Chapter1 basic structure of computers

Chapter 1. Basic
Structure of Computers

Functional Units
Figure 1.1. Basic functional units of a computer.
I/O Processor
Output
Memory
Input and
Arithmetic
logic
Control

Information Handled by a
Computer
 Instructions/machine instructions
 Govern the transfer of information within a computer as
well as between the computer and its I/O devices
 Specify the arithmetic and logic operations to be
performed
 Program
 Data
 Used as operands by the instructions
 Source program
 Encoded in binary code – 0 and 1

Memory Unit
 Store programs and data
 Two classes of storage
 Primary storage
 Fast
 Programs must be stored in memory while they are being executed
 Large number of semiconductor storage cells
 Processed in words
 Address
 RAM and memory access time
 Memory hierarchy – cache, main memory
 Secondary storage – larger and cheaper

Arithmetic and Logic Unit
(ALU)
 Most computer operations are executed in
ALU of the processor.
 Load the operands into memory – bring them
to the processor – perform operation in ALU
– store the result back to memory or retain in
the processor.
 Registers
 Fast control of ALU

Control Unit
 All computer operations are controlled by the control
unit.
 The timing signals that govern the I/O transfers are
also generated by the control unit.
 Control unit is usually distributed throughout the
machine instead of standing alone.
 Operations of a computer:
 Accept information in the form of programs and data through an
input unit and store it in the memory
 Fetch the information stored in the memory, under program control,
into an ALU, where the information is processed
 Output the processed information through an output unit
 Control all activities inside the machine through a control unit

The processor : Data Path and
Control
Two types of functional units:
elements that operate on data values (combinational)
 elements that contain state (state elements)

Five Execution Steps
Step nameStep name Action for R-typeAction for R-type
instructionsinstructions
Action for Memory-Action for Memory-
reference Instructionsreference Instructions
Action forAction for
branchesbranches
Action forAction for
jumpsjumps
Instruction fetch IR = MEM[PC]
PC = PC + 4
Instruction decode/ register
fetch
A = Reg[IR[25-21]]
B = Reg[IR[20-16]]
ALUOut = PC + (sign extend (IR[15-0])<<2)
Execution, address
computation, branch/jump
completion
ALUOut = A op B ALUOut = A+sign
extend(IR[15-0])
IF(A==B) Then
PC=ALUOut
PC=PC[31-
28]||(IR[25-
0]<<2)
Memory access or R-type
completion
Reg[IR[15-11]] =
ALUOut
Load:MDR =Mem[ALUOut]
or
Store:Mem[ALUOut] = B
Memory read completion Load: Reg[IR[20-16]] =
MDR

Review
 Activity in a computer is governed by instructions.
 To perform a task, an appropriate program
consisting of a list of instructions is stored in the
memory.
 Individual instructions are brought from the memory
into the processor, which executes the specified
operations.
 Data to be used as operands are also stored in the
memory.

A Typical Instruction
 Add LOCA, R0
 Add the operand at memory location LOCA to the
operand in a register R0 in the processor.
 Place the sum into register R0.
 The original contents of LOCA are preserved.
 The original contents of R0 is overwritten.
 Instruction is fetched from the memory into the
processor – the operand at LOCA is fetched and
added to the contents of R0 – the resulting sum is
stored in register R0.

Separate Memory Access and
ALU Operation
 Load LOCA, R1
 Add R1, R0
 Whose contents will be overwritten?

Connection Between the
Processor and the Memory
Figure 1.2. Connections between the processor and the memory.
Processor
Memory
PC
IR
MDR
Control
ALU
Rn 1
R1
R0
MAR
n general purpose
registers

Registers
 Instruction register (IR)
 Program counter (PC)
 General-purpose register (R0 – Rn-1)
 Memory address register (MAR)
 Memory data register (MDR)

Typical Operating Steps
 Programs reside in the memory through input
devices
 PC is set to point to the first instruction
 The contents of PC are transferred to MAR
 A Read signal is sent to the memory
 The first instruction is read out and loaded
into MDR
 The contents of MDR are transferred to IR
 Decode and execute the instruction

Typical Operating Steps
(Cont’)
Get operands for ALU
 General-purpose register
 Memory (address to MAR – Read – MDR to ALU)
Perform operation in ALU
Store the result back
 To general-purpose register
 To memory (address to MAR, result to MDR – Write)
During the execution, PC is
incremented to the next instruction

Interrupt
 Normal execution of programs may be preempted if
some device requires urgent servicing.
 The normal execution of the current program must
be interrupted – the device raises an interrupt
signal.
 Interrupt-service routine
 Current system information backup and restore (PC,
general-purpose registers, control information,
specific information)

Bus Structures
 There are many ways to connect different
parts inside a computer together.
 A group of lines that serves as a connecting
path for several devices is called a bus.
 Address/data/control

Bus Structure
 Single-bus
Figure 1.3. Singlebus structure.
MemoryInput Output Processor

Speed Issue
 Different devices have different
transfer/operate speed.
 If the speed of bus is bounded by the slowest
device connected to it, the efficiency will be
very low.
 How to solve this?
 A common approach – use buffers.

