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Number of Operands in an Instruction
From CompArch
The terminology for the number of input operands in an instruction is inconsistent.
For example, instructions of the form A+=B are often called 2-operand instructions. Which they are, from the point of view of instruction encoding. However, from the point of view of a simple micro-dataflow OOO machine, such an instruction really 3 operands in its micro-operation form: A.new := A.old + B. Whereas a move instruction like A:=B has 2-operands from both perspectives, albeit 1-input, 1-output.
| CLEAR reg | 1 operand | 1 output (1 implicit constant input) |
| INC reg | 1 operand | 1 input/output |
| MOV reg1 := reg2 | 2 operand | 1 input, 1 output |
| ADD in place reg1 += reg2 | 2 operand | 1 input/output, 1 input
(viz. 2 inputs, one of which also happens to be output) |
| ADD reg1 = reg2 + reg3 | 3 operand | 2 input, 1 output |
| FMA3 reg1 += reg2*reg3 | 3 operand | 2-input, 1-input/output
(viz. 3-inputs, one of which happens to be output) |
| FMA4 reg1 = reg2*reg3 + reg4 | 4 operand | 1 output, 3 input |
See Instructions with more than 2 inputs, 1 output.
Terms beyond 4-operands do not seem to be in common use, except possibly with notes of incredulity, e.g. "Do you realize that Itanic had 7-operand instructions?"
And except for instructions such as load-mutiple or store-multiple.
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