation.
•Status bits S3 to S7 are multiplexed with higher order address bits and the BHE signal.
Address is valid during T1 while status bits S3 to S7 are valid during T2 through T4.

Memory read cycle Memory write cycle
T1 T2 T3 Tw T4 T1 T2 T3 Tw T4
CLK

ALE

S2 – S0
A19-A16 S3-S7 A19-A16 S3-S7
Add/stat
BHE Bus reserve BHE
Add/data for Data In Data Out D15 – D0
A0-A15 D15-D0 A0-A15 D15-D0
RD/INTA
Ready
READY Ready
DT/R Wait Wait

DEN

WR Memory access time

General Bus Operation Cycle in Maximum Mode

Minimum Mode 8086 System
•In a minimum mode 8086 system, the microprocessor 8086 is operated in minimum
mode by strapping its MN/MX pin to logic 1.
•In this mode, all the control signals are given out by the microprocessor chip itself.
There is a single microprocessor in the minimum mode system.
•The remaining components in the system are latches, transreceivers, clock generator,
memory and I/O devices. Some type of chip selection logic may be required for selecting
memory or I/O devices, depending upon the address map of the system.
•Latches are generally buffered output D-type flip-flops like 74LS373 or 8282. They are
used for separating the valid address from the multiplexed address/data signals and are
controlled by the ALE signal generated by 8086.



•Transreceivers are the bidirectional buffers and some times they are called as data
amplifiers. They are required to separate the valid data from the time multiplexed
address/data signals.
•They are controlled by two signals namely, DEN and DT/R.
•The DEN signal indicates the direction of data, i.e. from or to the processor. The system
contains memory for the monitor and users program storage.
•Usually, EPROM are used for monitor storage, while RAM for users program storage. A
system may contain I/O devices.
•The working of the minimum mode configuration system can be better described in
terms of the timing diagrams rather than qualitatively describing the operations.
•The opcode fetch and read cycles are similar. Hence the timing diagram can be
categorized in two parts, the first is the timing diagram for read cycle and the second is
the timing diagram for write cycle.
•The read cycle begins in T1 with the assertion of address latch enable (ALE) signal and
also M / IO signal. During the negative going edge of this signal, the valid address is
latched on the local bus.
•The BHE and A0 signals address low, high or both bytes. From T1 to T4 , the M/IO
signal indicates a memory or I/O operation.
•At T2, the address is removed from the local bus and is sent to the output. The bus is
then tristated. The read (RD) control signal is also activated in T2.
•The read (RD) signal causes the address device to enable its data bus drivers. After RD
goes low, the valid data is available on the data bus.
•The addressed device will drive the READY line high. When the processor returns the
read signal to high level, the addressed device will again tristate its bus drivers.
•A write cycle also begins with the assertion of ALE and the emission of the address. The
M/IO signal is again asserted to indicate a memory or I/O operation. In T2, after sending
the address in T1, the processor sends the data to be written to the addressed location.
•The data remains on the bus until middle of T4 state. The WR becomes active at the
beginning of T2 (unlike RD is somewhat delayed in T2 to provide time for floating).
•The BHE and A0 signals are used to select the proper byte or bytes of memory or I/O
word to be read or write.
•The M/IO, RD and WR signals indicate the type of data transfer as specified in table
below.



T1 T2 T3 TW T4 T1

Clk

ALE

BHE S7 – S3
ADD / STATUS A19 – A16

ADD / DATA A15 – A0 Valid data D15 – D0

WR

DEN

DT / R

Write Cycle Timing Diagram for Minimum Mode
•Hold Response sequence: The HOLD pin is checked at leading edge of each clock
pulse. If it is received active by the processor before T4 of the previous cycle or during
T1 state of the current cycle, the CPU activates HLDA in the next clock cycle and for
succeeding bus cycles, the bus will be given to another requesting master.
•The control of the bus is not regained by the processor until the requesting master does
not drop the HOLD pin low. When the request is dropped by the requesting master, the
HLDA is dropped by the processor at the trailing edge of the next clock.

