assemblyriscvinstructionsriscv32

How is RISC-V neg instruction implemented?

How is the neg pseudo instruction implemented with only one sub?

I don't understand, as neg is R[rd] = -R[rs1]. But if I have sub, it is R[rs1] - something.

Solution

  • The "something" in this case is the zero register. but you're not subtracting that from the register, you're subtracting the register from that.

    The:

    neg rd, rs

    pseudo-instruction is meant to put the negation of rs into rd. The

    sub rd, zero, rs

    instruction subtracts rs from zero, placing the result into rd.

    rd := -rs        ; example: -(42)  -> -42
rd := 0 - rs     ;          0 - 42 -> -42

    Since -x is the same as 0 - x, they are equivalent.

    If you want a more comprehensive list of pseudo instructions and what they map to, here an image which details some, including the specific one you asked about:

    enter image description here