Heyo

I'm working on learning on some 6502 assembler and there's something i'm having a little trouble with.
I want to write a sub routine (for my c64) that will display a box in the screen. (Text only)
I want to position the box with x, y, w, h.
I'm just wondering what the easiest/most common way to access the video memory would be. I want to store a 16bit pointer to the current position I'm writing to so that I may increase both the lsB and msB.

Also if you happen to have a link or some code laying around that shows the basic way to work with 16 bit addition. That's be awesome. I don't understand where the data to add to the msB with a ADC instruction codes. I understand that the Carry bit is set, but this alone isn't enough to complete an Add.

Thanks a ton! It's awesome there's such a popular forum on the web for this classic CPU

There is all sorts of stuff around here that can help you out. Like finding Blackbeard's treasure, you need to do a little digging. As for using the 65xx instructions themselves, enough paper has been printed about that to sink a small ship.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

I thought we had something in the source code repository or in the wiki but I guess it's too basic.


On the 65816 you can handle both bytes at once, cutting out half the steps or more.

Hi Dirk
I thought it would be very easy to just google for the details of multi-byte arithmetic, but I didn't come up with any exact snippets for your question.

Two resources worth looking through: the tutorials section on this very site, and the responses to "I want to learn 6502 assembly" on metafilter.

You'd normally store a pointer in zero page, because of the various addressing modes which treat two adjacent bytes in zero page as a pointer, and because code to access zero page is shorter and faster.

Cheers
Ed

Edit: might also be worth looking at the pdf manual to 'the visible computer' which I posted about earlier - page 73 or so of the PDF.