An
instruction format defines the layout of the bits of an instruction in terms of
it constituent parts. An instruction format –
(i)
Must explicit operand is referenced
using one of the addressing modes.
(ii)
Must implicitly or explicitly,
indicate the addressing mode for each operand.
The most
basic design issue to be faced is the instruction format length. This decision
affects and is affected by memory size, memory organization, bus instruction
CPU complexity and CPU speed.
The other
issue is allocated the bits in that format the trade of here are complex.
Given
instruction length; there is clearly a tread- off between the number of
op-codes and the power of the addressing capability.
The
following interrelate factors go into determining the use of the address bits.
- Number of addressing modes
- Number of operands
- Register versus memory
- Number of register sets
- Address range
- Address granularity
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