rs1_index
#define rs1 ((register_t *)frame)[kinst_regoff(frame, rs1_index)]
if (funct == 0b100 && rs1_index != 0) {
#define rs1 ((register_t *)frame)[kinst_regoff(frame, rs1_index)]
#define rs1_lval (rs1_index != 0 ? rs1 : 0)