Re: [stella] time to stick me with a fork?

 Subject: Re: [stella] time to stick me with a fork? From: KirkIsrael@xxxxxxxxxxxxx Date: 14 Mar 2004 21:54:52 -0000
```> At 01:41 PM 3/14/2004 +0000, you wrote:
>
> >Subpixel Joust Pong comes to mind. Everyone on the group said it
> >was the right thing to do, and it made all the difference in
> >the world, but 16 bit math still makes me go crosseyed sometimes.
> >(I understand why you clear the carry before addition, but owe
> >it to myself to spend some time w/ the 6502 emulator and figure
> >out why you need to set the carry before subtraction.)
> In a nutshell - the carry bit is essentially the 9th bit of accumulator
> (possibly other math functions of the cpu as well - it's been a number of
> years since I've touched 6502 programming) so, when your addition results
> in a value greater than 255 decimal, the 9th bit (carry) catches the
> carryover. In a subtraction, it PREVENTS an underflow. (in other words,
> when you set the 9th bit of the accumulator to 1 it becomes impossible for
> your subtraction to go below zero, or to become negative.)

Ok, maybe I'm starting to get it.

I find the carry for addition easy to understand--
not enough room for the bits, one gets pushed into
the carry, and then if you do another addition
thereafter, that'll get added in to the right.

I think what messed me up with subtraction, is this
idea that the carry bit was...like, a bit whose meaning
was...numeric? But REALLY, it's more of a flag. Because
in subtraction, the mental metaphor of the carry bit as
a "numeric bit" breaks down, because if it's not set,
that means the result of what's happening at the rightmost
bits is one LESS than what you'd expect.

I know that's a rough, vaguely handwaving description that
misses the "why" (like, I think there's a more elegant, deeper
reason why they didn't reverse the meaning, so you always
Cleared Carry before any math) but I'm getting the basics.

--
KirkIsrael@xxxxxxxxxxxxx    http://kisrael.com
You Shall Not Subject Your God To Market Forces!
--Book of Om, Chap. IV v.6b  ("Small Gods",Terry Pratchett)

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