The CPU - Outline.

The CPU - Outline.

Components of CPU Englander p. 166-167

Registers and their role p. 167-169

The operation of memory p. 170-176

The fetch execute cycle p. 176-177

Buses p. 177-183

Instruction sets p. 183-197

CPU components

Three major components of the CPU: ALU, Control unit, and Registers.

The Arithmetic Logic Unit (ALU) performs all calculations, operating on binary data with arithmetic and logical functions.

Arithmetic operations include additions, division, exponentials, logarithms.

Logical functions include comparisions (< > = ) and transformations

CPU components

Control Unit fetches and interprets instructions from memory, deciding what action has to be taken

Control Unit executes instructions that do not require logical or arithmetic operations eg JUMP or HALT instruction

Control Unit generates signals to coordinate all other CPU components, and moves data and instructions between registers.

CPU components

Registers - very fast memory providing temporary storage places for data and instructions. (May be in control unit or ALU. )

Registers are often assigned special roles - eg the accumulator which holds the result of calculations.

Other important registers - Program counter (instruction pointer)(PC) Memory address register (MAR) Memory data(buffer) register (MDR) Program status register (PSW) Instruction register (IR)

ALU

ControlUnit

CPU

Accumulator

InstructionRegister

Program Counter

PrimaryMemory

BUS

Data is moved between CPU components and from memory into registers along buses - shared communication pathways

Control UnitBUS

BUS

BUS

The CPU operates by performing an Instruction Cycle consisting of 2 main parts :

Fetch + Execute. CPU reads (fetches) instructions from

memory CPU decodes and ‘executes’ instruction.

START

Execute the Instruction

Fetch the next instruction

Fetch-execute cycle

The fetched instruction is loaded in to the IR and the program counter (PC) keeps track of which instruction is to be fetched next.

A single instruction is broken down further into micro-instructions before it can be executed. For example it must initially be broken down into opcode and operands.

Fetch-Execute Cycle: Fetch

• Program Counter -> Memory Address Register

• Data -> Memory Data Register

• Memory Data Register -> Instruction Register Execute (load)

• IR (op+address) -> Memory Address Register

• Memory Data Register -> Accumulator

• Program Counter + 1 -> PC

Fetch-execute cycle

Instruction sets

Each CPU type has a unique collection of instructions which it can execute - known as its instruction set.

The instruction set must provide for:

Arithmetic e.g. + - * / Comparisons e.g. < > = Data movement e.g LOAD,STORE, MOVE Program control e.g. JUMP, CALL, RETURN

Also instructions to implement boolean logic, operate on stacks, and multimedia data

System Clock

System clock generates regular pulses to synchronize all system events and determine the speed at which processing can occur.

Each fetch-execute instruction cycle divided into states, which are one clock pulse long. Most instructions require multiple steps, and so require several clock pulses to complete.

Some individual steps (e.g. a memory access) take longer & may require additional clock pulses to complete – these clock cycles spent waiting are called wait states

System Clock

The clock speed of a CPU determines how often a new instruction is executed, and is measured in MHz or GHz: 1.7GHz means that a computer executes 1,700,000,000 instructions per second!

However, all recent microprocessors overlap the fetching, decoding and execution of a number of instructions at same time. This is called pipelining.

Therefore, clock speed not necessarily an accurate measure of performance, and other measurements are required (to be covered later).

Evolution of Intel Microprocessors.

Name: 8008 80386 80486 Pentium Pentium4

Year: 1972 1985 1989 1993 2000

I. set 66 154 235 >300 >400

Clock <1mh 33mh 50mh 133mh 1.3GH

Speed

System Clock

Memory addressing

The “address” is not the “data”• address bus• data bus

Size matters….• 2, 4, 8, 16, 32, 64 bit address buses • M=2^k where k=the width of the address bus• Word size determined by bus and register size.

Types of memory

examples: RAM - random access memory DRAM - dynamic RAM, requires refresh SRAM - static RAM, no refresh & faster ROM - read only memory PROM - programmable ROM EPROM - erasable, programmable ROM

BusesData travels between components (CPU, memory &

I/O devices) of the computer along communication paths called buses. (Note: can also have buses external to system)

Bus is made up of multiple lines, which may be wires or conductors on a printed board.

Bus may carry data between two components only (point-to-point) or it may be shared between many (multipoint). Need to control bus access in multipoint buses to prevent collisions between signals.

Buses

Multipoint / system bus

CPU

videocontroller

memory

disk controller

Point-to pointbus

ALU

Control unit

Buses may be serial (one bit at time) or parallel (multiple bits at time. Most internal buses are parallel, most external are serial.

Bus LinesBus lines classified as data, address, control and

power lines. Depending on the function of the bus, it may have all or some of these types.

Current system bus will have >100 lines, including all 4 types

Data lines provide path for moving data (including instructions) between system components. It’s width usually same or multiple of CPU word size (eg 64 bit CPU usually has 64 or 128 bit data bus) to enhance system performance

MEMORY LOCATIONS

00

01

10

11

1 or 0

1 or 0

Possible combinations = 4 (or 22 )

00, 01, 10, 11

2 line addressBUS

Address lines used by the CPU to carry a digital code which uniquely identifies each memory location and I/O port. It’s size dictates maximum addressable memory. eg 8 bit address bus allows only 28 memory locations

Evolution of Intel Microprocessor

Name: 8080 80286 80386 Pentium PII

Year: 1974 1980 1985 1993 1997

Data Bus 8 16 32 64 64

Address 20 24 32 32 36

bus width

Addressable 1MB 16MB 4GB 4GB 64G

Memory

Bus lines

Control lines include

memory read/write & I/O read/write transfer acknowledge bus request/grant interrupt request/acknowledge clock used to synchronize operations

Bus clock speed and data bus width together determine theoretical max data transfer rate of bus eg 64 bits * 100 mhz

Power lines carry electrical power to the module they are communicating with.

Review Little-Man Computer

Review fetch-execute cycle

www.cems.uwe.ac.uk/~rstephen/courses/UQI108S1

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