Clk

HOLD

HLDA

Bus Request and Bus Grant Timings in Minimum Mode System



Maximum Mode 8086 System •In the maximum mode, the 8086 is operated by
strapping the MN/MX pin to ground.
•In this mode, the processor derives the status signal S2, S1, S0. Another chip called bus
controller derives the control signal using this status information.
•In the maximum mode, there may be more than one microprocessor in the system
configuration.
•The components in the system are same as in the minimum mode system.
•The basic function of the bus controller chip IC8288, is to derive control signals like RD
and WR ( for memory and I/O devices), DEN, DT/R, ALE etc. using the information by
the processor on the status lines.
•The bus controller chip has input lines S2, S1, S0 and CLK. These inputs to 8288 are
driven by CPU.
•It derives the outputs ALE, DEN, DT/R, MRDC, MWTC, AMWC, IORC, IOWC and
AIOWC. The AEN, IOB and CEN pins are specially useful for multiprocessor systems.
•AEN and IOB are generally grounded. CEN pin is usually tied to +5V. The significance
of the MCE/PDEN output depends upon the status of the IOB pin.
•If IOB is grounded, it acts as master cascade enable to control cascade 8259A, else it
acts as peripheral data enable used in the multiple bus configurations.
•INTA pin used to issue two interrupt acknowledge pulses to the interrupt controller or to
an interrupting device.
•IORC, IOWC are I/O read command and I/O write command signals respectively. These
signals enable an IO interface to read or write the data from or to the address port.
•The MRDC, MWTC are memory read command and memory write command signals
respectively and may be used as memory read or write signals.
•All these command signals instructs the memory to accept or send data from or to the
bus.
•For both of these write command signals, the advanced signals namely AIOWC and
AMWTC are available.
•Here the only difference between in timing diagram between minimum mode and
maximum mode is the status signals used and the available control and advanced
command signals.



Clk DEN
S0 DT/ R Control bus
S1 8288 IORC
S2 IOWT
AEN MWTC
Reset Reset S0
Clk IOB
S1 CEN AL MRDC
Generator Clk
S2
RDY 8284 Ready + 5V

8086
CLK
AD6-AD15 A/D Address bus
A16-A19 Latches
A
dd
DT/R bu
BHE A0
DIR
Data CS0H CS0L RD CS WR RD
buffer WR
DEN G Memory Peripheral

Data bus

Maximum Mode 8086 System.

•R0, S1, S2 are set at the beginning of bus cycle.8288 bus controller will output a pulse
as on the ALE and apply a required signal to its DT / R pin during T1.
•In T2, 8288 will set DEN=1 thus enabling transceivers, and for an input it will activate
MRDC or IORC. These signals are activated until T4. For an output, the AMWC or
AIOWC is activated from T2 to T4 and MWTC or IOWC is activated from T3 to T4.
•The status bit S0 to S2 remains active until T3 and become passive during T3 and T4.
•If reader input is not activated before T3, wait state will be inserted between T3 and T4.
•Timings for RQ/ GT Signals:
The request/grant response sequence contains a series of three pulses. The request/grant
pins are checked at each rising pulse of clock input.
•When a request is detected and if the condition for HOLD request are satisfied, the
processor issues a grant pulse over the RQ/GT pin immediately during T4 (current) or T1
(next) state.
•When the requesting master receives this pulse, it accepts the control of the bus, it sends
a release pulse to the processor using RQ/GT pin.



One bus cycle
T1 T2 T3 T4 T1

Clk

ALE

S2 – S0 Active Inactive Active

Add/Status BHE, A19 – A16 S7 – S3

Add/Data A15 – A0 D15 – D0

MRDC

DT / R

DEN

Memory Read Timing in Maximum Mode



One bus cycle
T1 T2 T3 T4 T1

Clk

ALE

S2 – S0 Active Inactive Active

ADD/STATUS BHE S7 – S3

ADD/DATA A15-A0 Data out D15 – D0
AMWC or AIOWC

MWTC or IOWC

DT / R high
DEN

Memory Write Timing in Maximum mode.

Clk

RQ / GT

Another master CPU grant bus Master releases
request bus access

RQ/GT Timings in Maximum Mode.
Minimum Mode Interface
•When the Minimum mode operation is selected, the 8086 provides all control signals
needed to implement the memory and I/O interface.



