basic structure of computers

Chapter 1. Basic
Structure of Computers

Functional Units
Arithmetic
Input and
logic

Memory

Output Control

I/O Processor

Figure 1.1. Basic functional units of a computer.

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
 Storeprograms 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 name Action for R-type Action for Memory- Action for Action for
instructions reference Instructions branches jumps

Instruction fetch IR = MEM[PC]
PC = PC + 4

Instruction decode/ register A = Reg[IR[25-21]]
fetch B = Reg[IR[20-16]]
ALUOut = PC + (sign extend (IR[15-0])<<2)

Execution, address ALUOut = A op B ALUOut = A+sign IF(A==B) Then PC=PC[31-
computation, branch/jump extend(IR[15-0]) PC=ALUOut 28]||(IR[25-
completion 0]<<2)

Memory access or R-type Reg[IR[15-11]] = Load:MDR =Mem[ALUOut]
completion ALUOut or
Store:Mem[ALUOut] = B

Memory read completion Load: Reg[IR[20-16]] =
MDR

Basic Operational
Concepts

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
Memory

MAR MDR
Control

PC R0

R1
Processor
IR

ALU
Rn 1

n general purpose
registers

Figure 1.2. Connections between the processor and the memory.

Registers
 Instruction
register (IR)
 Program counter (PC)

 General-purpose register (R – R )
0 n-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

Input Output Memory Processor

Figure 1.3. Singlebus structure.

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 Cache
memory memory Processor

Bus

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

N×S
T=
R
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
 Tradeoffbetween 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

Running time on the reference computer
SPEC rating =
Running time on the computer under test
n 1
SPEC rating = (∏SPECi ) n

i =1

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

basic structure of computers

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