Variable Roll Over

SailerMan

What is the most efficient way to calculate a byte variable rolling over....

What I mean is... Let's say I have a byte Variable...

I increment it by one and the Value goes from 0 to 1 the difference is +1. The if I decrement by 2. I want to get -1, not 255. How can I make this happen without a bunch of If then statements??



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Paul Baker

It depends on the platform you are using, but are you aware that in 8 bit signed arithmetic $FF is -1? If you are working with signed 8 bit numbers, the range is -128 ($80) to +127 ($7F), you can't express 255 in an 8 bit signed value.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

metron9

The 7th bit of the byte will always be 1 for negative numbers 128 thru 255 and 0 for numbers 0 to 127 so checking that bit tells you if its negative or positive
In assembler the N flag would be set on a decrement from 0 to 255 so branching can be done without an extra compare or if then type statement.

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Think outside the BOX!