•The minimum mode signal can be divided into the following basic groups: address/data
bus, status, control, interrupt and DMA.
•Address/Data Bus: these lines serve two functions. As an address bus is 20 bits long
and consists of signal lines A0 through A19. A19 represents the MSB and A0 LSB. A
20bit address gives the 8086 a 1Mbyte memory address space. More over it has an
independent I/O address space which is 64K bytes in length.
•The 16 data bus lines D0 through D15 are actually multiplexed with address lines A0
through A15 respectively. By multiplexed we mean that the bus work as an address bus
during first machine cycle and as a data bus during next machine cycles. D15 is the MSB
and D0 LSB.
•When acting as a data bus, they carry read/write data for memory, input/output data for
I/O devices, and interrupt type codes from an interrupt controller.
Vcc GND

INTR
A0-A15,A16/S3 – A19/S6
INTA
Interrupt
Address / data bus
interface
TEST

D0 – D15
NMI
8086
MPU ALE
RESET
BHE / S7

M / IO Memory I/O
HOLD controls
DMA DT / R
interface
HLDA RD

WR

Vcc
DEN
Mode select
READY
MN / MX

CLK clock

Block Diagram of the Minimum Mode 8086 MPU
•Status signal:
The four most significant address lines A19 through A16 are also multiplexed but in this
case with status signals S6 through S3. These status bits are output on the bus at the same
time that data are transferred over the other bus lines.
•Bit S4 and S3 together from a 2 bit binary code that identifies which of the 8086 internal
segment registers are used to generate the physical address that was output on the address
bus during the current bus cycle.
•Code S4S3 = 00 identifies a register known as extra segment register as the source of
the segment address.



•Status line S5 reflects the status of another internal characteristic of the 8086. It is the
logic level of the internal enable flag. The last status bit S6 is always at the logic 0 level.

S4 S3 Segment Register

0 0 Extra

0 1 Stack

1 0 Code / none

1 1 Data

Memory segment status codes.
•Control Signals:
The control signals are provided to support the 8086 memory I/O interfaces. They
control functions such as when the bus is to carry a valid address in which direction data
are to be transferred over the bus, when valid write data are on the bus and when to put
read data on the system bus.
•ALE is a pulse to logic 1 that signals external circuitry when a valid address word is on
the bus. This address must be latched in external circuitry on the 1-to-0 edge of the pulse
at ALE.
•Another control signal that is produced during the bus cycle is BHE bank high enable.
Logic 0 on this used as a memory enable signal for the most significant byte half of the
data bus D8 through D1. These lines also serves a second function, which is as the S7
status line.
•Using the M/IO and DT/R lines, the 8086 signals which type of bus cycle is in progress
and in which direction data are to be transferred over the bus.
•The logic level of M/IO tells external circuitry whether a memory or I/O transfer is
taking place over the bus. Logic 1 at this output signals a memory operation and logic 0
an I/O operation.
•The direction of data transfer over the bus is signaled by the logic level output at DT/R.
When this line is logic 1 during the data transfer part of a bus cycle, the bus is in the
transmit mode. Therefore, data are either written into memory or output to an I/O device.



•On the other hand, logic 0 at DT/R signals that the bus is in the receive mode. This
corresponds to reading data from memory or input of data from an input port.
•The signal read RD and write WR indicates that a read bus cycle or a write bus cycle is
in progress. The 8086 switches WR to logic 0 to signal external device that valid write or
output data are on the bus.
• On the other hand, RD indicates that the 8086 is performing a read of data of the bus.
During read operations, one other control signal is also supplied. This is DEN ( data
enable) and it signals external devices when they should put data on the bus.
•There is one other control signal that is involved with the memory and I/O interface.
This is the READY signal.
•READY signal is used to insert wait states into the bus cycle such that it is extended by
a number of clock periods. This signal is provided by an external clock generator device
and can be supplied by the memory or I/O sub-system to signal the 8086 when they are
ready to permit the data transfer to be completed.
•Interrupt signals: The key interrupt interface signals are interrupt request (INTR) and
interrupt acknowledge ( INTA).
•INTR is an input to the 8086 that can be used by an external device to signal that it need
to be serviced.
•Logic 1 at INTR represents an active interrupt request. When an interrupt request has
been recognized by the 8086, it indicates this fact to external circuit with pulse to logic 0
at the INTA output.
•The TEST input is also related to the external interrupt interface. Execution of a WAIT
instruction causes the 8086 to check the logic level at the TEST input.
•If the logic 1 is found, the MPU suspend operation and goes into the idle state. The 8086
no longer executes instructions, instead it repeatedly checks the logic level of the TEST
input waiting for its transition back to logic 0.
•As TEST switches to 0, execution resume with the next instruction in the program. This
feature can be used to synchronize the operation of the 8086 to an event in external
hardware.
•There are two more inputs in the interrupt interface: the nonmaskable interrupt NMI and
the reset interrupt RESET.
•On the 0-to-1 transition of NMI control is passed to a nonmaskable interrupt service
routine. The RESET input is used to provide a hardware reset for the 8086. Switching
RESET to logic 0 initializes the internal register of the 8086 and initiates a reset service
routine.
•DMA Interface signals:The direct memory access DMA interface of the 8086
minimum mode consist of the HOLD and HLDA signals.
•When an external device wants to take control of the system bus, it signals to the 8086
by switching HOLD to the logic 1 level. At the completion of the current bus cycle, the
8086 enters the hold state. In the hold state, signal lines AD0 through AD15, A16/S3
through A19/S6, BHE, M/IO, DT/R, RD, WR, DEN and INTR are all in the high Z state.
The 8086 signals external device that it is in this state by switching its HLDA output to
logic 1 level.
Maximum Mode Interface
•When the 8086 is set for the maximum-mode configuration, it provides signals for
implementing a multiprocessor / coprocessor system environment.