Performance
 The most important measure of a computer is
how quickly it can execute programs.
 Three factors affect performance:
 Hardware design
 Instruction set
 Compiler

Performance
 Processor time to execute a program depends on the hardware
involved in the execution of individual machine instructions.
Main
memory Processor
Bus
Cache
memory
Figure 1.5. The processor cache.

Performance
 The processor and a relatively small cache
memory can be fabricated on a single
integrated circuit chip.
 Speed
 Cost
 Memory management

Processor Clock
 Clock, clock cycle, and clock rate
 The execution of each instruction is divided
into several steps, each of which completes
in one clock cycle.
 Hertz – cycles per second

Basic Performance Equation
 T – processor time required to execute a program that has been
prepared in high-level language
 N – number of actual machine language instructions needed to
complete the execution (note: loop)
 S – average number of basic steps needed to execute one
machine instruction. Each step completes in one clock cycle
 R – clock rate
 Note: these are not independent to each other
R
SN
T
×
=
How to improve T?

Pipeline and Superscalar
Operation
 Instructions are not necessarily executed one after
another.
 The value of S doesn’t have to be the number of
clock cycles to execute one instruction.
 Pipelining – overlapping the execution of successive
instructions.
 Add R1, R2, R3
 Superscalar operation – multiple instruction
pipelines are implemented in the processor.
 Goal – reduce S (could become <1!)

Clock Rate
 Increase clock rate
 Improve the integrated-circuit (IC) technology to make
the circuits faster
 Reduce the amount of processing done in one basic step
(however, this may increase the number of basic steps
needed)
 Increases in R that are entirely caused by
improvements in IC technology affect all
aspects of the processor’s operation equally
except the time to access the main memory.

CISC and RISC
 Tradeoff between N and S
 A key consideration is the use of pipelining
 S is close to 1 even though the number of basic steps
per instruction may be considerably larger
 It is much easier to implement efficient pipelining in
processor with simple instruction sets
 Reduced Instruction Set Computers (RISC)
 Complex Instruction Set Computers (CISC)

Compiler
 A compiler translates a high-level language program
into a sequence of machine instructions.
 To reduce N, we need a suitable machine
instruction set and a compiler that makes good use
of it.
 Goal – reduce N×S
 A compiler may not be designed for a specific
processor; however, a high-quality compiler is
usually designed for, and with, a specific processor.

Performance Measurement
 T is difficult to compute.
 Measure computer performance using benchmark programs.
 System Performance Evaluation Corporation (SPEC) selects and
publishes representative application programs for different application
domains, together with test results for many commercially available
computers.
 Compile and run (no simulation)
 Reference computer
∏=
=
=
n
i
n
iSPECratingSPEC
ratingSPEC
1
1
)(
under testcomputeron thetimeRunning
computerreferenceon thetimeRunning

Multiprocessors and
Multicomputers
 Multiprocessor computer
 Execute a number of different application tasks in parallel
 Execute subtasks of a single large task in parallel
 All processors have access to all of the memory – shared-memory
multiprocessor
 Cost – processors, memory units, complex interconnection networks
 Multicomputers
 Each computer only have access to its own memory
 Exchange message via a communication network – message-
passing multicomputers

Chapter 2. Machine
Instructions and
Programs

Objectives
 Machine instructions and program execution,
including branching and subroutine call and return
operations.
 Number representation and addition/subtraction in
the 2’s-complement system.
 Addressing methods for accessing register and
memory operands.
 Assembly language for representing machine
instructions, data, and programs.
 Program-controlled Input/Output operations.

