Hardware Slides 14

8/3/2019 Hardware Slides 14

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DOC 112: Computer Hardware Lecture 14 Slide 1

Lecture 14:

Let's Put together a Manual Processor

(part 1)


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Inside every computer there is at least one processor(CPU) which can take an instruction some operands andproduce a result.

Processors can be operated in various ways: As a peripheral of another computer

(array processor or graphics processor).

One instruction at a time (e.g. a calculator).

A Manual Processor


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Block Diagram of a Processor

Both the data and the instruction will be binary numbers.The processing will be a sequence of one or more stepscontrolled by a clock.

Manual

Processor

Data

Instruction

Result

Operate


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Block Diagram of the 8 bit Processor

Manual

Processor

Data or

Instructions

8 Result8

Carry (out)

Operate

We will build an 8-bit processor, and will have acommon input stream for data and instructions.

Both data and instructions will be organised in 8-bit

bytes


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Example: Average of two numbers.

Result = (A+B)/2

The following steps must be carried out:

1. Set up the processor to receive the first number from theinput stream and save it in a register called A

2. Set up the processor to receive the second number from theinput stream and save it in a register called B

How does a simple processor work?



3. Set the processor to add register A to register B. andthen shift the result right by one bit. The operation isexecuted and the results may be stored internally in a resultsregister.

4. Set up the processor to load the results on the outputregister.

The processor is a sequential digital circuit

How does a simple processor work?



Designing a Processor

To design a general purpose processor, we need (at least):

Two data registers, A and B

A Results register

A register to hold the operation code (Instruction register(IR)).

Circuits to do arithmetic and shifting

The results register may be augmented with a one-bit registerwhich holds the carry out bit.


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The processor Data Paths

A (8 bits)

B (8 bits)

Res (8 bits)

Instruction (8 bits)

C bit

INPUT

OUTPUT

Arithmetic

and shiftoperations

8

8

8

8 8

8


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Internal Registers and the Data Paths

Notice that:

on the data paths diagram there is no information aboutwhen the data transfers occur. The diagram shows only thepossible paths where data can be transferred.

the arithmetic and shift operations are done bycombinational circuits without more registers. Thefunction that is computed is controlled by the bits in theinstruction register.

the processor is unable to execute further operations on theresults.


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The Arithmetic - Logic Unit (4 bits)

Hardware design packages provide us with Arithmetic-Logic Units of different precisions, for example 4 bit:

The select lines determine what function of A and Bappears on the output


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The Arithmetic - Logic Unit (operations)

This table assumes that Cin=0.

When Cin=1, three arithmetic operations become:

011 : results = A plus B plus 1 010 : results = A minus B minus 1

001 : results = B minus A minus 1

Selection Bits/Results

000 001 010 011 100 101 110 111

000 A - B A B 1111zero minus plus xor or and -1

A or B A BB - A A + Bminus


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Designing an ALU

An arithmetic logic unitis a simplecombinatorial circuitthat can be built fromcomponents that wehave already designed.

To provide thearithmetic functions weuse adders andsubtractors.


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Designing an ALU

The logic functionsare provided by simplegates.

We need a multiplexerfor each bit of the ALUwe are designing.


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Designing an ALU

To finish the job we needone more multiplexer toprovide the carry out.


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The Arithmetic - Logic Unit (8 bits)

We could design anALU for any bit sizewe want, but often it isuseful to proceedfunctionally.

For example we canbuild an 8 bit ALU outof two four bit

circuits.


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1

The Control of the C(in) Bit

We can use a 2-to-1 multiplexer to allow two different carryinput bits. The Carry(in) may come from Carry(out), this helpsmultiple precision.

It can also be set to logic 1 in order to use the extra threearithmetic functions. Clearing the C flip-flop will supply alogic 0.

Cin Cout

8 bitALU

3F/alu (3 bits)

A 8 8

B 8 8AB

Out8

Cy/outC

F/cy

MUX


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The Shifter Circuit

Earlier in the course we discussed a specialisedregister, called the shift register, which could shiftits contents let or right depending on a multiplexerselection.

We could incorporate such a shift register in ourdesign, but, to save on circuitry, we will make use ofa simpler shifter which does not store the result.


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The Binary Shifter

The Binary Shifter circuit is a combinational circuit. It has eight input data bits plus a carry in, and 2

function selection bits which determine the data out.

Shifter8

Data in

CYin

Data out

8

F/shift

2


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Important Shifting Functions

ROTATE RIGHT

ARITHMETIC RIGHT SHIFT

ARITHMETIC LEFT SHIFT MSB LSB Cy

MSB LSB

MSB LSB


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The Shifter - basic design

The basic shifter will perform four operations dependingon its two control inputs. These are:

00 Hold

01 Arithmetic shift left (with carry in) 10 Arithmetic shift right

11 Rotate right

It is implemented simply using multiplexers. There isone multiplexer per bit.


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The Shifter - Basic design

An eight bit shifter is shown, but clearly the designcan be easily scaled up to any size.

00: Hold01: Shift left with carry10: Arithmetic shift right11: Rotate right


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Increasing the Shifter functionality

There are many different possible shifts thatprogrammers may want to use. One possibility ourdesign doesn't do is:

Logical Shift Right

This could be done by using first an ALU operation(Data AND 11111110) followed by a rotate right,but that would involve more processing steps andtherefore be slow.

MSB LSB0


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Increasing the shifter functionality

An alternative would be to add another multiplexer toimplement the shift directly. This would mean a fasterprocessor but at the cost of more hardware.

Shifter8

Data in

Carry in

Data out

MUX

F/cin

F/shift

2

Data in[7]

0

8

i h hif f i li


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Increasing the shifter functionality

More functionality can be provided by making the carrymultiplexer four way.

Shifter8

Input Output

Input[0]0

CYin

00 -- unchanged01 -- left shift10 -- right shift11 -- right shift

00 -- Input[0]01 -- 010 -- Input[7]11 - Carry in

F/Cin

F/shft

Input[7]MUX

8


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Function Table of the Shifter

A variety of shifts is now provided, but functions[0,0,0] and [1,0,0] are the same.


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The Shifter is Built From Multiplexers


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The 8-bit Processor Data Paths

SHIFTER

ALU

A

B

CY-in

CY-out

Res

MUX

1

IR7

Data In

Carry Out

Data Out

8

F-ALU

F-Shifter

SelA

SelC

RES

C

B

A

IR0

IR

MUX

8

8

8

8

8 8

ClkA

ClkBClkR

ClkC

Control Unit

ClkIR

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Hardware Slides 14