CENTRAL PROCESSING UNIT
The part of computer that do data processing operations is called
central processing unit (CPU)
The CPU is made of 3 parts:
1.
Registers: stores intermediate data generated during execution.
2.
ALU: performs required micro operations.
3. Control Unit: controls
transfer of data among registers and instruct ALU to perform correct operation.
Now Study about GENERAL
REGISTER ORGANIZATION.
For example to perform
operation
R1 ß
R2 + R3
The control then provides
1)
MUXA select R2
2)
MUXB select R3
3)
OPR in ALU
operation for ADD
4)
SELD to direct destination register R1
CONTROL WORD
There are 14 selection inputs
in the unit and their combined value specifies control word.
SEL A
3
|
SELB
3
|
SELD
3
|
OTR
5
|
3
bits to select A source
3 bits to select B source
3 bits to select destination register
5 bits to select operation required
on them
SEL A
|
SEL B
|
SEL D
|
|
000
|
INPUT
|
INPUT
|
NONE
|
001
|
R1
|
R1
|
R1
|
010
|
R2
|
R2
|
R2
|
011
|
R3
|
R3
|
R3
|
100
|
R4
|
R4
|
R4
|
101
|
R5
|
R5
|
R5
|
110
|
R6
|
R6
|
R6
|
111
|
R7
|
R7
|
R7
|
OPERANDS
00000
|
TRANSFER A
|
00001
|
INCREMENT A
|
00010
|
ADD A+B
|
00101
|
SUB A-B
|
00110
|
DECREMENT A
|
01000
|
AND A AND B
|
01010
|
OR A AND B
|
01100
|
XOR A AND B
|
01110
|
COMPLEMENT A
|
10000
|
SHIFT RIGHT A
|
11000
|
SHIFT LEFT A
|
ALU
1)
ALU provides arithmetic operations (ADD, SUB,
INCA, DECA)
2)
Logic operations (AND, OR, XOR, COMA)
3)
Shift operations
(SHLA , SHRA).
4)
And Transfer operation (TSFA)
EXAMPLES
R1 ß R2 – R3
Field SELA SELB SELD
OPR
Symbol R2
R3 R1 SUB
CW 010 011
001 00101
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