Other instruction formats: non-MIPS

Other possible formats

older formats were designed to minimize the number of bits in an instruction

3-register format (MIPS)

addu    $r10,$r8,$r9

2-register format (CISC)

add2 $r1, $r2

semantics:

R[1] = R[1] + R[2]

(like += in C)

same register used for source AND target

fewer bits necessary

1-register format (accumulator)

add1 $r2

semantics:

Acc = Acc + R[2]

accumulator: special register used to hold results, implicit in instruction

0-register format (stack)

add0

semantics:

Stack[Top-4] = Stack[Top] + Stack[Top-4]

Top = Top - 4

replace top of stack with sum of top 2 values

requires push and pop operations

must go back to memory to reuse value