•By multiprocessor environment we mean that one microprocessor exists in the system
and that each processor is executing its own program.
•Usually in this type of system environment, there are some system resources that are
common to all processors.
•They are called as global resources. There are also other resources that are assigned to
specific processors. These are known as local or private resources.
•Coprocessor also means that there is a second processor in the system. In this two
processor does not access the bus at the same time.
•One passes the control of the system bus to the other and then may suspend its operation.
•In the maximum-mode 8086 system, facilities are provided for implementing allocation
of global resources and passing bus control to other microprocessor or coprocessor.
INIT
Multi Bus
S0 BUSY
S1 CBRQ
S2 8289 BPRO
LOCK Bus BPRN
CRQLCK
CLK RESB BREQ
Vcc GND SYSB/RESB
ANYREQ CLK AEN IOB BCLK

INTR LOCK CLK AEN IOB
S0 MRDC
TEST CLK AEN IOB MWTC
S1 S0
NMI AMWC
S2 S1 8288 Bus IORC
RESET S2 controller IOWC
DEN AIOWC
DT/ R INTA
8086 MPU ALE MCE / PDEN
DEN
DT / R
ALE
A0-A15,
A16/S3-A19/S6
MN/MX
D0 – D15
BHE
RD
READY
QS1, QS0
Local bus control
RQ / GT1 RQ / GT0 8086 Maximum mode Block Diagram
•8288 Bus Controller – Bus Command and Control Signals:
8086 does not directly provide all the signals that are required to control the memory,
I/O and interrupt interfaces.
•Specially the WR, M/IO, DT/R, DEN, ALE and INTA, signals are no longer produced
by the 8086. Instead it outputs three status signals S0, S1, S2 prior to the initiation of
each bus cycle. This 3- bit bus status code identifies which type of bus cycle is to follow.
•S2S1S0 are input to the external bus controller device, the bus controller generates the
appropriately timed command and control signals.



Status Inputs
CPU Cycles 8288
S2 S1 S0 Command
0 0 0 Interrupt Acknowledge INTA
0 0 1 Read I/O Port IORC
0 1 0 Write I/O Port IOWC, AIOWC
0 1 1 Halt None
1 0 0 Instruction Fetch MRDC

1 0 1 Read Memory MRDC

1 1 0 Write Memory MWTC, AMWC
1 1 1 Passive None

Bus Status Codes
•The 8288 produces one or two of these eight command signals for each bus cycles. For
instance, when the 8086 outputs the code S2S1S0 equals 001, it indicates that an I/O read
cycle is to be performed.
•In the code 111 is output by the 8086, it is signaling that no bus activity is to take place.
•The control outputs produced by the 8288 are DEN, DT/R and ALE. These 3 signals
provide the same functions as those described for the minimum system mode. This set of
bus commands and control signals is compatible with the Multibus and industry standard
for interfacing microprocessor systems.
•The output of 8289 are bus arbitration signals:
Bus busy (BUSY), common bus request (CBRQ), bus priority out (BPRO), bus priority
in (BPRN), bus request (BREQ) and bus clock (BCLK).
•They correspond to the bus exchange signals of the Multibus and are used to lock other
processor off the system bus during the execution of an instruction by the 8086.
•In this way the processor can be assured of uninterrupted access to common system
resources such as global memory.
•Queue Status Signals: Two new signals that are produced by the 8086 in the maximum-
mode system are queue status outputs QS0 and QS1. Together they form a 2-bit queue
status code, QS1QS0.
•Following table shows the four different queue status.



