Posts Tagged ‘6502’

COMFY-6502: hosting at github

May 6, 2008

Mostly to support my own development on multiple computers, I’ve moved my Common Lisp port of COMFY-6502 to github. I chose the name cl-comfy-6502 for the repository, although the “cl” prefix is a bit ugly.

The current snapshot of COMFY-6502.

COMFY-6502: snapshot

March 5, 2008

Even though it is hardly a finished product, I’ve posted a snapshot of my current CL conversion of COMFY-6502 Not that I expect a huge pent-up demand for the code, but it might be slightly more interesting than the output examples. Thanks to Henry Baker for allowing the release.

The page also contains a link to the TODO file describing my next steps. (Most immediately, my attempt to include JMP elision was messy enough to convince me I need a different class to represent opcodes+arguments as a unit.)

COMFY-6502: a slight correction

February 11, 2008

After all the work to write a post showing off COMFY-6502’s ability to reproduce the Red Book tone routine, I made a slight mistake in translation. The code I presented differed ever-so-slightly from the Red Book routine in the case where the Y-register reached zero.
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COMFY-6502: work in progress

February 5, 2008

As I’ve mentioned before, I’ve become intrigued by Baker’s COMFY assembler, and have been working on porting it to Common Lisp, and making it a bit more powerful in the link stage.

One metric to judge the success of this kind of “medium-level” language is how well it compiles compared to hand-written assembler code. For the 6502, there are a few examples of code created by wizards like Steve Wozniak, which you can find copies of around the web, the largest being the Apple II monitor, the Integer Basic interpreter, and some medium-sized ones like the Apple II 6502 step/trace, the mini-assembler, the floating-point “Wozpack,” and the Sweet-16 virtual machine.

This kind of code has a lot of quirks that make it hard to straight-forwardly translate: lots of shared “tail code”, branches known by the programmer to always be taken (to save a precious extra byte consumed by an unconditional JMP), and the classic “fake RTS trick”, pushing a return address picked from a table onto the stack, then using an RTS instead of a zero-page indirect JMP. Common in Woz’s code is a further shortening of the code by arranging the destination addresses to all be in the same 256-byte page, so the high-order byte is a constant. Some of these ideas will likely be possible with intelligent macros, combined with address labels and computation on those labels. I’m puzzling a bit over how to optimize the “same page” condition: whether to include a “link-time assert”, which will issue an error if the code is emitted so as to cross a page, or an even more intelligent “link-time computation” which, given the current available memory space, can choose a location that meets the constraints.
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Another interesting mid-level assembler

December 17, 2007

While I’m still digesting COMFY in my “ample spare time,” a post on comp.sys.apple2 mentioned some assemblers from “Ron” having a similar flavor: SPL for the 6502 and PIC0 for PIC 10F2xx microcontrollers. They are apparently implemented in Python, and inspired by Forth. They emit assembler.

Baker’s COMFY: a few notes

November 3, 2007

I’ve been working a bit with Baker’s COMFY-6502 code; a few notes of what I have learned so far.

First, a couple of tiny bugs in the genbrc; the code miscounts the size of the branch instructions, meaning that (- l 2) should be a (+ l 2) and so on. I found it handy to enumerate each clause of the cond. Each clause handles a particular case, such as when the “lose” continuation can be reached by a short branch instruction, while the “win” continuation is far enough away to require an absolute jump.

Second, genbrc, genbr, and compile all provide the address of the resulting “continuation” as the return value. This is perhaps clear when one traces out all the recursion, but it isn’t explicitly mentioned. One interesting case is genbrc when the “win” and “lose” branch destinations are the same: one could simply emit an unconditional branch to that destination, and return the address of that branch but actually returning the destination works just as well. (In the emit routine, if the continuation does not happen to be the next instruction, the unconditional branch is, after all, emitted, moving the continuation to the front of the instruction stream, ready for the non-branching instruction to be emitted just in front of it.)

As for “upgrades” to the package, I have been focused up to this point on moving the knowledge of 6502 opcodes and addressing modes from magic decimal numbers to symbolic processing. Instead of simply emitting a decimal opcode, I have been changing the code to emit symbolic opcodes, such as (ADC ABSOLUTE), with routines to reduce these symbolic forms to the appropriate opcode, including checking for invalid opcodes. Baker’s original code will happily emit opcodes with addressing modes not supported by the chip. Currently, the compiler emits these symbolic codes into the code vector, awaiting a processing step to convert them to the decimal equivalent. Ideally, one would detect invalid addressing modes in the compilation stage, not the post-processing stage.

Some other changes I am contemplating are allowing for symbolic jump destinations, so that object code can be relocated and external labels could be used to relax the restrictions of the current scheme, which requires manually sequencing compilation and storing of addresses.