Re: Help with confirmation on a small bit of assembly?

["Michael J. Mahon" <mjmahon@aol.com>]

David Empson wrote:
> Michael J. Mahon <mjmahon@aol.com> wrote:
>
>
> > David Empson wrote:
> >
> >
> > > The 6502's carry flag is "backwards" for a subtract: CLC, SBC will
> > > subtract one more, while SEC, SBC will not. This also affects the logic
> > > of compare-and-branch sequences: BCC is "branch if less than" (unsigned)
> > > on a 6502, but it is "branch if greater than or equal" on other
> > > processors.
> >
> > I see it as *more* consistent, in that Carry = NOT Borrow.
> >
> > In the hardware, it means that SBC need only 1's complement its operand
> > and add, since the carry being set will add 1 for the 2's complement.
>
>
> I wasn't suggesting that it was illogical, it is just different from
> what someone might be used to on another processor.
>
> It simplifies the 6502's ALU, since it only needs to implement a single
> add operation with alternative inputs.

Exactly--and the RISC architect in me *loves* that gate-saving symmetry.
;-)

-michael

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