>I'm working on a sort routine, sorting 16 bit values that can span the
>full "unsigned" range. To compare two values in the sort, I'm subtracting
>one from another: if the result is negative, then the second is larger
>than the first.
>
>I started with the 16-bit subtract routine in Myke's book:
>
>movf      r2hi, w
>subwf     r1hi, w
>movwf     temphi
>movf      r2lo, w
>subwf     r1lo, w
>btfss     status, c
>     decf      temphi
>
>; check for negative value
>btfsc     temphi, 7
>     goto      Greater_Than
>....
>
>but this is a "signed" routine in the sense that it looks at the high bit
>of the result to see if it's negative, and because he does the
>subtraction "backward", the czrry flag is useless. But I'm using full
>"unsigned" 16 bit values, so for instance if I'm subtracting 0x0001 from
>0x8001, the result will get reported as negative, since the high bit is
>set, even though the number didn't "roll over."
>
>What I want is a 16 bit subtract that will let me check the carry flag at
>the end, to see if the number rolled over.
>
>Here's what I think, and I'm looking for confirmation that this will
>work: I can subtract the low bytes first, and if carry is set (result is
>"negative") then decrement the high byte of the number I'm subtracting
>FROM (basically perform the carry). Then I subtract the high bytes, and
>if the carry is set after that, then the result is negative (meaning that
>the number I'm subtracting is larger than the number I'm subtracting
>from). That's the brute force and awkwardness approach, but it seems
>logically sound to me. Did I miss something?
>
>Even if that will work, is there a more elegant (read: faster) way to do
>this? Somehow I suspect there is :-)
>
>Thanks for any help,
>
>Dave Johnson
>

> -----Original Message-----
> From: Dave Johnson [.....djohnsonKILLspam@spam@SIRIUS.COM]
> Sent: Friday, February 12, 1999 10:00 AM
> To: PICLISTKILLspamMITVMA.MIT.EDU
> Subject: unsigned 16-bit subtract
>
>
> I'm working on a sort routine, sorting 16 bit values that can span the
> full "unsigned" range. To compare two values in the sort, I'm
> subtracting
> one from another: if the result is negative, then the second is larger
> than the first.
>