SUB — subtract

sub rd, rs1, rs2

Computes rd = rs1 - rs2. Overflow wraps modulo XLEN. There is no base subi; use addi with a negative immediate when it fits.

sub a0, a0, a1
addi sp, sp, -16       # effectively subtract 16

x86 connection: x86 sub is a two-operand instruction and sets flags; RISC-V sub has three operands and does not.