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Section
06 Part 05 The BVC & BVS Instructions |
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An overflow of good
converts to bad.
~William Shakespeare |
Introduction
These
two are conditional branches that will branch depending on the V 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 BVC
Instruction
BVC Branch on oVerflow Clear
If
the V 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 V flag is
set, the instruction is ignored.
Examples
This
is based on whether or not overflow occurred:
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cmpi.w #$0020,d0 bvc.s FlagVIsClear move.w #$0000,d0 FlagVIsClear: |
Well
pretend d0 contains 2400801E, the CMP is word, so only the 801E
is taken into account. 801E - 0020 = 7FFE. Since were subtracting from a negative
value, the result is suppose to be negative.
But, since the result has turned positive, we have an impossible result
(i.e. overflow). The V flag has been
set.
The BVS
Instruction
BVS Branch on oVerflow Set
If
the V 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 V flag is
clear, the instruction is ignored.
Examples
This
is the opposite of BVC:
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cmpi.w #$0020,d0 bvs.s FlagVIsSet move.w #$0000,d0 FlagVIsSet: |
Again,
using the same example, with d0 containing 2400801E. 801E - 0020 = 7FFE, the result is impossible,
overflow has occurred. The V flag is
set.