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+16 votes

The microinstructions stored in the control memory of a processor have a width of 26 bits. Each microinstruction is divided into three fields: a micro-operation field of 13 bits, a next address field ($X$), and a MUX select field ($Y$). There are 8 status bits in the input of the MUX.


How many bits are there in the $X$ and $Y$ fields, and what is the size of the control memory in number of words?

  1. 10, 3, 1024
  2. 8, 5, 256
  3. 5, 8, 2048
  4. 10, 3, 512
asked in CO & Architecture by Veteran (59.4k points)
retagged by | 2.5k views

I am not  getting how the size of control memory$=1024$?

I am ok with calculating $X$ and $Y$.But i think

control memory size=number of different microinstruction * size of each micro instruction 

                               =$1024 *26 \,\,bits=26624bits$=$3328 Bytes$=$3328 words$ (if i consider word size = 1 Byte)

If the solution is correct then this solution is wrong



I think the word size is considered to be 26bits.

 @sourav, Control memory is nothing but the collection of control words, in this question next address field is 10 which means there are 1024 control words are there.


Notice in this question Load and increment both are connected to MUX output. But at a time only one will be active.
@chhotu ... so does is affect the width of micro-instruction ?? I  also have the same doubt like @ Sourav X = 10 ,so their are 2^10 microinstructions are present so control memory size should be 1024 * 26 bit.
what is the purpose of mux here?what functionality do the status bits provide?
Read last line of question carefully. We just need to say size in words, that is how many words will be there, question is not asking for absolute size i.e. in bits or bytes.

Hope this is clear.

1 Answer

+26 votes
Best answer
$x + y + 13 = 26  \rightarrow (1)$
$y = 3$ // y is no of bits used to represent 8 different states of multiplexer $ \rightarrow (2)$
$x$ is no of bits required represent size of control memory
$x = 10$ from (1) and (2)

size of control memory $= 2^x = 2^{10}= 1024$
answered by Veteran (54.8k points)
selected by
is this there in portion?
Could you please explain how the control memory is calculated ?

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