Consider a CPU where all the instructions require 7 clock cycles to complete…
2008
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, 7
- D.
135, 10
Attempted by 75 students.
Show answer & explanation
Correct answer: D
Answer — Control word = 135 bits, Control Address Register (CAR) = 10 bits.
Why (step-by-step):
Control signals = 125 → need 125 bits in horizontal control word (one bit per signal).
Total microinstructions required = instructions × cycles = 140 × 7 = 980 microinstructions.
CAR width = smallest n with 2n≥9802^n \ge 9802n≥980.
29=5122^9 = 51229=512 (too small)
210=1024≥9802^{10} = 1024 \ge 980210=1024≥980 → so n = 10 bits.
Control word size = control-bits + address-field = 125 + 10 = 135 bits.
A video solution is available for this question — log in and enroll to watch it.