Q5. Assume a computer having 31 registers and a main memory that are connected t

Question

Q5. Assume a computer having 31 registers and a main memory that are connected through a common bus. Assume the computer ALU can perform 16 different operations (arithmetic, shift and logic operations). The control signals are Load, Clear, and Increment controls for each register, memory read and write signals, bus select signals and ALU operation control signals. Assume the design has 250 distinct control words. Assume also that each microinstruction requires 7 bits for branch condition and branch type (i.e., similar to the CD and BR bits used in your textbook)

a. What is the size of the control store if you use a pure horizontal microinstruction format?

b. What is the size of the control store if you use a pure vertical microinstruction format?

c. Which one of the above two options will be more costly in terms of chip area?

d. Which one of the above two options will produce a faster control unit?

text book https://archive.org/details/1993ComputerSystemArchitecture3rdEditionM.MorrisMano

Explanation / Answer

(a) 250 bits

Solution: In the horizontal microinstruction format, each control signal is represented by a single bit in the control word. So, if the design has 250 control signals, it requires 250 bits in each control word to store the control the bits.

(b) 7.9657842847

Solution:  log2 (number of distinct control words) = log2 250 = 7.9657842847

(c) Vertical microinstruction consumes less space.

(d) Horizontal microinstruction produces faster control unit.

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