Machine instructions

Question

Q15 I am having confusion regarding indirect bit(mode bit) .Are mode bit used in deciding number of operations possible?

Answer 1

b) 16

128 Registers: 7 bits is required to specify the register operand

256 M Memory : 28 bits is required to specify the memory operand

Total Length of the Instruction = 40 bits

So remaining 5 bits can be used to specify the opcode. So totally 32 different instructions are possible. Generally instructions with indirect addressing mode use specific notation to distinguish from direct addressing mode.

For eg : LD @A

Assuming this notation takes 1 bit. Something like 0 to specify direct addressing mode and  1 to specify indirect addressing mode. we are left 4 bits for opcode. Hence 2⁴ = 16 instructions are totally possible.

Comments

How 28 bits are required ??

256M bytes first covert into bits ? Right?

Please explain!!

Answer 2

Can you specify the book, from where you have taken this question??