QS1 QS0 Queue Status

0 (low) 0 No Operation. During the last clock cycle, nothing was
taken from the queue.
0 1 First Byte. The byte taken from the queue was the first byte
of the instruction.
1 (high) 0 Queue Empty. The queue has been reinitialized as a result
of the execution of a transfer instruction.
Subsequent Byte. The byte taken from the queue was a
1 1 subsequent byte of the instruction.

Queue status codes
•Local Bus Control Signal – Request / Grant Signals: In a maximum mode
configuration, the minimum mode HOLD, HLDA interface is also changed. These two
are replaced by request/grant lines RQ/ GT0 and RQ/ GT1, respectively. They provide a
prioritized bus access mechanism for accessing the local bus.
Internal Registers of 8086
•The 8086 has four groups of the user accessible internal registers. They are the
instruction pointer, four data registers, four pointer and index register, four segment
registers.
•The 8086 has a total of fourteen 16-bit registers including a 16 bit register called the
status register, with 9 of bits implemented for status and control flags.
•Most of the registers contain data/instruction offsets within 64 KB memory segment.
There are four different 64 KB segments for instructions, stack, data and extra data. To
specify where in 1 MB of processor memory these 4 segments are located the processor
uses four segment registers:
•Code segment (CS) is a 16-bit register containing address of 64 KB segment with
processor instructions. The processor uses CS segment for all accesses to instructions
referenced by instruction pointer (IP) register. CS register cannot be changed directly.
The CS register is automatically updated during far jump, far call and far return
instructions.
•Stack segment (SS) is a 16-bit register containing address of 64KB segment with
program stack. By default, the processor assumes that all data referenced by the stack
pointer (SP) and base pointer (BP) registers is located in the stack segment. SS register
can be changed directly using POP instruction.
•Data segment (DS) is a 16-bit register containing address of 64KB segment with
program data. By default, the processor assumes that all data referenced by general
registers (AX, BX, CX, DX) and index register (SI, DI) is located in the data segment.
DS register can be changed directly using POP and LDS instructions.
•Accumulator register consists of two 8-bit registers AL and AH, which can be
combined together and used as a 16-bit register AX. AL in this case contains the low-
order byte of the word, and AH contains the high-order byte. Accumulator can be used
for I/O operations and string manipulation.



•Base register consists of two 8-bit registers BL and BH, which can be combined together
and used as a 16-bit register BX. BL in this case contains the low-order byte of the word,
and BH contains the high-order byte. BX register usually contains a data pointer used for
based, based indexed or register indirect addressing.
•Count register consists of two 8-bit registers CL and CH, which can be combined
together and used as a 16-bit register CX. When combined, CL register contains the low-
order byte of the word, and CH contains the high-order byte. Count register can be used
in Loop, shift/rotate instructions and as a counter in string manipulation,.
•Data register consists of two 8-bit registers DL and DH, which can be combined
together and used as a 16-bit register DX. When combined, DL register contains the low-
order byte of the word, and DH contains the high-order byte. Data register can be used as
a port number in I/O operations. In integer 32-bit multiply and divide instruction the DX
register contains high-order word of the initial or resulting number.
•The following registers are both general and index registers:
•Stack Pointer (SP) is a 16-bit register pointing to program stack.
•Base Pointer (BP) is a 16-bit register pointing to data in stack segment. BP register is
usually used for based, based indexed or register indirect addressing.
•Source Index (SI) is a 16-bit register. SI is used for indexed, based indexed and register
indirect addressing, as well as a source data address in string manipulation instructions.
•Destination Index (DI) is a 16-bit register. DI is used for indexed, based indexed and
register indirect addressing, as well as a destination data address in string manipulation
instructions.
Other registers:
•Instruction Pointer (IP) is a 16-bit register.
•Flags is a 16-bit register containing 9 one bit flags.
•Overflow Flag (OF) - set if the result is too large positive number, or is too small
negative number to fit into destination operand.
•Direction Flag (DF) - if set then string manipulation instructions will auto-decrement
index registers. If cleared then the index registers will be auto-incremented.
•Interrupt-enable Flag (IF) - setting this bit enables maskable interrupts.
•Single-step Flag (TF) - if set then single-step interrupt will occur after the next
instruction.
•Sign Flag (SF) - set if the most significant bit of the result is set.
•Zero Flag (ZF) - set if the result is zero.
•Auxiliary carry Flag (AF) - set if there was a carry from or borrow to bits 0-3 in the AL
register.
•Parity Flag (PF) - set if parity (the number of "1" bits) in the low-order byte of the
result is even.
•Carry Flag (CF) - set if there was a carry from or borrow to the most significant bit
during last result calculation.
Addressing Modes
•Implied - the data value/data address is implicitly associated with the instruction.
•Register - references the data in a register or in a register pair.
•Immediate - the data is provided in the instruction.
•Direct - the instruction operand specifies the memory address where data is located.



