Bob J. <rocketbob <at> gmail.com> writes:

> Silly question:
> 
> Two variables, A containing -1 and b containing 1:
> rega = b'11111111'
> regb = b'00000001'
> 
> rega is a signed integer, regb is unsigned.
> 
> How do I do a comparision using cpfslt on these two?  Do I just complement
> rega then do the comparison?  I don't believe this is correct, but maybe I
> am not thinking it all the way through.  rega obviously can be a positive
> integer also so that would throw that out.  So thinking aloud there, do I
> check the sign bit then complement if its negative, then do the comparision?

You must convert both to the same type, and it must be a type that computes
correctly on binary (2's complement) substraction, i.e. not packed bcd or
anything like that. The representation for a binary unsigned number lower than
128 is the same as a binary signed number larger than -1 and smaller than 128.
So they are the same type and you just substract I think. The condition is regb
< 128.

If cpfslt is the macro shown here:

  http://www.staton.us/electronics/rc_servo/tmp1820c.asm

then somthing like this should work:

  movlw rega
  cpfslt regb

Peter P.

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