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Section
04 Part 03 – The NEG Instruction |
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“Polarity, or action and reaction, we meet in every part of nature; in darkness and light; in heat and cold; in the ebb and flow of water; in male and female; in the equation of quantity and quality; in the fluids of the animal body; in the systole an” ~Ralph Waldo Emerson |
Introduction
NEG – NEGate
This
instruction will reverse the polarity of the destination operand, from positive to negative,
or from negative to positive.
Examples
In
the previous part, we were looking at signed data for positive and
negative. This instruction will allow
you to change a positive number into a negative number, or vise-versa.
Now
let’s take this example below:
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neg.b d0 |
A byte of d0 is negated.
You’ll
notice that the byte in d0 was changed from 02 to FE, basically +2 to -2.
Here,
you’ll notice that the byte E4 has
changed to 1C, basically -1C to +1C.
Positive
to Negative, Negative to Positive, and that’s the general rule. However, there are certain cases where
negation will have no effect, for example:
Using
the 00 example, 00 is neutral as far as negate
is concerned. It represents nothing, so
there’s nothing to negate. The 80 example on the other hand is
in a similar predicament, but it’s harder to explain.
As
you can see, 80 seems
to have its own “neutral” position, being the exact opposite of 00:
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... FC FD FE FF 00 01 02 03 04 ... |
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... 7C 7D 7E 7F 80 81 82 83 84 ... |
It is
of course important to remember, that 00
is still technically positive, while 80
is still technically negative.
The
NEG instruction can be used on memory, directly or by using an address register
in various ways:
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neg.w $00004000 neg.l (a2) neg.w (a2)+ neg.b -(a2) neg.w $0C(a2) |
You cannot of course use NEG on
address registers directly:
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neg.l a2 |