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For a 6502, what is the assembly convention for calling a 16 bit word into the accumulator from memory to increment as a variable? (lemmy.world)

submitted 1 week ago by j4k3@lemmy.world to c/asklemmy@lemmy.world

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I figured one of y'all would just know seeing how ubiquitous the 6502 is. I'm not following any tutorial at the moment and it has been too long since I last mucked around in assembly. I have the programming manual but it is convoluted between the 6502 and 65C816 stuff.

I'm messing with the little Easy 6502 emulator (Flatpak/FlatHub). I want to nest a couple of loops. I should probably just use the stack, but I went down this rabbit hole and damn it I want to find the rabbit!

I want something like (crude):

define lineL $1000
define lineH $1001

LDA $00
STA $lineL
LDA $02
STA $lineH

Hopefully I have that endian right... So now I have a 2 byte word starting at address 0×1000 and loaded with 0x0200. I want to increment this value as a 16 bit variable up to 0x05FF. What I am struggling with is which addressing mode indexes like this or if this must be manually implemented (– which does not seem right to me).

I'll figure it out on my own in the next couple of hours regardless. It is more fun to chat and see the spectrum of knowledge here, or maybe not, either way, ya don't know if ya don't try. Skool me Woz

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[–] FigMcLargeHuge@sh.itjust.works 8 points 1 week ago* (last edited 1 week ago) (1 children)

~~It has been a while for me, but I think you are going to have to do this all manually. The 6502 is 8bit and you can't load a 16bit word into the accumulator. Load A $lineL, increment A, store A, then compare it to 0 and beq a routine to increment $lineH, and then also compare that to your ending number. If you are wanting to end in 0x05FF you might also have to put in a compare when $lineL gets to 05 to see if $lineH is FF. I would have to find my atari books, or do some googling.~~

Edit: see my reply.

[–] FigMcLargeHuge@sh.itjust.works 4 points 1 week ago (1 children)

Just going to reply to myself here. I think I am completely wrong on my original reply. You should use ADC. If the byte overflows the carry bit will be set, and then you can test with BCC or BCS.

http://www.6502.org/users/obelisk/6502/reference.html#ADC

"ADC - Add with Carry

This instruction adds the contents of a memory location to the accumulator together with the carry bit. If overflow occurs the carry bit is set, this enables multiple byte addition to be performed."

[–] j4k3@lemmy.world 2 points 1 week ago (1 children)

Yeah this is the answer I found too.

[–] FigMcLargeHuge@sh.itjust.works 4 points 1 week ago

I was thinking about it, and since you want to count to 05FF, it might be better to load 05FF into the two bytes and use SBC and count down until you hit 0000. Then the 05 isn't causing more code to test for.

[–] abbadon420@sh.itjust.works 3 points 1 week ago

I watched a whole lot of Ben Eater and Usagi Electrics, but I still can't write any assembly. It's more fun to watch than to do imho.

[–] nettie@lemmy.world 2 points 1 week ago (1 children)

Sorry, I can't offer any help, it's been 40 years since I did 6052, on a BBC B!! Would love to know what your interest in it is?

[–] j4k3@lemmy.world 2 points 1 week ago

FORTH, and as a stepping stone into more complex hardware... again. I kinda did this once before and am getting back into it.

I am just goofing around with the emulator. I would actually like to try breadboarding something without any external connection and build it up into an interpreter. Basically try to build my own basic FORTH like threaded language from scratch. It is ambitious for my skill level. I actually have the chips to build this with a 6502, 65C816, Z80, 6809, 68010, 68008, and 8085, along with a bunch of peripherals. I also have several chips running various embedded versions of FORTH. I never managed to pick up stack programming well enough. I got so caught up in just getting FORTH onto the various chips and then getting a sensor or two working, but never built anything complex or beyond tutorial scopes. Now I am thinking in terms of the fundamentals without all the extra overhead... or trying to. There are a bunch of questions in the back of my head that I would like to answer with a functional understanding. Like what are the real limits of fuzzing for unknown hidden instructions and registers, how some red team exploits work in practice, or reasons bare metal programming is uncommon on current CPU hardware... and several other lingering curiosities.

I wish I could just jump into some 3k page documentation for a modern SoC and have a chance of doing anything useful, but it seems one needs to understand a great deal of implied context and methodologies. So I go to the source of the hardware that the present leaders played with as kids, figuring I might follow the breadcrumbs to understand them.