Department

Computer ScienceCourse Code

CSC258H1Professor

Steve EngelsThis

**preview**shows half of the first page. to view the full**1 pages of the document.**PROCESSOR COMPONENTS

Microprocessors

o S0 far, we’ve been talking about making devices, such as adders,

counters and registers

o The ultimate goal is to make a microprocessor

A digital device that processes input, can store values and

produces output, according to a set of on-board instructions

Processor

o Deconstructing processor – consider them piece by piece

o Microprocessors

These devices are a combination of the units that we’ve

discussed so far:

Registers to store values

Adders and shifters to process data

Finite state machines to control the process

Arithematic Logic Unit (ALU) – the “Arithmetic Thing”

o The first microprocessor applications were calculators

Recall the unit on adders and Subtractors

These are part of a larger structure call the arithmetic logic unit

(ALU)

o This larger stricture is responsible for the processing of all data values

in a basic CPU

o The ALU performs all of the arithematic operations covered in this

course so far, and logical operations as well (AND, OR, NOT, etc.)

Input A and B = data inputs for the operations

The select bits indicate which operations is being performed

S2 is a mode select bit, indicating whether the ALU is in

arithmetic or logic mode

The carry bit Cin is used in operations such as incrementing an

input value or the overall result

Output signals V, C, N & Z indicate special conditions in the

arithmetic results

V: overflow condition, the result of the operation could

not be stored in the n bits of G meaning that the result is

incorrect

C: carry-

out bit

N:

negative

indicator

Z: Zero-

condition

indicator

The Arithematic side of ALU

o Fundamentally, this side is made of an add/subtractor unit

o ALU block diagram

In addition to data inputs and outputs, this circuit also has:

Outputs indicating the different conditions

Inputs specifying the operation to perform (similar to

sub)

o Beyond addition and subtraction, many more operations can be

performed by manipulating what is addeded to input A,

o Arithemtic operations; if the input logic circuit on the left side:

Sends B straight through to the adder

Result of addition operation: G = A + B

Replace B with all ones

Result of addition operation: G = A – 1

Replace B with

Result of addition operation: G = A – B – 1

Replace B with all zeros

Result of addition operation: G = A

Instead of sub singal, the operation you want is signaled using

the select bits S0 & S1

o Full operation selection

Select

Input

Operation

S1

S0

Y

Cin = 0

Cin = 1

0

0

All 0s

G = A (transfer)

G = A + 1 (increment)

0

1

B

G = A + B (add)

G = A + B + 1

1

0

G = A +

= A – B – 1

G = A +

+ 1

= A – B (subtract)

1

1

All 1s

G = A – 1

(decrement)

G = A (transfer)

Based on the values on the select bits and the carry bit, we can

perform any number of basic arithmetic operations by

manipulating what value is added to A

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