Post by thread:Subtraction Carry

I'm sure Andy Warren will want to take a shot at this, but just in case he is busy -- Byon Garrabrant <spam_OUTbyonTakeThisOuTNETCOM12.NETCOM.COM> wrote: >What does the carry flag represent after a PIC subtraction? I've >heard from one place that it should be thought of as a "non-negative >result" flag, but that doesn't always work. The PSIM simulator gave >the following reslts for these values: > >-4 - -2 = -2 No Carry >-4 - 2 = -6 Carry > Byon, PICs don't do signed arithmetic. The basic instructions treat each opperand as an 8 bit absolute value. -4 - -2 : The PIC sees this as b'11111100' - b'11111110' which clearly negative, hence, no carry. -4 - 2 : b'11111100' - b'00000010' which is clearly non-negative, hence a carry. -- Bob Fehrenbach Wauwatosa, WI .....bfehrenbKILLspam@spam@execpc.com

>What does the carry flag represent after a PIC subtraction? I've >heard from one place that it should be thought of as a "non-negative >result" flag, but that doesn't always work. I just pulled some code from my simulator. The subtract on a PIC is the following sequence. j = arg + ((~ W) + 1) ; if ((j & 0x100) != 0) set_carry(); else clr_carry(); w = j & 0xff; The important thing is the increment takes place before the add on a PIC, this does not happen in many processors. The case to look at is when you subtract W that contains a 0xff from 0 . Subtracts can be lots of fun. Compare the subtracts on 6805, COP8, Z8, and 6502. Walter Banks http://www.bytecraft.com