Riyadh Philanthropic Society For Science

Prince Sultan College For WomanDept. of Computer & Information Sciences

CS 251Introduction to Computer Organization

& Assembly Language

Lecture 3(Computer System Organization)

Processors

Processors 2

From text book: 2.1, 2.1.1, 2.1.2

The Von Neumann Machine

CPU Organization

Instruction Execution

Timing

Control Unit

CISC vs. RISC

Design Principles for modern Computers

Outline

The Von-Neumann Machine

3Processors

The first version of modern computer designed by John Von-Neumann

It is the basis for the majority of digital computer designs

Its main features are:

Using binary arithmetic

Representing the program in a digital form inside the computer’s memory

Organizing the steps of the computer operation into a cycle of decode-fetch-

execute.

Central Processing Unit (CPU)

4Processors

It is the brain of the computer

Its function is to execute programs stored in main memory by fetching instructions, decoding them, and executing them.

CPU

Main Memory

(IOP) O-DI-D

RegistersControl UnitALU

It consists of 3 main components

Arithmetic Logic Unit (ALU)

Control Unit

Registers

CPU – Internal Components

5Processors

Internal components of the CPU:

I. ALU: Arithmetic & Logic Unit responsible for manipulating data

II. Control Unit: Directs the flow of information. It controls where every bit of data goes

III. Registers: High speed memory locations used to store temporary results & certain control information

CPU

MARControl

Unit

MDR

PC

IR

R0

R1

:::

Rn-1

ALU

Registers

Processors 6

CPU – Data path

(A) (B)

ALU

A + B)

A + B

A

B

Registers

ALU input register

ALU output register

ALU input bus

Processors 7

The figure corresponds to the data path of a typical Von-Neumann machine

Not all designs have the input registers A & B or the output register.

It consists of:

Registers (Typically 1 to 32)

ALU

Several busses connecting the pieces

CPU – Data path (Cont.)

Processors 8

1. The registers feed into the ALU input registers A & B

2. The input registers hold the input data while the ALU does the execution

3. The ALU performs simple operations such as addition, subtraction, and Boolean

logic on its inputs

4. The output value is fed into the ALU output register

5. The result is then stored in one of the registers in the register stack

6. The value of the register could be then saved in memory if required

7. The cycle repeats a gain for any other operation in the ALU

CPU – Data path Flow Sequence

Processors 9

CPU – Data path Features

The data path is very important in all machines

Not all machines have the input registers A & B or the output register

The process of running two operands through the ALU and storing the result is

called the data path cycle.

The faster the data path cycle is, the faster the machine runs.

Processors 10

CPU – Data path Instructions

Most instructions can be divided into one of the two categories:

Register – Register Instructions:

Involve instructions between registers

Register – memory Instructions:

Allow memory words to be fetched into registers, where they can be used as ALU

inputs in subsequent instructions.

Allow registers to be stored back into memory.

Processors 11

Machine Instruction Format• A machine instruction has two parts:

•Opcode• Operands

Specify the type of operation Immediate data or address

Opcode Operand 1 Operand 2

16 bits

4 bits 6 bits 6 bits

Processors 12

CPU – Registers There are three main registers in a typical ALU:

Program Counter Register (PC):

It holds the address of the next instruction to be fetched for execution

Instruction Register (IR):

It holds the instruction currently being executed

Accumulator Register (AC):

It accumulates arithmetic results

Processors 13

Instruction Cycle

The processing required for an instruction is called an instruction cycle.

Instruction cycle = fetch-decode-execute cycle

The program is executed one instruction at a time.

