Abscissa

Are there any slight (or major) differences in writing to that chip? Or is it pin-for-pin identical with burning as well as reading? EDIT: Nevermind, I found the AT28C64 datasheet. Thanks for the help

(I'm not sure if this is in the right section...) Is there an EEPROM that (at least for reading) is pin-compatible with the 2732 EPROM normally used with the 2600 PCBs in the AtariAge store? If so, if there a datasheet, or any other information on it?

Right, yea. Although I guess that means that even though I'm going to use the SBC instead of ADC, I should still clear the flag before starting the loop, right? (I realize I won't need it in the loop, since the loop itself won't be causing a carry.)

Took me a minute to realize why you said the carry flag instead of the zero flag . But, that's a good call, I'll try that. EDIT: Aha! That's it! I tried clearing the carry flag before each "adc #$FF", and it worked: lda #37 VBlankLoop sta WSYNC clc adc #$FF; Add two's complement of 1 bne VBlankLoop Obviously, I'll still use the subtract instruction instead. But thanks, thought I was going nuts for a minute

Pardon if this is a stupid question... I've come across a strange thing (I'm using the Z26 emulator, if it makes any difference). At first, I had my VBlank loop look like this: ldx #37 VBlankLoop sta WSYNC dex bne VBlankLoop (It's a slighly modified form of what was in one of Andrew Davie's tutorials.) I was using the X register, and needed X to be 0 later on anyway, so I just looped by decrementing from 37 to 0. But then, I wanted to switch it to use A. So I changed it to this, but this now makes the screen go blank: lda #37 VBlankLoop sta WSYNC adc #$FF; Decrement, by adding the two's complement of 1 bne VBlankLoop (This was before I remembered there was a subtract instruction ) But then it worked again when I changed it to this: lda #37 VBlankLoop sta WSYNC adc #$FE; Decrement, by adding the one's complement of 1 (aka the two's complement of 2) bne VBlankLoop Which seems to tell me one of three things: 1. I've run into a strange bug in either Z26 or the 6502/6507 (doubtful) 2. The 6502 (or at least the 6507 variant) uses one's compliment arithmetic instead of two's complement (which I would find very surprising, since I didn't think one's complement was ever used in something this common) 3. I screwed up something somewhere else (entirely plausable ) But this other variant I made seems to make #1 or #2 more likely than #3: lda #37 VBlankLoop sta WSYNC sbc #1; Decrement, by subtracting 1 bne VBlankLoop That code does work, unlike adding $FF. So what do the experts think? Am I nuts for not realizing that the Atari 2600 is a one's complement machine, or have I probably just screwed up something else? (Or is there some other thing going on?) EDIT: I guess this board doesn't like bold within a code block.