control unit organization

CONTROL UNIT ORGANIZATION

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• Instruction Execution• The CPU executes a sequence of instructions.• The execution of an instruction is organized as an instruction

cycle: it is performed in several steps;• Each step is executed as a set of several micro operations.• The task performed by any micro operation falls in one of the

following categories:• - Transfer data from one register to another;• - Transfer data from a register to an external interface (system bus);

• - Transfer data from an external interface to a register;• - Perform an arithmetic or logic operation, using registers for

input and output.

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CONTROL UNIT ORGANIZATION • Instruction Execution

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• Microoperations and Control Signals• In order to allow the execution of a microoperation,

one or several control signals have to be issued• Examples:• a) signals for transferring content of register R0 to R1:• R0out, R1in• b) signals for adding content of Y to that of R0 (result in

Z):• R0out, Add, Zin• The CPU executes an instruction as a sequence of

control steps.

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• Microoperations and Control Signals• Instruction:• Add (R3), R1

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Step Action

1 PCout, MAR in , Read,Select4,Add, Zin

2 Zout, PC in , Y in , WMFC

3 MDRout, IR in

4 R3out , MAR in , Read

5 R1out , Y in , WMFC

6 MDRout, SelectY,Add, Zin

7 Zout, R1in , End

Figure7.6. Control sequenceforexecutionof theinstructionAdd (R3),R1.


• The first (three) control steps are identical for each instruction; they perform instruction fetch and increment the PC.

• The following steps depend on the actual instruction (stored in the IR).

• If a control step issues a read, the value will be available in the MDR after one additional step.

• Several micro operations can be performed in the same control step if they don’t conflict

• (for example, only one of them is allowed to output on the bus)

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CONTROL UNIT ORGANIZATION • Control Unit• The basic task of the control unit :• - For each instruction the control unit causes the CPU to go

through a sequence of control steps;• - in each control step the control unit issues a set of signals• The control unit is driven by the processor clock.• The signals to be generated at a certain moment depend on:• - the actual step to be executed;• - the condition and status flags of the processor;• - the actual instruction executed;• - external signals received on the system bus (e.g. interrupt

signal)

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CONTROL UNIT ORGANIZATION • To execute instructions, the processor must

have some means of generating the control signals needed in the proper sequence.

• Three Techniques : • Hardwired control ,• Microprogrammed control and• PLA control • Hardwired system can operate at high speed;

but with little flexibility.

Hardwired control• Each step in the sequence of execution is completed in

one clock period• A counter is used to keep track of the control steps• The control signals are determined by the following

information• Contents of the control step counter• Contents of the instruction register• Contents of the condition code flags• External input signal, such as MFC and interrupt

requests

Control Unit Organization

Figure 7.10. Control unit organization.

CLKClock

Control step

IRencoder

Decoder/

Control signals

codes

counter

inputs

Condition

External

Detailed Block Description

Hardwired control• The step decoder provides a separate signal line for

each step or time slot in the control sequence• The output of the instruction decoder consists of

separate line for each machine instruction.• For any instruction loaded in the IR one of the

output line INS1 through INSm is set 1 • All other lines are set 0• Input signal to the decoder are combined to

generate the individual control signals Yin, Pcout, ADD End So on

Hardwired control• An example of how the encoder generates the Zin

control signal for the processor organization show below

• Implementation of the logic function• Zin = T1 + T6 • ADD + T4 • BR + …• this signal is asserted during time slot T1 for all

instructions• During T6 for an Add instruction • During T4 for an unconditional branch instruction

and so on

Generating Zin

• Zin = T1 + T6 • ADD + T4 • BR + …

Figure 7.12. Generation of the Zin control signal for the processor in Figure 7.1.

T1

AddBranch

T4 T6

Hardwired control

• Another example shown below

• A circuit that generate the End control signal from the logic function

• End = T7 • ADD + T5 • BR + (T5 • N + T4 • N) • BRN +…

Generating End

• End = T7 • ADD + T5 • BR + (T5 • N + T4 • N) • BRN +…

Figure 7.13. Generation of the End control signal.

T7

Add BranchBranch<0

T5

End

NN

T4T5

Hardwired control

• The End instruction starts a new instruction fetch cycle by resetting the control signal step counter to its starting value

• The RUN set to 1 causes the counter to be incremented by one at the end of every clock cycle

• When RUN=0 the counter stops counting• It is needed when ever WMFC signal is issued, the

processor to wait the reply from memory

Microprogrammed Control

Microprogrammed Control• Control signals are generated by a program similar to machine language

programs.• Control Word (CW); micro routine; microinstruction

Microprogrammed Control

Step Action

1 PCout, MAR in , Read,Select4,Add, Zin

2 Zout, PC in , Y in , WMFC

3 MDRout, IR in

4 R3out , MAR in , Read

5 R1out , Y in , WMFC

6 MDRout, SelectY,Add, Zin

7 Zout, R1in , End

Figure7.6. Control sequenceforexecutionof theinstructionAdd (R3),R1.

Micro programmed Control

• Control word: (CW)- is a word whose individual bits represent the various control signal

• Each control steps in the control sequence defines a unique combinations of 0s and 1s in the CW

• Select Y is represented by select =0• Select 4 by select 1• Micro routine :Collection of CW to a machine

instructions• Micro instruction :Each control word

Overview

• Control store

One functioncannot be carriedout by this simpleorganization.

Micro programmed Control• The micro routines for all instructions are stored in

a special memory called control store• The control unit can generate signals by sequentially

reading CWs from the control store• To read Cws sequentially from control word a micro

program counter (μPC) is used • Every time a new instruction is loaded in the IR• Then μPC automatically incremented by the clock• Thus the control signal are generated various parts

of the processor

PLA Control• Programmable logic array PLA• PLA is LSI device that can implemented any

complex combinational circuits• It is used to reduce the number of Ics and the no

of interconnection wires

PLA Control

PLA Control• An external sequence register establishes the

present state of the control circuit• The PLA output determine which • 1) Micro operations should be initiated, • 2) Depending on the external input conditions • 3) The present state of sequence register• At the same time other PLA outputs determine

the next state of the sequence register• Unwanted connections are "blown"


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PLA

ANDArray

ORArray

••••

••

••••••

•••

n InputLines

m Output LinesK WordLines

PLA

A

A’

Inputs

B

B’

C

C’

AND ARRAY

OR ARRAY

F0 F1 F2 F3

Outputs

A’B’

AC’

B

BC’

AC

Internal Structure of PLA


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control unit organization

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