Number, Arithmetic
Operations, and
Characters

Signed Integer
 3 major representations:
Sign and magnitude
One’s complement
Two’s complement
 Assumptions:
4-bit machine word
16 different values can be represented
Roughly half are positive, half are negative

Sign and Magnitude
Representation
0000
0111
0011
1011
1111
1110
1101
1100
1010
1001
1000
0110
0101
0100
0010
0001
+0
+1
+2
+3
+4
+5
+6
+7-0
-1
-2
-3
-4
-5
-6
-7
0 100 = + 4
1 100 = - 4
+
-
High order bit is sign: 0 = positive (or zero), 1 = negative
Three low order bits is the magnitude: 0 (000) thru 7 (111)
Number range for n bits = +/-2n-1
-1
Two representations for 0

One’s Complement
Representation
 Subtraction implemented by addition & 1's complement
 Still two representations of 0! This causes some problems
 Some complexities in addition
0000
0111
0011
1011
1111
1110
1101
1100
1010
1001
1000
0110
0101
0100
0010
0001
+0
+1
+2
+3
+4
+5
+6
+7-7
-6
-5
-4
-3
-2
-1
-0
0 100 = + 4
1 011 = - 4
+
-

Two’s Complement
Representation
0000
0111
0011
1011
1111
1110
1101
1100
1010
1001
1000
0110
0101
0100
0010
0001
+0
+1
+2
+3
+4
+5
+6
+7-8
-7
-6
-5
-4
-3
-2
-1
0 100 = + 4
1 100 = - 4
+
-
 Only one representation for 0
 One more negative number than positive
number
like 1's comp
except shifted
one position
clockwise

Binary, Signed-Integer
Representations
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
1
1
0
0
1
1
0
0
0
0
1
1
0
0
1
1
1
0
1
0
1
0
1
0
0
1
0
1
0
1
0
1
1+
1
2+
3+
4+
5+
6+
7+
2
3
4
5
6
7
8
0+
0
1+
2+
3+
4+
5+
6+
7+
0+
7
6
5
4
3
2
1
0
1+
2+
3+
4+
5+
6+
7+
0+
7
6
5
4
3
2
1
b3 b2b1b0
Sign and
magnitude 1' s complement 2' s complement
B V alues represented
Figure 2.1. Binary, signedinteger representations.
Page 28

Addition and Subtraction – 2’s
Complement
4
+ 3
7
0100
0011
0111
-4
+ (-3)
-7
1100
1101
11001
4
- 3
1
0100
1101
10001
-4
+ 3
-1
1100
0011
1111
If carry-in to the high
order bit =
carry-out then ignore
carry
if carry-in differs from
carry-out then overflow
Simpler addition scheme makes twos complement the most common
choice for integer number systems within digital systems

2’s-Complement Add and
Subtract Operations
1 1 0 1
0 1 1 1
0 1 0 0
0 0 1 0
1 1 0 0
1 1 1 0
0 1 1 0
1 1 0 1
0 0 1 1
1 0 0 1
0 1 0 1
1 1 1 0
1 0 0 1
1 1 1 1
1 0 0 0
0 0 1 0
0 0 1 1
0 1 0 1
4+( )
2( )
3+( )
2( )
8( )
5+( )
+
+
+
+
+
+
1 1 1 0
0 1 0 0
1 0 1 0
0 1 1 1
1 1 0 1
0 1 0 0
6( )
2( )
4+( )
3( )
4+( )
7+( )
+
+
(b)
(d)1 0 1 1
1 1 1 0
1 0 0 1
1 1 0 1
1 0 0 1
0 0 1 0
0 1 0 0
0 1 1 0
0 0 1 1
1 0 0 1
1 0 1 1
1 0 0 1
0 0 0 1
0 0 1 0
1 1 0 1
0 1 0 1
0 0 1 0
0 0 1 1
5( )
2+( )
3+( )
5+( )
2+( )
4+( )
2( )
7( )
3( )
7( )
6+( )
3+( )
1+( )
7( )
5( )
7( )
2+( )
3( )
+
+

(a)
(c)
(e)
(f)
(g)
(h)
(i)
(j)
Figure 2.4. 2'scomplement Add and Subtract operations.
Page 31

Overflow - Add two positive numbers to get a
negative number or two negative numbers to
get a positive number
5 + 3 = -8 -7 - 2 = +7
0000
0001
0010
0011
1000
0101
0110
0100
1001
1010
1011
1100
1101
0111
1110
1111
+0
+1
+2
+3
+4
+5
+6
+7-8
-7
-6
-5
-4
-3
-2
-1
0000
0001
0010
0011
1000
0101
0110
0100
1001
1010
1011
1100
1101
0111
1110
1111
+0
+1
+2
+3
+4
+5
+6
+7-8
-7
-6
-5
-4
-3
-2
-1

Overflow Conditions
5
3
-8
0 1 1 1
0 1 0 1
0 0 1 1
1 0 0 0
-7
-2
7
1 0 0 0
1 0 0 1
1 1 0 0
1 0 1 1 1
5
2
7
0 0 0 0
0 1 0 1
0 0 1 0
0 1 1 1
-3
-5
-8
1 1 1 1
1 1 0 1
1 0 1 1
1 1 0 0 0
Overflow Overflow
No overflow No overflow
Overflow when carry-in to the high-order bit does not equal carry out

