MadeEasy Test Series: CO & Architecture - MicroProgramming

Question

Q.27

Consider a CPU where 150 instructions take 8 clock cycles each to complete the execution. A horizontal microprogrammed control unit has to generate 125 control signals. What is the minimum size of control word? _______

 

Answer given is 136 (multiple of 8 bit)

Actual No of bits required is 133. Do we always need multiple of 8  ? Here issue is that they have not specified unit (Bit or byte) , Neither they have given that answer should be multiple of 8 , if it is in bit !

Comments

1 Instruction contains Many microinstructions.

Each microinstruction consists for One or more MIcrooperations (eg, LOAD STORE) which corresponds to one Control signal where each control signal is 1 bit.

In control memory each word contains 1 Microinstruction.

Ref : Morris mano 

Format of Control Word  

Flags bits

Control signals

Next address bits

Here,

150 Instructions each taking 8 cycles.

One Microinstruction is executed in one Clock cycle

Ref : http://faculty.washington.edu/lcrum/Archives/TCSS372AS07/Slides05_29.ppt

Slide 8

Total Microinstructions = 150 * 8  = 1200

# Next address bits = log(1200) = 11.

# control signals bits = # control signals  = 125 (Since Horizontal )

# Flags bits = 0 (No info given)

MInimum SIze of control signal  = 11+125 = 136

 

Answer 1

Horizontal Control Word Format is

Branch condition

Flags

Control Signal Field

CM Address

Nothing is mention in question about Branch condition.

so number of bits for Branch Condition = 0

Same with flag, number of bits for flags = 0

Control field indicates the control signal which is 125 Control Signal.Since Horizontal control Word uses decode binary format to represent control signal i.e. 1 bit per control signal.

So number of bits required for Control Signal Field = 125 bits

Given Number of Instruction = 150 and number of cycle/instruction = 8

So total number of cycle required in 150 instructions = 150 * 8 = 1200 cycle

In Control Unit 1 micro operation require 1 cycle so total number of micro operation = 1200 micro operation

1 micro operation require 1 Control Memory Word Storage.

So total size of control memory  = 1200 Control Memory Word

number of bits required to represent Control Memory Address = Ceil( log₂ 1200 ) = 11 bits

Minimum Size of Control Word  = 0 + 0 + 125 + 11 =136 bits

** Control Word Size is not always mandatory to be multiple of 8. Actually there is no need of specifying bit or bytes in this question.

Comments

Can you give reference from where you studied This Horizontal Control Word Format & This no of cycles required ? Need it badly !

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i have read it from Made Easy class room notes :)

Any way you can get this all this at page no 5  http://www.ecs.csun.edu/~cputnam/Comp546/Stallings-Appendices/20-Microprogrammed.pdf

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One machine instruction initiates several micro instructions. It is 1 micro instruction that requires 1 Control Memory Word Storage. One micro instruction can contain several control signals that executes micro operations...source morris mano

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does it always have to be in one cycle only. every micro instruction. u assumed it as it is not given or it should be always like that.

Answer 2

150*8 =1200 micro instructions

so 11 address bits required

control word length = 11 + 125 = 136