The execution of the program is nothing more than a repeated sequence of

fetching, decoding, & executing individual instructions, until the program executes a

Halt or until a fetal error occurs

Processors 14

Instruction Execution

Obtain instruction from program storageInstruction

Fetch

Instruction Decode

Operand fetch

Execute

Result Store

Next Instruction

Determine required actions

Locate and obtain operand data

Compute result value and status

Deposit result in storage

Determine successor instruction

Processors 15

1. Fetch the next instruction from memory into the instruction register

2. Change the program counter to point to the following instruction

3. Determine the type of instruction just fetched

4. If the instruction uses a word in memory, determine where it is.

5. Fetch the word, if needed, into the CPU register

6. Execute the instruction

7. Go to step 1 to begin executing the following instruction

Instruction Cycle – In details

Processors 16

ExampleMain Memory

979899100101

357358359

25

Load contents of 100 into AC

Add contents of 101 to AC

Store contents of AC to 100

360361

Program code

void main(void){ : : a = 2; b = 5; a = a + b;}

Processors 17

ExampleMain Memory

979899100101

357358359

25

Load contents of 100 into AC

Add contents of 101 to AC

Store contents of AC to 100

360361

357

Start

PC

AC

IR

Processors 18

ExampleCPU

PC

AC

IR

Main Memory

979899100101

357358359

25

Load contents of 100 into AC

Add contents of 101 to AC

Store contents of AC to 100

360361

Fetch

357

Fetch

Load 100 in AC

358

2

A register

Processors 19

ExampleCPU

Load 100 in AC

PC

AC

IR

Main Memory

979899100101

357358359

25

Load contents of 100 into AC

Add contents of 101 to AC

Store contents of AC to 100

360361

Execute

358

Execute

2

2

A register

Processors 20

ExampleCPU

2

Load 100 in AC

PC

AC

IR

Main Memory

979899100101

357358359

25

Load contents of 100 into AC

Add contents of 101 to AC

Store contents of AC to 100

360361

Fetch

358

Fetch

Add 101 to AC

359

5

A register

Processors 21

ExampleCPU

Add 101 to AC

PC

AC

IR

Main Memory

979899100101

357358359

25

Load contents of 100 into AC

Add contents of 101 to AC

Store contents of AC to 100

360361

Execute

359

Execute

2 + 5 = 7

5

A register

Processors 22

ExampleCPU

2 + 5 = 7

Add 101 to AC

PC

AC

IR

Main Memory

979899100101

357358359

25

Load contents of 100 into AC

Add contents of 101 to AC

Store contents of AC to 100

360361

Fetch

359

Fetch

Store AC in 100

Processors 23

ExampleCPU

7

Store AC in 100

PC

AC

IR

Main Memory

979899100101

357358359

25

Load contents of 100 into AC

Add contents of 101 to AC

Store contents of AC to 100

360361

Execute

359

Execute

7

Processors 24

ExampleCPU

7

Store AC in 100

PC

AC

IR

Main Memory

979899100101

357358359

75

Load contents of 100 into AC

Add contents of 101 to AC

Store contents of AC to 100

360361

Halt

359

Processors 25

Instruction Execution Can we run a program that imitate the function of the CPU?

Yes, by using an interpreter.

This machine can be much simpler & less expensive than a hardware processor.

A hardware machine should also be provided (in order to run the interpreter)

Processors 26

Timing

Voltage

Time

Train of clock pulses

To ensure that the steps of an instruction are carried out in an orderly fashion a clock circuit controls the processor by generating a train of clock pulses.

Processors 27

Timing (Cont.) The time interval between two pulses is known as a clock period

The clock rate or speed corresponds to the number of pulses per second,

measured in Mega Hertz (MHz). 1 MHz = 1 million cycles per second

The computer circuits perform an operation on the clock pulse

Each step in the instruction execution cycle requires one or more clock periods

If the clock circuit is sped up, the processor could be made to operate faster.

However, each processor has a rated maximum clock speed over which it can’t

operate properly

Processors 28

The control unit of the microprocessor is the one responsible for controlling the

sequencing of events needed for the execution of an instruction, as well as the

timing of this sequence of events.

It obtains instructions from memory in the proper order, and sends them to IR.

Decode them, and issue the necessary timing and control signals to all other

devices so that the instructions can be executed correctly.

The control unit is complemented by a clock or timing generator that helps it

trigger the occurrence of each event at the correct point in time.

Control Unit (CU)

Processors 29

There are two major types of control organization:

Hardwired control

Microprogrammed control

RISC vs. CISC

This control determines the instruction set of the computer and it is mainly split

into two main categories:

RISC: (Reduced Instruction Set Computer)

CISC: (Complex Instruction set Computer)