Sign Extension
 Task:
 Given w-bit signed integer x
 Convert it to w+k-bit integer with same value
 Rule:
 Make k copies of sign bit:
 X ′ = xw–1 ,…, xw–1 , xw–1 , xw–2 ,…, x0
k copies of MSB
• • •X
X ′ • • • • • •
• • •
w
wk

Sign Extension Example
short int x = 15213;
int ix = (int) x;
short int y = -15213;
int iy = (int) y;
Decimal Hex Binary
x 15213 3B 6D 00111011 01101101
ix 15213 00 00 C4 92 00000000 00000000 00111011 01101101
y -15213 C4 93 11000100 10010011
iy -15213 FF FF C4 93 11111111 11111111 11000100 10010011

Memory Locations,
Addresses, and
Operations

Memory Location, Addresses,
and Operation
 Memory consists
of many millions of
storage cells,
each of which can
store 1 bit.
 Data is usually
accessed in n-bit
groups. n is called
word length.
second word
first word
Figure 2.5. Memory words.
n bits
last word
i th word
•
•
•
•
•
•

and Operation
 32-bit word length example
(b) Four characters
charactercharactercharacter character
(a) A signed integer
Sign bit: for positive numbers
for negative numbers
ASCIIASCIIASCIIASCII
32 bits
8 bits 8 bits 8 bits 8 bits
b31 b30 b1 b0
b31 0=
b31 1=
•
•
•

and Operation
 To retrieve information from memory, either for one
word or one byte (8-bit), addresses for each location
are needed.
 A k-bit address memory has 2k
memory locations,
namely 0 – 2k
-1, called memory space.
 24-bit memory: 224
= 16,777,216 = 16M (1M=220
)
 32-bit memory: 232
= 4G (1G=230
)
 1K(kilo)=210
 1T(tera)=240

and Operation
 It is impractical to assign distinct addresses
to individual bit locations in the memory.
 The most practical assignment is to have
successive addresses refer to successive
byte locations in the memory – byte-
addressable memory.
 Byte locations have addresses 0, 1, 2, … If
word length is 32 bits, they successive words
are located at addresses 0, 4, 8,…

Big-Endian and Little-Endian
Assignments
2
k
4 2
k
3 2
k
2 2
k
1 2
k
42
k
4
0 1 2 3
4 5 6 7
0 0
4
2
k
1 2
k
2 2
k
3 2
k
4
3 2 1 0
7 6 5 4
Byte addressByte address
(a) Bigendian assignment (b) Littleendian assignment
4
Word
address
•
•
•
•
•
•
Figure 2.7. Byte and word addressing.
Big-Endian: lower byte addresses are used for the most significant bytes of the word
Little-Endian: opposite ordering. lower byte addresses are used for the less significant
bytes of the word

and Operation
 Address ordering of bytes
 Word alignment
 Words are said to be aligned in memory if they
begin at a byte addr. that is a multiple of the num
of bytes in a word.
 16-bit word: word addresses: 0, 2, 4,….
 32-bit word: word addresses: 0, 4, 8,….
 64-bit word: word addresses: 0, 8,16,….
 Access numbers, characters, and character
strings

Memory Operation
 Load (or Read or Fetch)
 Copy the content. The memory content doesn’t change.
 Address – Load
 Registers can be used
 Store (or Write)
 Overwrite the content in memory
 Address and Data – Store
 Registers can be used

Instruction and
Instruction
Sequencing

“Must-Perform” Operations
 Data transfers between the memory and the
processor registers
 Arithmetic and logic operations on data
 Program sequencing and control
 I/O transfers

Register Transfer Notation
 Identify a location by a symbolic name
standing for its hardware binary address
(LOC, R0,…)
 Contents of a location are denoted by placing
square brackets around the name of the
location (R1←[LOC], R3 ←[R1]+[R2])
 Register Transfer Notation (RTN)

Assembly Language Notation
 Represent machine instructions and
programs.
 Move LOC, R1 = R1←[LOC]
 Add R1, R2, R3 = R3 ←[R1]+[R2]

CPU Organization
 Single Accumulator
 Result usually goes to the Accumulator
 Accumulator has to be saved to memory quite
often
 General Register
 Registers hold operands thus reduce memory
traffic
 Register bookkeeping
 Stack
 Operands and result are always in the stack

