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assemblyadditionsparc

sparc assembly - add instruction with i=0 or i=1

From the Sparc architecture manual, page 110 for Add instruction:

"If i = 0, ADD and ADDcc compute “R[rs1] + R[rs2]”. If i = 1, they compute “R[rs1] + sign_ext(simm13)”. In either case, the sum is written to R[rd]."

When reading the assembly, how can I know whether i=0 or i=1 ? It doesn't look like there is any change in the mnemonic/opcode as it appears in the disassembled code.

Solution

  • The "i" indicates an "immediate" value in the instruction. An immediate is a constant. So you'll see something like this:

    add %g1, 59, %g1

    That means "add the constant 59 to g1 and put the result in g1".

    When i=0, it means that the parameter is not an immediate. So it's a register! You will see this in the assembly or disassembly:

    add %g1, %o3, %g1

    And that means add registers g1 and o3 and put the result in o3.