GATE IT 2008 | Question: 39

Question

Consider a CPU where all the instructions require $7$ clock cycles to complete execution. There are $140$ instructions in the instruction set. It is found that $125$ control signals are needed to be generated by the control unit. While designing the horizontal microprogrammed control unit, single address field format is used for branch control logic. What is the minimum size of the control word and control address register?

• A. $125, 7$
• B. $125, 10$
• C. $135, 9$
• D. $135, 10$

Comments

single address field format is used for branch control logic                    

What is the significance of this line in the question?

---

@sachinmittal1 sir in this question if we consider microinstruction and control word same then answer is 135 ,10

and if we consider different then answer is 125 ,10 what should we consider

Answer 1

1. Control Word Size:

• In horizontal microprogramming, each control signal needs a dedicated bit in the control word.
• 125 control signals + 10 bits for branch control logic (single address field) = 135 bits

2. Control Address Register Size:

• The control address register (CAR) holds the address of the next microinstruction to be executed.
• The number of microinstructions is equal to the number of clock cycles per instruction * the number of instructions.
• Number of microinstructions = 7 clock cycles/instruction * 140 instructions = 980 microinstructions
• Bits needed to address 980 microinstructions = log2(980) ≈ 9.9 bits, but we need at least 10 bits to represent all microinstructions.

Therefore, the minimum size of the control word is 135 bits, and the minimum size of the control address register is 10 bits.