Instruction Formats
 Three-Address Instructions
 ADD R1, R2, R3 R1 ← R2 + R3
 Two-Address Instructions
 ADD R1, R2 R1 ← R1 + R2
 One-Address Instructions
 ADD M AC ← AC + M[AR]
 Zero-Address Instructions
 ADD TOS ← TOS + (TOS – 1)
 RISC Instructions
 Lots of registers. Memory is restricted to Load & Store
Opcode Operand(s) or Address(es)

Instruction Formats
Example: Evaluate (A+B) ∗ (C+D)
 Three-Address
1. ADD R1, A, B ; R1 ← M[A] + M[B]
2. ADD R2, C, D ; R2 ← M[C] + M[D]
3. MUL X, R1, R2 ; M[X] ← R1 ∗ R2

Instruction Formats
 Two-Address
1. MOV R1, A ; R1 ← M[A]
2. ADD R1, B ; R1 ← R1 + M[B]
3. MOV R2, C ; R2 ← M[C]
4. ADD R2, D ; R2 ← R2 + M[D]
5. MUL R1, R2 ; R1 ← R1 ∗ R2
6. MOV X, R1 ; M[X] ← R1

Instruction Formats
 One-Address
1. LOAD A ; AC ← M[A]
2. ADD B ; AC ← AC + M[B]
3. STORET ; M[T] ← AC
4. LOAD C ; AC ← M[C]
5. ADD D ; AC ← AC + M[D]
6. MUL T ; AC ← AC ∗ M[T]
7. STOREX ; M[X] ← AC

Instruction Formats
 Zero-Address
1. PUSH A ; TOS ← A
2. PUSH B ; TOS ← B
3. ADD ; TOS ← (A + B)
4. PUSH C ; TOS ← C
5. PUSH D ; TOS ← D
6. ADD ; TOS ← (C + D)
7. MUL ; TOS ←
(C+D)∗(A+B)
8. POP X ; M[X] ← TOS

Instruction Formats
 RISC
1. LOAD R1, A ; R1 ← M[A]
2. LOAD R2, B ; R2 ← M[B]
3. LOAD R3, C ; R3 ← M[C]
4. LOAD R4, D ; R4 ← M[D]
5. ADD R1, R1, R2 ; R1 ← R1 + R2
6. ADD R3, R3, R4 ; R3 ← R3 + R4
7. MUL R1, R1, R3 ; R1 ← R1 ∗ R3
8. STOREX, R1 ; M[X] ← R1

Using Registers
 Registers are faster
 Shorter instructions
 The number of registers is smaller (e.g. 32
registers need 5 bits)
 Potential speedup
 Minimize the frequency with which data is
moved back and forth between the memory
and processor registers.

Instruction Execution and
Straight-Line Sequencing
R0,C
B,R0
A,R0
Movei + 8
Begin execution here Movei
ContentsAddress
C
B
A
the program
Data for
segment
program
3instruction
Addi + 4
Figure 2.8. A program for C ← [Α] + [Β].
Assumptions:
- One memory operand
per instruction
- 32-bit word length
- Memory is byte
addressable
- Full memory address
can be directly specified
in a single-word instruction
Two-phase procedure
-Instruction fetch
-Instruction execute
Page 43

Branching
NUMn
NUM2
NUM1
R0,SUM
NUMn,R0
NUM3,R0
NUM2,R0
NUM1,R0
Figure 2.9. A straightline program for adding n numbers.
Add
Add
Move
SUM
i
Move
Add
i 4n+
i 4n 4+
i 8+
i 4+
•
•
•
•
•
•
•
•
•

Branching
N,R1Move
NUMn
NUM2
NUM1
R0,SUM
R1
"Next" number to R0
Figure 2.10. Using a loop to add n numbers.
LOOP
Decrement
Move
LOOP
loop
Program
Determine address of
"Next" number and add
N
SUM
n
R0Clear
Branch>0
•
•
•
•
•
•
Branch target
Conditional branch

Condition Codes
 Condition code flags
 Condition code register / status register
 N (negative)
 Z (zero)
 V (overflow)
 C (carry)
 Different instructions affect different flags

Conditional Branch
Instructions
 Example:
 A: 1 1 1 1 0 0 0 0
 B: 0 0 0 1 0 1 0 0
A: 1 1 1 1 0 0 0 0
+(−B): 1 1 1 0 1 1 0 0
1 1 0 1 1 1 0 0
C = 1
S = 1
V = 0
Z = 0

Status Bits
ALU
V Z S C
Zero Check
Cn
Cn-1
Fn-1
A B
F

Generating Memory Addresses
 How to specify the address of branch target?
 Can we give the memory operand address
directly in a single Add instruction in the
loop?
 Use a register to hold the address of NUM1;
then increment by 4 on each pass through
the loop.

