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IDL in Instructions

Instruction definitions in spec/std/isa/inst/ use IDL to formally specify execution behavior via the operation() key. See also IDL in CSRs and Scope for how IDL source is organized across .idl files.

Instruction operation()​

Instruction definitions in spec/std/isa/inst/ use IDL to formally specify execution behavior via the operation() key. The IDL executes at Instruction scope when the instruction executes on a hart.

operation() has no arguments and no return value, though decode variables are automatically populated from the encoding before execution begins.

Example instruction operation (add.yaml)
add:
  # ...
  encoding:
    # ...
    variables:
    - name: xs2
      location: 24-20
    - name: xs1
      location: 19-15
    - name: xd
      location: 11-7
  operation(): |
    X[xd] = X[xs1] + X[xs2];

In the operation() body, the decoded fields (xs1, xs2, xd) are automatically available as variables. The $encoding and $pc builtins are also available.