GATE CSE 2006 | Question: 09, ISRO2009-35

Question

A CPU has $24$-$bit$ instructions. A program starts at address $300$ (in decimal). Which one of the following is a legal program counter (all values in decimal)?

• A. $400$
• B. $500$
• C. $600$
• D. $700$

Answer 1

After each instruction cycle the PC needs to be updated to point to the next instruction in memory.

INSTRUCTION SIZE : 24BITS or 3Bytes

Address of next instruction : 303 then 306 and so on.

So it is an AP series and only 600 comes under this series. So answer is C

Answer 2

Actually the question is incomplete. It should be mentioned in the question that the

CPU is byte-addressible so that the PC will increment byte by byte. Now because the instruction is 3byte, the PC has to increment 3 times.

So the answer is 600, since it is the only option which is multiple of 3.

Answer 3

Each instruction is 24 bits ..so we need three cells it means that 1 cell consists of 8 bits so
I1: 300 -302
I2: 303-305
I3:306-308

.

.

.

.

continue like this so we can see here PC has a multiplicative of 3. So 600 is the answer.

Answer 4

When nothing is given, we always have to assume byte addressability.

So, 1 instruction = 3 B.

From mem location 300, load 99 instructions. We'll be on mem location 597, and PC would point to 600.

Or simply, look for multiples of 3.

Option C