x86dec.b
LBT intrinsic · group: x86 Integer
Synopsis
Instruction: x86dec.b rj
CPU Flags: LBT
Description
x86-style decrement: subtract 1 from the 8-bit value in rj. Update EFLAGS (AF, OF, PF, ZF, SF). Preserve CF. Only EFLAGS (LBT4) is updated; the GPR is not modified.
Operation
uint8_t v = (uint8_t)a;
uint8_t r = v - 1;
// CF preserved from input EFLAGS.CF
EFLAGS.PF = parity_even((uint8_t)r);
EFLAGS.AF = ((v ^ 1 ^ r) & 0x10) != 0;
EFLAGS.ZF = r == 0;
EFLAGS.SF = (int8_t)r < 0;
EFLAGS.OF = v == (uint8_t)INT8_MIN;
Tested on real machine.
Latency and Throughput
| CPU | µarch | Latency | Throughput (IPC) |
|---|---|---|---|
| 3C5000 | LA464 | 2 | 0.5(1/2) |
| 3C6000 | LA664 | 2 | 0.5(1/2) |