SixtyPical

SixtyPical is a very low-level programming language, similar to 6502 assembly, with static analysis through type-checking and abstract interpretation.

It is a work in progress, currently at the proof-of-concept stage.

It is expected that a common use case for SixtyPical would be retroprogramming for the Commodore 64 and other 6502-based computers such as the VIC-20, the Apple ][+, and the NES.

Many SixtyPical instructions map precisely to 6502 opcodes. However, SixtyPical is not an assembly language: the programmer does not have total control over the layout of code and data in memory. Some 6502 opcodes have no SixtyPical equivalent, while some have an equivalent that acts in a slightly different (but intuitively related) way. And some commands are unique to SixtyPical.

sixtypical is the reference implementation of SixtyPical. It is written in Haskell. It can currently parse and check a SixtyPical program, and can emit an Ophis assembler listing for it.

SixtyPical itself is distributed under a BSD-style open-source license, while the example SixtyPical programs in the eg directory are in the public domain. See the file LICENSE for more information.

Quick Start

If you have ghc, Ophis, and VICE 2.4 installed, clone this repo, cd into it, and run

./loadngo.sh eg/game.60p

The Big Idea(s)

Typed Addresses

SixtyPical distinguishes several kinds of addresses: those that hold a byte, those that hold a word (in low-byte-high-byte sequence), those that are the beginning of a table of bytes, and vectors (those that hold a word pointer to a machine-language routine.) It prevents the program from accessing them in certain ways. For example, these are illegal:

reserve byte lives
reserve word score
routine do_it {
    lda score        // no! can't treat word as if it were a byte
    lda lives, x     // no! can't treat a byte as if it were a table
}

Abstract Interpretation

SixtyPical tries to prevent the program from using data that has no meaning.

The instructions of a routine are analyzed using abstract interpretation. One thing we specifically do is determine which registers and memory locations are not affected by the routine. For example, the following:

routine do_it {
    lda #0
    jsr update_score
    sta vic_border_colour    // uh... what do we know about reg A here?
}

...is illegal unless one of the following is true:

• the A register is declared to be a meaningful output of update_score
• update_score was analyzed and determined to not change the value of the A register

The first case must be done with an explicit declaration on update_score. The second case will be be inferred using abstract interpretation of the code of update_score.

Structured Programming

SixtyPical eschews labels for code and instead organizes code into blocks.

Instead of the assembly-language subroutine, SixtyPical provides the routine as the abstraction for a reusable sequence of code. A routine may be called, or may be included inline, by another routine. The body of a routine is a block.

Along with routines, you get if, repeat, and with constructs which take blocks. The with construct takes an instruction like sei and implicitly (and unavoidably) inserts the corresponding cli at the end of the block.

Abstract interpretation extends to if blocks. The two incoming contexts are merged, and any storage locations poisoned in either context are considered poisoned in the result context. (A similar case applies to repeat and with, but these are different too as there is only one block and it is always executed at least once.)

Declarations can have block scope. Such declarations may only be used within the block in which they are declared. reserved storage inside a block is not, however, like a local variable (or auto in C); rather, it is more like a static in C, except the value at that address is not guaranteed to be retained between invokations of the block. This is intended to be used for temporary storage. In addition, if analysis of the call graph indicates that two such temporary addresses are never used simultaneously, they may be merged to the same address. (This is, however, not yet implemented, and may not be implemented for a while.)

Pseudo-Instructions

Along with instructions which map to the 6502 instruction set, SixtyPical supplies some instructions which are slightly more abstract and powerful. For lack of a better term, I'm calling them "pseudo-instructions" here. (But I would really like a better term.)

In a macro assembler, these pseudo-instructions would be implemented with macros. However, macros, being textual-substitution-based, are a pain to analyze. By providing the functions as built-in instructions, we can easily work them into the type system. Also, there are some macros that are so common and useful that it makes sense for them to be built-ins, with standardized, prescriptive names.

Such pseudo-instructions are:

• copy, which copies a value from one storage location to another. This is a typesafe way to copy 16-bit words and vectors. In the future, it may handle 8-bit values and immediate values too.
• save, which is not yet implemented. Intended to be used in with blocks when you want to save a value but you don't want to use the stack. Pairs well with block-level temporary reserved addresses.

"It's a Partial Solution"

SixtyPical does not attempt to force your typed, abstractly interpreted program to be absolutely watertight. In assembly language on an 8-bit microprocessor, you will sometimes need to do dangerous and tricky things, like self-modifying code and cycle-counting, in order to accomplish a sophisticated effect, like a raster interrupt trick.

For that reason, sixtypical does not attempt to emit a fully-formed Ophis assembler source. Instead, it expects you to mix its output with some raw Ophis assembler to make a complete program. This "mixin" may contain as much unchecked assembler code as you like. An example is provided in the lib directory which adds a prelude that makes the resulting program runnable from Commodore BASIC 2.0 and stores uninitialized data at $C000.

In addition, various checks are not attempted (such as tracking the usage of an indirect indexed table) and other checks may be subverted (for example by locateing two variables with two different types of storage at the same address.)

In summary, SixtyPical helps you write a very-nearly-assembly-level program which is a bit more "solid" than raw assembly, but it still expects you to know what you're doing down there.

For More Information

For more information, see the docs (which are written in the form of Falderal literate test suites. If you have falderal on your executable search path, you can run the tests with ./test.sh.)

• Checking
• Analyzing
• Emitting
• Instruction Support

Internals

Some (OK, a lot) of the Haskell code is kind of gross and non-idiomatic. The parser, in particular, could not be described as "elegant". There could definitely be more higher-order functions defined and used. At the same time, I'm really not a fan of pointless style — I prefer it when things are written out explicitly and pedantically. Still, there are places where an added foldr or two would not be unwelcome...

The 6502 semantics, which are arguably RISC-like (load/store architecture) are translated into an intermediate representation which is arguably CISC-like. For example, lda, sta, ldx, and tax all become kinds of COPY internally. This internal instruction set is much smaller than the 6502's, and thus is usually easier to analyze. It would also be easier to adapt to other instruction sets, such as the Z80 or the 8086.

Notes

This is not quite the right place for this, but I need to write it down somewhere:

6502 machine code supports an indirect jmp, but not an indirect jsr. But an indirect jsr is very easy to simulate with an indirect jmp. Instead of

launch:
    copy whatever to vector
    jsr (vector)
    ...

Just say

launch:
    copy whatever to vector
    jsr indirect_jsr
    ...

indirect_jsr:
    jmp (vector)

Then the rts at the end of your routine pointed to by vector will return you to where you jsred.

Because the above is so easy to write, SixtyPical will probably not support a jsr (vector) form (unless it would somehow make analysis easier, but it probably won't.)

TODO

• Addressing modes — indexed mode on more instructions
• Rename and lift temporaries in nested blocks
• Tail-recursion optimization
• word 100 to promote an otherwise 8-bit literal to a 16-bit value
• jmp routine
• Enforce that jmps come at ends of blocks(?)
• outputs on externals
• Routine is a kind of StorageLocation? (Location)?
• Test that pha restores the A register
• Test poisonining of flags
• Test output of flags