Not Exactly Translating The Dead Sea Scrolls, But...

Hello,

I'm trying to provide an instruction-accurate port of some code that runs on a 6502 CPU to x86. I'm pretty good on the x86 assembly side (though don't really know FPU opcodes or anything added since 1996), but know zip about the 6502 side of life.

At the moment I'm building a table that provides the x86 equivalent of 6502 instructions. Most are pretty straightforward (e.g. ADC = ADC), but others are a bit more of a mystery.

CLI clears the "interrupt disable" flag on a 6502, but the flag works the opposite (enable) on an Intel, so the equivalent is STI, yes?

I also have no clues about "decimal mode" on the 6502, whether that translates to anything at all on the Intel. Any ideas?

Quite handy that it only has three registers though (X, Y and SP), makes things fairly economical. ;)

Quote:

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I also have no clues about "decimal mode" on the 6502,

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Do you mean Binary Coded Decimal? Have you looked at some of the information from this site: 6502.org?

Jim

Ah, BCD. That makes sense. I know there are some x86 opcodes that deal with BCD but there's no "decimal flag" on x86, is there?

Yeah. Instead you use x86 usual arithmetic opcodes in conjunction with x86 supported BCD opcodes (AAA, AAS, DAA, DAS, etc). It will be your task to know that the stored values are in BCD format. But can you avoid doing BCD on the x86? I'd rather skip it entirely and perform normal binary operations. You'll just have to concern yourself with translating the values from the 6502 BCD code (here's a full description).

In any case, a good introductory tutorial for BCD in x86 is here. It's a whole lot of inefficient cycles.

As for STI, yes. Or POPF, depending on your needs.

tbh the code doesn't feature any BCD opcodes, but I'm a bit obsessive when it comes to completeness of a solution. :o
I'm quite pleased, this doesn't look as difficult as I thought it might be.

I'm more familiar with the Z80 but both have some similarities (well the 6510 more than the 6502). But you are right, it's pretty straightforward (and fun!). You'll only have to worry with the x86 side of things. 8bit assembly is the "Hello World" of assemblies. I just hope the program that you are trying to translate doesn't have any access to graphics memory. You will have to scrap entirely that part of the code.