In a computer instruction form at the instruction length is

In a computer instruction form at, the instruction length is 12bits and the size of an address field is 4 bits. Is it possible to have:

13 2-address instructions

46 1-address instructions

30 0-address instructions

using the specified format? You must show

your design or argument to justify your

answer to get credit.

Solution

Instruction length =12 bits

size of 1 address field = 4 bits

1. 13 two address instruction

4 bits opcode 4 bits Address 1 4 bits Address 2

we are left with 4 bits for opcode . 4 bits can store 2⁴ opcodes or 16 opcodes . So 13 opcodes can be easily accomodated. We can have 13 two address instructions.

2. 46 one address instructions

Out of 12 bits we require 4 bits for one address. We are left with 8 bits for opcodes. 8 bits can store 2⁸ opcodes or 256 opcodes. So it is possible to have 46 one address instructions.

3. 30 0 address instructions

The 30 0 address instructions can be easily supported by the 12 bits instruction format as we do not require bits for address.