IF

Skip over the next instruction unless a condition is true according to the CPU flags.

Opcode	Bytes	Cycles	Form	Satisfied when...
$02	1	1	IF ZERO	ZF=1
$03	1	1	IF NOT ZERO	ZF=0
$04	1	1	IF NEGATIVE	NF=1
$05	1	1	IF NOT NEGATIVE	NF=0
$06	1	1	IF OVERFLOW	OF=1
$07	1	1	IF NOT OVERFLOW	OF=0
$08	1	1	IF LESS UNSIGNED	CF=0
$09	1	1	IF NOT LESS UNSIGNED	CF=1
$0A	1	1	IF GREATER UNSIGNED	CF=1 and ZF=0
$0B	1	1	IF NOT GREATER UNSIGNED	CF=0 or ZF=1
$0C	1	1	IF LESS	NF≠OF
$0D	1	1	IF NOT LESS	NF=OF
$0E	1	1	IF GREATER	ZF=0 and NF=OF
$0F	1	1	IF NOT GREATER	ZF=1 or NF≠OF

Control flag	Output
skip (SF)	If the condition is not satisfied, then SF is 1; otherwise it is 0.

Notes

• If the condition is false, then the subsequent instruction will be skipped by setting the skip flag (SF).

• Skipped instructions only require 1 CPU cycle.

Examples

MATH::SIGN()

Here's how the Hybrix compiler implements MATH::SIGN(), which returns 1 if I:0 is positive, -1 if it is negative, or returns 0 if I:0 is zero. Since MOVE does not alter the flags, a single COMPARE operation can set the flags for two different IF instructions:

    COMPARE I:0, 0
    IF GREATER
    MOVE I:0, 1
    IF NOT GREATER
    SHIFT RIGHT SIGNED I:0, 31

KERNEL::MEMSET_BYTE()

IF instructions are very often used with JUMP to implement loops. Here's an example from the Hybrix framework:

MODULE KERNEL
  . . .
  FUNC MEMSET_BYTE(TARGET: INT, VALUE: BYTE, NUM_BYTES: INT)
    CHOMBIT
  END FUNC
  . . .
END MODULE

Here, IF jumps out of the loop when the TARGET register reaches TARGET_END:

@KERNEL.KERNEL.MEMSET_BYTE:
    PUSH FP
    MOVE FP, SP
    ADD SP, 8

    # I:-17 ARG_TARGET
    # B:-13 ARG_VALUE
    # I:-12 ARG_NUM_BYTES
    # I:-8 RETURN IP
    # I:-4 FP

    # I:0 TARGET
    # I:4 TARGET_END

    MOVE I:0, I:-17

    MOVE I:4, I:-17
    ADD I:4, I:-12

@KERNEL.KERNEL.MEMSET_BYTE.L_2:
    COMPARE I:0, I:4
    IF NOT LESS
    JUMP @KERNEL.KERNEL.MEMSET_BYTE.L_3

    STORE [I:0], B:-13
    ADD I:0, 1

    JUMP @KERNEL.KERNEL.MEMSET_BYTE.L_2

@KERNEL.KERNEL.MEMSET_BYTE.L_3:
    ADD SP, -8
    POP FP
    POP IP
    FILL 4