David Cary <PICLIST@mitvma.mit.edu> wrote:

> What is the "correct" Carry result after a subtraction ? I have
> some friends that are making tiny CPUs inside FPGAs, and I'm sure
> they want to pick whatever Carry flag result makes the software
> guys happy. Which would be ... ?

    ... Which would be the result that makes sense for the NEXT byte
    in a multi-byte subtraction.

    The actual state of the carry after a subtraction doesn't
    matter; the important thing is that if you implement a
    subtract-with-carry instruction, the description of the
    carry-out has to match the description of the carry-in.

    For instance... The 6502 indicates a borrow with a CLEAR
    carry-out, and all subtractions use the carry-in to indicate
    whether there was a borrow from a previous subtraction... So
    before performing a one-byte subtraction on the 6502, you must
    explicitly SET the carry.  If you're doing a multi-byte
    subtraction, your code is simply:

        Set Carry
        Subtract Low Byte
        Subtract Middle Byte
        Subtract High Byte

    On the PICs, the carry also happens to be cleared by a borrow,
    but (unless you're using a device with a subtract-with-borrow
    instruction) it makes absolutely no difference whatsoever; the
    code to do a multi-byte subtraction has to explicitly look at the
    carry bit, and a "SKPC/INCFSZ" takes just as much space and time
    as a "SKPNC/INCFSZ"

    Some processors (like the 8051) use the opposite convention
    (borrow = carry set), but that's no more or less efficient as the
    6502 convention; on an 8051, the first line of the above example
    just changes to "Clear the carry".

    Personally, I prefer the "borrow = carry clear" convention (as
    in the 6502 and PIC), but that's just personal preference.

    -Andy


=== Andrew Warren --- aiw@cypress.com
=== Staff Systems Engineer, IPD
=== Cypress Semiconductor Corporation

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