29 October 2013Birkbeck College, U. London1 Introduction to Computer Systems Lecturer: Steve Maybank Department of Computer Science and Information Systems.

29 October 2013 Birkbeck College, U. London 1

Introduction to Computer Systems

Lecturer: Steve MaybankDepartment of Computer Science and

Information [email protected]

Autumn 2013

Week 5b: CPU and the Machine Cycle

29 October 2013 Birkbeck College, U. London 2

Action of a Computer 1

Recall Tom and his boxes

Instruction: add the contents of box a and box b, put the result in box c.

The part of a computer corresponding to Tom is the central processing unit (CPU)

10a cb d e f g

5 12 -3 -1 111

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Action of a Computer 2

Tom has to:

Read the contents 10 of box a and the contents 5 of box b.

Add together 10 and 5 to get 15.

Write 15 in box c.

10a cb d e f g

5 12 -3 -1 111

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Summary of the Actions of the CPU

Obtain an instruction

Read data from memory

Calculate

Write data to memory

The CPU requires its own memory for storing instructions and data

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Parts of a Computer

arithmetic/logicunit

register unit

central processing unit

:

registers

bus

mainmemory

control unit

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Central Processing Unit

Register: small amount of very fast read/write memory, e.g. enough to store one number.

Arithmetic/Logic Unit: operates on data in registers, e.g. addition, subtraction.

Control Unit: controls the ALU.

ALU

ControlUnit

Register Unit

.

.

.

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Concepts Places for storing bit strings:

registers, main memory. Transfer of bit strings from one

place to another. Operations on bit strings: produce

a new bit string from one or more given bit strings.

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Some Actions of a Computer

ALUCPU

registers

busmain

memory

1. Take a bit string from main memory and put it in a register.2. Take a bit string from a register and put it in main memory.3. Move a bit string from one register to another.4. Take bit strings bi from registers Ri, i=1, 2. Interpret b1, b2 as two’s complement representations of integers, add them and put the result in the register R3.

control unit

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Bus

Wires connecting CPU to main memory. Read: CPU sends read signal and

address of memory cell. Write: CPU sends write signal, address,

and data to be written. Length ~15cm. Speed of light = 30cm

in 1 ns. Time for CPU to access a register ~ 2.5 ns.

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Logical Structure of Main Memory

List of cells or words addressed from 0 to 2n-1, where n is a large integer.

All cells contain the same number of bytes, e.g. 1 byte.

Example: if a register contains 16 bits then the largest memory that can be addressed using the register has 216 cells.

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Memory Access Times

Access Time

Size

CPU registers

2.5ns 256 bit

RAM 60 ns ~1 GB

Hard disk

15 ms 2 TB

Tape 10min 10 TB

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Stored Program Concept The program is stored in main memory

along with the data. The control unit reads both instructions

and data from the main memory. This is the von Neumann architecture.

Advantage: the program can be changed easily.

Disadvantage: high traffic on the bus. This is the von Neumann bottleneck.

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The Machine Cycle

Fetch nextinstruction frommemory to CPU

Decode thebit pattern

Execute theinstruction

FetchDecodeExecute

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CPU and Main Memory in More Detail

Address | Cells

00

01

.

.

FF

Arithmetic/logic unit

control unit

Registers

.

.

0

F

1Program counter

Instruction register

Bus

Main memoryCentral processing unit

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Special Registers

Program counter: contains the address of the next instruction to be executed.

Instruction register: holds the instruction which is being executed.

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Adding Values Stored in Memory

1. Get the first value from memory and place it in a register.

2. Get the second value from memory and place it in another register.

3. Activate the addition circuitry with the registers in 1 and 2 providing the inputs and another register to hold the result.

4. Store the result in memory.5. Stop.

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Cache Memory High speed memory in the CPU. Size

~100 KB. Stores data and instructions recently

accessed from main memory. Cache read/write is faster than main

memory read/write. Disadvantage: overhead of keeping the

cache consistent with main memory.

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System Clock

Circuit generating pulses at regular intervals.

The computer’s activities are in step with the pulses, e.g. 5 pulses for an addition.

Current clock speeds ~ 2 GHz. (=2 pulses per nanosecond).

Clock speed only a rough guide to performance, e.g. instructions per second.

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Motherboard

Main circuit board of the PC. Contains the central processing unit

(CPU) and the bus. Components, e.g. memory, hard drive

interface, plug into the motherboard to obtain power and communicate with the CPU.

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Example of a Motherboard

http://en.wikipedia.org/wiki/Motherboard