Addressing Modes
 Implied
 AC is implied in “ADD M[AR]” in “One-Address”
instr.
 TOS is implied in “ADD” in “Zero-Address” instr.
 Immediate
 The use of a constant in “MOV R1, 5”, i.e. R1 ←
5
 Register
 Indicate which register holds the operand
Opcode Mode ...

Addressing Modes
 Register Indirect
 Indicate the register that holds the number of the
register that holds the operand
MOV R1, (R2)
 Autoincrement / Autodecrement
 Access & update in 1 instr.
 Direct Address
 Use the given address to access a memory
location
R1
R2 = 3
R3 = 5

Addressing Modes
 Indirect Address
 Indicate the memory location that holds the
address of the memory location that holds the
data
AR = 101
100
101
102
103
104
0 1 0 4
1 1 0 A

100
101
102
103
104
0
1
2
Addressing Modes
 Relative Address
 EA = PC + Relative Addr
AR = 100
1 1 0 A
PC = 2
+
Could be Positive
or Negative
(2’s Complement)

Addressing Modes
 Indexed
 EA = Index Register + Relative Addr
100
101
102
103
104
AR = 100
1 1 0 A
XR = 2
+
Could be Positive
or Negative
(2’s Complement)
Useful with
“Autoincrement” or
“Autodecrement”

Addressing Modes
 Base Register
 EA = Base Register + Relative Addr
100
101
102
103
104
BR = 100
0 0 0 A
AR = 2
+
Could be Positive
or Negative
(2’s Complement)
Usually points
to the beginning
of an array
0 0 0 5
0 0 1 2
0 1 0 7
0 0 5 9

Addressing Modes
 The different
ways in which
the location of
an operand is
specified in
an instruction
are referred to
as addressing
modes.
Name Assem bler syn tax Addressing function
Immediate #Value Operand = Value
Register Ri EA = Ri
Absolute(Direct) LOC EA = LOC
Indirect (Ri ) EA = [Ri ]
(LOC) EA = [LOC]
Index X(Ri) EA = [Ri ] + X
Basewith index (Ri ,Rj ) EA = [Ri ] + [Rj ]
Basewith index X(Ri,Rj ) EA = [Ri ] + [Rj ] + X
and offset
Relative X(PC) EA = [PC] + X
Autoincrement (Ri )+ EA = [Ri ] ;
Increment Ri
Autodecrement (Ri ) Decrement Ri ;
EA = [Ri]
−

Indexing and Arrays
 Index mode – the effective address of the operand
is generated by adding a constant value to the
contents of a register.
 Index register
 X(Ri): EA = X + [Ri]
 The constant X may be given either as an explicit
number or as a symbolic name representing a
numerical value.
 If X is shorter than a word, sign-extension is needed.

Indexing and Arrays
 In general, the Index mode facilitates access
to an operand whose location is defined
relative to a reference point within the data
structure in which the operand appears.
 Several variations:
(Ri, Rj): EA = [Ri] + [Rj]
X(Ri, Rj): EA = X + [Ri] + [Rj]

Relative Addressing
 Relative mode – the effective address is determined
by the Index mode using the program counter in
place of the general-purpose register.
 X(PC) – note that X is a signed number
 Branch>0 LOOP
 This location is computed by specifying it as an
offset from the current value of PC.
 Branch target may be either before or after the
branch instruction, the offset is given as a singed
num.

Additional Modes
 Autoincrement mode – the effective address of the operand is
the contents of a register specified in the instruction. After
accessing the operand, the contents of this register are
automatically incremented to point to the next item in a list.
 (Ri)+. The increment is 1 for byte-sized operands, 2 for 16-bit
operands, and 4 for 32-bit operands.
 Autodecrement mode: -(Ri) – decrement first
R0Clear
R0,SUM
R1
(R2)+,R0
Figure 2.16. The Autoincrement addressing mode used in the program of Figure 2.12.
Initialization
Move
LOOP Add
Decrement
LOOP
#NUM1,R2
N,R1Move
Move
Branch>0

Types of Instructions
 Data Transfer Instructions
Name Mnemonic
Load LD
Store ST
Move MOV
Exchange XCH
Input IN
Output OUT
Push PUSH
Pop POP
Data value is
not modified