•Register indirect - instruction specifies a register containing an address, where data is
located. This addressing mode works with SI, DI, BX and BP registers.
•Based:- 8-bit or 16-bit instruction operand is added to the contents of a base register
(BX or BP), the resulting value is a pointer to location where data resides.
•Indexed:- 8-bit or 16-bit instruction operand is added to the contents of an index register
(SI or DI), the resulting value is a pointer to location where data resides
•Based Indexed:- the contents of a base register (BX or BP) is added to the contents of
an index register (SI or DI), the resulting value is a pointer to location where data resides.
•Based Indexed with displacement:- 8-bit or 16-bit instruction operand is added to the
contents of a base register (BX or BP) and index register (SI or DI), the resulting value is
a pointer to location where data resides.
Memory •Program, data and stack memories occupy the same memory space. As the
most of the processor instructions use 16-bit pointers the processor can effectively
address only 64 KB of memory.
•To access memory outside of 64 KB the CPU uses special segment registers to specify
where the code, stack and data 64 KB segments are positioned within 1 MB of memory
(see the "Registers" section below).
•16-bit pointers and data are stored as:
address: low-order byte
address+1: high-order byte
•Program memory - program can be located anywhere in memory. Jump and call
instructions can be used for short jumps within currently selected 64 KB code segment,
as well as for far jumps anywhere within 1 MB of memory.
•All conditional jump instructions can be used to jump within approximately +127 to -
127 bytes from current instruction.
•Data memory - the processor can access data in any one out of 4 available segments,
which limits the size of accessible memory to 256 KB (if all four segments point to
different 64 KB blocks).
•Accessing data from the Data, Code, Stack or Extra segments can be usually done by
prefixing instructions with the DS:, CS:, SS: or ES: (some registers and instructions by
default may use the ES or SS segments instead of DS segment).
•Word data can be located at odd or even byte boundaries. The processor uses two
memory accesses to read 16-bit word located at odd byte boundaries. Reading word data
from even byte boundaries requires only one memory access.
•Stack memory can be placed anywhere in memory. The stack can be located at odd
memory addresses, but it is not recommended for performance reasons (see "Data
Memory" above).
Reserved locations:
•0000h - 03FFh are reserved for interrupt vectors. Each interrupt vector is a 32-bit pointer
in format segment: offset.
•FFFF0h - FFFFFh - after RESET the processor always starts program execution at the
FFFF0h address.
Interrupts
The processor has the following interrupts:



•INTR is a maskable hardware interrupt. The interrupt can be enabled/disabled using
STI/CLI instructions or using more complicated method of updating the FLAGS register
with the help of the POPF instruction.
•When an interrupt occurs, the processor stores FLAGS register into stack, disables
further interrupts, fetches from the bus one byte representing interrupt type, and jumps to
interrupt processing routine address of which is stored in location 4 * <interrupt type>.
Interrupt processing routine should return with the IRET instruction.
•NMI is a non-maskable interrupt. Interrupt is processed in the same way as the INTR
interrupt. Interrupt type of the NMI is 2, i.e. the address of the NMI processing routine is
stored in location 0008h. This interrupt has higher priority then the maskable interrupt.
•Software interrupts can be caused by:
•INT instruction - breakpoint interrupt. This is a type 3 interrupt.
•INT <interrupt number> instruction - any one interrupt from available 256 interrupts.
•INTO instruction - interrupt on overflow
•Single-step interrupt - generated if the TF flag is set. This is a type 1 interrupt. When the
CPU processes this interrupt it clears TF flag before calling the interrupt processing
routine.
•Processor exceptions: Divide Error (Type 0), Unused Opcode (type 6) and Escape
opcode (type 7).
•Software interrupt processing is the same as for the hardware interrupts.


Microprocessors8085

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