Rust on the MOS 6502: Beyond Fibonacci

Post by dhosek » Tue, Sep 21, 2021, 1:25 PM UTC

cmrdporcupine wrote: post #6

royjacobs wrote: post #2

That is so cool. I saw some posts about LLVM-MOS a while ago, but at that point I thought it would be just another in a fairly long list of attempts to try and get LLVM to output 6502 instructions. I never expected it to come together this well! Especially considering that the author of the article mentions there were so many issues with LLVM-AVR, you'd expect them to exist in LLVM-MOS as well. Apparently not! I gues…

I haven't looked at this closely, but 6502 really doesn't lend itself to C compilation. Three registers, only one of which works with the ALU, awkward immovable stack, etc. The 65816 is a better target (moveable direct page and stack and some wider registers), but also awkward with its register mode switching.

Post by royjacobs » Tue, Sep 21, 2021, 1:29 PM UTC

dhosek wrote: post #13

gergoerdi wrote: post #10

Earlier quoted context omitted.

From what I understand, LLVM-MOS treats large parts of the zero page as virtual ("imaginary") registers, so you have no shortage of that ( https://llvm-mos.org/wiki/Imaginary_registers ). Then, sufficiently advanced compiler technology improves the stack situation ( https://llvm-mos.org/wiki/C_calling_convention ).

6502 assembly has the distinct advantage of having special page-0 instructions for reading/writing from memory, including, if I recall correctly, the ability to take a 2-byte sequence and treat it as a 16-bit value (or was that in the AppleSoft ROM?)

Post by royjacobs » Tue, Sep 21, 2021, 1:38 PM UTC

Someone wrote: post #8

cmrdporcupine wrote: post #6

Earlier quoted context omitted.

I haven't looked at this closely, but 6502 really doesn't lend itself to C compilation. Three registers, only one of which works with the ALU, awkward immovable stack, etc. The 65816 is a better target (moveable direct page and stack and some wider registers), but also awkward with its register mode switching.

Not only C, any language that thinks there’s other things than global state. If all your functions are void foo(void) and you don’t use local variables (or your language doesn’t support recursion, in which case all locals can be given a fixed address), targeting 6502 is fine (it also helps if you avoid floating point, use 8-bit variables where possible, etc) Not supporting recursion also means you can statically comp…

Post by royjacobs » Tue, Sep 21, 2021, 1:41 PM UTC

dhosek wrote: post #15

royjacobs wrote: post #14

Earlier quoted context omitted.

You can treat 2 bytes (not just in the zero page, though) as indirect jump addresses, yes. Doing something like "JMP ($2345)" will jump to whatever $2345/$2346 is pointing to.

It's a little amazing how much 6502 assembler sticks with me 35 years later. But only a little. I didn't have the money to buy an assembler or the skill to write one so I would write out my programs in long-hand on graph paper and hand-assemble them before entering hex codes manually. While not the most efficient process, it did do a good job of encoding things into long-term memory.

Post by leeter » Tue, Sep 21, 2021, 2:03 PM UTC

cmrdporcupine wrote: post #6

royjacobs wrote: post #2

That is so cool. I saw some posts about LLVM-MOS a while ago, but at that point I thought it would be just another in a fairly long list of attempts to try and get LLVM to output 6502 instructions. I never expected it to come together this well! Especially considering that the author of the article mentions there were so many issues with LLVM-AVR, you'd expect them to exist in LLVM-MOS as well. Apparently not! I gues…

I haven't looked at this closely, but 6502 really doesn't lend itself to C compilation. Three registers, only one of which works with the ALU, awkward immovable stack, etc. The 65816 is a better target (moveable direct page and stack and some wider registers), but also awkward with its register mode switching.