Data Transfer Instructions
Mode Assembly Register Transfer
Direct address LD ADR AC ← M[ADR]
Indirect address LD @ADR AC ← M[M[ADR]]
Relative address LD $ADR AC ← M[PC+ADR]
Immediate operand LD #NBR AC ← NBR
Index addressing LD ADR(X) AC ← M[ADR+XR]
Register LD R1 AC ← R1
Register indirect LD (R1) AC ← M[R1]
Autoincrement LD (R1)+ AC ← M[R1], R1 ← R1+1

Data Manipulation Instructions
 Arithmetic
 Logical & Bit Manipulation
 Shift
Name Mnemonic
Increment INC
Decrement DEC
Add ADD
Subtract SUB
Multiply MUL
Divide DIV
Add with carry ADDC
Subtract with borrow SUBB
Negate NEG
Name Mnemonic
Clear CLR
Complement COM
AND AND
OR OR
Exclusive-OR XOR
Clear carry CLRC
Set carry SETC
Complement
carry
COMC
Enable interrupt EI
Name Mnemonic
Logical shift right SHR
Logical shift left SHL
Arithmetic shift right SHRA
Arithmetic shift left SHLA
Rotate right ROR
Rotate left ROL
Rotate right through
carry
RORC

Program Control Instructions
Name Mnemonic
Branch BR
Jump JMP
Skip SKP
Call CALL
Return RET
Compare
(Subtract)
CMP
Test (AND) TST
Subtract A – B but
don’t store the result
1 0 1 1 0 0 0 1
0 0 0 0 1 0 0 0
0 0 0 0 0 0 0 0
Mask

Conditional Branch
Instructions
Mnemonic Branch Condition Tested Condition
BZ Branch if zero Z = 1
BNZ Branch if not zero Z = 0
BC Branch if carry C = 1
BNC Branch if no carry C = 0
BP Branch if plus S = 0
BM Branch if minus S = 1
BV Branch if overflow V = 1
BNV
Branch if no
overflow
V = 0

I/O
 The data on which the instructions operate
are not necessarily already stored in memory.
 Data need to be transferred between
processor and outside world (disk, keyboard,
etc.)
 I/O operations are essential, the way they are
performed can have a significant effect on the
performance of the computer.

Program-Controlled I/O
Example
 Read in character input from a keyboard and
produce character output on a display
screen.
 Rate of data transfer (keyboard, display, processor)
 Difference in speed between processor and I/O device
creates the need for mechanisms to synchronize the
transfer of data.
 A solution: on output, the processor sends the first
character and then waits for a signal from the display
that the character has been received. It then sends the
second character. Input is sent from the keyboard in a
similar way.

Example
DATAIN DATAOUT
SIN SOUT
Keyboard Display
Bus
Figure 2.19 Bus connection for processor, keyboard, and display.
Processor
- Registers
- Flags
- Device interface

Example
 Machine instructions that can check the state
of the status flags and transfer data:
READWAIT Branch to READWAIT if SIN = 0
Input from DATAIN to R1
WRITEWAIT Branch to WRITEWAIT if SOUT = 0
Output from R1 to DATAOUT

Example
 Memory-Mapped I/O – some memory
address values are used to refer to peripheral
device buffer registers. No special
instructions are needed. Also use device
status registers.
READWAIT Testbit #3, INSTATUS
Branch=0 READWAIT
MoveByte DATAIN, R1

Example
 Assumption – the initial state of SIN is 0 and the
initial state of SOUT is 1.
 Any drawback of this mechanism in terms of
efficiency?
 Two wait loopsprocessor execution time is wasted
 Alternate solution?
 Interrupt

Home Work
 For each Addressing modes mentioned
before, state one example for each
addressing mode stating the specific benefit
for using such addressing mode for such an
application.

Stack Organization
SP
Stack Bottom
Current
Top of Stack
TOS
 LIFO
Last In First Out
0
1
2
3
4
7
8
9
10
5
6
Stack
0 0 5 5
0 0 0 8
0 0 2 5
0 0 1 5
0 1 2 3
FULL EMPTY

Stack Organization
SP
Stack Bottom
Current
Top of Stack
TOS
 PUSH
SP ← SP – 1
M[SP] ← DR
If (SP = 0) then (FULL ← 1)
EMPTY ← 0
0
1
2
3
4
7
8
9
10
5
6
Stack
0 0 5 5
0 0 0 8
0 0 2 5
0 0 1 5
0 1 2 3
FULL EMPTY
1 6 9 0
1 6 9 0Current
Top of Stack
TOS

Stack Organization
SP
Stack Bottom
Current
Top of Stack
TOS
 POP
DR ← M[SP]
SP ← SP + 1
If (SP = 11) then (EMPTY ← 1)
FULL ← 0
0
1
2
3
4
7
8
9
10
5
6
Stack
0 0 5 5
0 0 0 8
0 0 2 5
0 0 1 5
0 1 2 3
FULL EMPTY
1 6 9 01 6 9 0
Current
Top of Stack
TOS

