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Section
06 Part 03 – The BEQ & BNE Instructions |
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“A gentleman would be ashamed
should his deeds not match his words.” ~Confucius |
Introduction
These
two are conditional branches that will branch depending on the Z flag. It should be noted that these branch
instructions have two sizes; .s for short and .w for word. For details see Section 05 Part 03 (The BRA Instruction).
The BEQ
Instruction
BEQ – Branch on EQual
If
the Z flag of the CCR is set, the destination operand will be added to the PC, and the 68k will continue reading at the
new offset held in PC. If the Z flag is
clear, the instruction is ignored.
Examples
You’ve
seen this one used various times in a few examples, it is quite simple:
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cmpi.w #$0020,d0 beq.s FlagZIsSet move.w #$0000,d0 FlagZIsSet: |
The CMP
instruction runs a comparison of 0020
and the word inside d0:
The
BEQ instruction will only branch if the Z flag is set. If it is cleared, it will ignore the branch
and continue.
The BNE
Instruction
BNE – Branch on Not Equal
If
the Z flag of the CCR is clear, the destination operand will be added to the PC, and the 68k will continue reading at the
new offset held in PC. If the Z flag is
set, the instruction is ignored.
Examples
This
is the opposite of BEQ:
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cmpi.w #$0020,d0 bne.s FlagZIsClear move.w #$0000,d0 FlagZIsClear: |
The CMP
instruction runs a comparison of 0020
and the word inside d0:
The
BNE instruction will only branch if the Z flag is cleared. If it is set, it will ignore the branch and
continue.