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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 !
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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


 

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2 Answers

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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( log1200 ) = 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.

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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.
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150*8 =1200 micro instructions

so 11 address bits required

control word length = 11 + 125 = 136