Question

A 2-address computer has 65 instructions, 32 Registers, 4MB memory, and three flags negative, zero,  and positive (N, Z, P). Assume length of each instruction is 32 bits. Find a format for BR instruction

Answer 1

Answer :- The BR instruction is used for brancing to a particular program location. So with BR instruction what we need is the address of the memory where we want to go.
The computer has 65 instructions i.e. for opcode we need atleast 7 bits. Since 2⁷ = 128 if we choose 6 bits then computer can have only 64 instruction max.
For 4MB of memory we need 22 bits since 2²² = 4 x 1 MB = 4 MB.

Thus the instruction format can be-
ZZZZZZZ RRR AA_AAAA_AAAA_AAAA_AAAA

where Z implies the 7-bit opcode, R imples bits are reserved and not being used, and A implies 22-bit address for 4MB of memory. So overall 32-bit instruction. The reserved 3 bits can be used for instruction like BEQ, BNE, BGE etc.