0
1
2
102
202
201
200
100
101
Stack Organization
 Memory Stack
 PUSH
SP ← SP – 1
M[SP] ← DR
 POP
DR ← M[SP]
SP ← SP + 1
PC
AR
SP

Reverse Polish Notation
 Infix Notation
A + B
 Prefix or Polish Notation
+ A B
 Postfix or Reverse Polish Notation (RPN)
A B +
A ∗ B + C ∗ D A B ∗ C D ∗ +
RPN
(2) (4) ∗ (3) (3) ∗
+
(8) (3) (3) ∗ +
(8) (9) +
17

 Example
(A + B) ∗ [C ∗ (D + E) + F]
(A B +) (D E +) C
∗
∗F +

 Stack Operation
(3) (4) ∗ (5) (6) ∗ +
PUSH 3
PUSH 4
MULT
PUSH 5
PUSH 6
MULT
ADD
3
4
12
5
6
30
42

Logical Shifts
 Logical shift – shifting left (LShiftL) and shifting right
(LShiftR)
CR00
before:
after:
0
1
0 0 01 1 1 . . . 11
0 0 1 1 1 000
(b) Logical shift right LShiftR #2,R0
(a) Logical shift left LShiftL #2,R0
C R0 0
before:
after:
0
1
0 0 01 1 1 . . . 11
1 10 . . . 00101
. . .

Arithmetic Shifts
C
before:
after:
0
1
1 1 00 0 1 . . . 01
1 1 0 0 1 011
(c) Arithmetic shift right AShiftR #2,R0
R0
. . .

Rotate
Figure 2.32.  Rotate instructions.
CR0
before:
after:
0
1
0 0 01 1 1 . . . 11
1 0 1 1 1 001
(c) Rotate right without carry RotateR   #2,R0
(a) Rotate left without carry RotateL    #2,R0
C R0
before:
after:
0
1
0 0 01 1 1 . . . 11
1 10 . . . 10101
C
before:
after:
0
1
0 0 01 1 1 . . . 11
1 0 1 1 1 000
(d) Rotate right with carry RotateRC   #2,R0
R0
. . .
. . .
(b) Rotate left with carry RotateLC   #2,R0
C R0
before:
after:
0
1
0 0 01 1 1 . . . 11
1 10 . . . 00101

Multiplication and Division
 Not very popular (especially division)
 Multiply Ri, Rj
Rj ← [Ri] х [Rj]
 2n-bit product case: high-order half in R(j+1)
 Divide Ri, Rj
Rj ← [Ri] / [Rj]
Quotient is in Rj, remainder may be placed in R(j+1)

Encoding of
Machine
Instructions

Encoding of Machine
Instructions
 Assembly language program needs to be converted into machine
instructions. (ADD = 0100 in ARM instruction set)
 In the previous section, an assumption was made that all
instructions are one word in length.
 OP code: the type of operation to be performed and the type of
operands used may be specified using an encoded binary pattern
 Suppose 32-bit word length, 8-bit OP code (how many instructions
can we have?), 16 registers in total (how many bits?), 3-bit
addressing mode indicator.
 Add R1, R2
 Move 24(R0), R5
 LshiftR #2, R0
 Move #$3A, R1
 Branch>0 LOOP
OP code Source Dest Other info
8 7 7 10
(a) Oneword instruction

Encoding of Machine
Instructions
 What happens if we want to specify a memory
operand using the Absolute addressing mode?
 Move R2, LOC
 14-bit for LOC – insufficient
 Solution – use two words
(b) Twoword instruction
Memory address/Immediate operand
OP code Source Dest Other info

Encoding of Machine
Instructions
 Then what if an instruction in which two operands
can be specified using the Absolute addressing
mode?
 Move LOC1, LOC2
 Solution – use two additional words
 This approach results in instructions of variable
length. Complex instructions can be implemented,
closely resembling operations in high-level
programming languages – Complex Instruction Set
Computer (CISC)

Encoding of Machine
Instructions
 If we insist that all instructions must fit into a single
32-bit word, it is not possible to provide a 32-bit
address or a 32-bit immediate operand within the
instruction.
 It is still possible to define a highly functional
instruction set, which makes extensive use of the
processor registers.
 Add R1, R2 ----- yes
 Add LOC, R2 ----- no
 Add (R3), R2 ----- yes

Chapter1 basic structure of computers

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Chapter1 basic structure of computers