On Thu, Sep 18, 1997 at 01:24:45PM -0400, Frank Dalton wrote:

> Does anyone have efficient code that will simply
> COMPARE two 16-bit SIGNED integers without too
> much temporary register usage?

Not really a newbie question - it's harder than it looks.
I'm giving away the family jools here :-)

The problem is that subtraction of signed numbers can cause
overflow; the solution is to make them unsigned, i.e. add
32768 to each number, thus bringing them into the range 0-65535,
and now you can subtract without fear of overflow, using the carry
flag as the sign of the result. Adding 32768 is as easy as toggling
the most significant bit of the number. Here's sample (compiler-generated)
code for testing i <= j which is actually turned around to be
!(j < i)

;x.c: 6: if(i <= j)
        movf    _j+1,w          ;get the most significant byte
        xorlw   128             ;make it an unsigned number
        movwf   btemp           ;save
        movf    _i+1,w          ;ditto the other operand
        xorlw   128
        subwf   btemp,w         ;subtract to compare
        movf    _i,w            ;get ready to compare low bytes
        btfsc   3,2             ;if hi bytes not equal, we're done
        subwf   _j,w
        btfss   3,0             ;now C bit is set if i <= j


> Is there anywhere I can look for the answers to
> these kinds of questions so that I don't have to

Nope, this is the right place.

Of course you can save all this heartache by using a comp... oops,
better not be too blatant!

Cheers, Clyde

--
Clyde Smith-Stubbs               |            HI-TECH Software
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