[A83] Re: tst + tstio instructions
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[A83] Re: tst + tstio instructions
>> From: "Ronald Teune" <rtwolf@gmx.net>
>>
>> > The m and p flags are minus and positive, which merely reflect the sign
>of
>> > the result. Note that this is distinctly different from the carry, as
>> > adding 2 to -3 still results in the m flag being set even though the
>carry
>> > is not. And p is true when m isn't true.
>
>> Well, I don't understand carry too, actually.
>
>The P/M flag has the same value as bit 7 of A. If it's set (negative
result)
>then the P/M flag is set, and you test that with M. If it isn't set
>(positive result) then the P/M flag isn't set (how obvious :-) and you test
>that with P.
>
>The carry you can see as the 9th bit of A. When A overflows (larger than
>$FF) then the carry is set. When the result is below 0 then the carry is
set
>either. Remember your first math classes. Then you had things like this
(but
>now in binary)
>
>ca 7 6 5 4 3 2 1 0
> 0 1 1 1 1 1 1 1 1
>+ 0 0 0 0 0 0 0 1
>------------------
> 1 0 0 0 0 0 0 0 0
>
>ca 7 6 5 4 3 2 1 0
> 0 0 0 0 0 0 0 0 0
>- 0 0 0 0 0 0 0 1
>------------------
> 1 1 1 1 1 1 1 1 1
>
>You see, the carry is set when a bit has been 'carried' over.
Ah. I already thought it would be something like this, but this makes it
clearer.
>In rotations and shifts, the carry has the same value as the bit that was
>shifted away, carried away.
Cool!
Hey, now you can make a HRC-'bit' by doing a few rol's or however they're
called and test the c/nc and then set (there was a command for it, wasn't
it?) the z/nz flag.
>Tijl Coosemans
>
>