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, 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.

   You Shall Not Subject Your God To Market Forces!
        --Book of Om, Chap. IV v.6b  ("Small Gods",Terry Pratchett)

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