SixtyPical

Version 0.16. Work-in-progress, everything is subject to change.

SixtyPical is a 6502-like programming language with advanced static analysis.

"6502-like" means that it has similar restrictions as programming in 6502 assembly (e.g. the programmer must choose the registers that values will be stored in) and is concomitantly easy for a compiler to translate it to 6502 machine language code.

"Advanced static analysis" includes abstract interpretation, where we go through the program step by step, tracking not just the changes that happen during a specific execution of the program, but sets of changes that could possibly happen in any run of the program. This lets us determine that certain things can never happen, which we can then formulate as safety checks.

In practice, this means it catches things like

• you forgot to clear carry before adding something to the accumulator
• a subroutine that you call trashes a register you thought was preserved
• you tried to read or write a byte beyond the end of a byte array
• you tried to write the address of something that was not a routine, to a jump vector

and suchlike. It also provides some convenient operations based on machine-language programming idioms, such as

• copying values from one register to another (via a third register when there are no underlying instructions that directly support it); this includes 16-bit values, which are copied in two steps
• explicit tail calls
• indirect subroutine calls

The reference implementation can analyze and compile SixtyPical programs to 6502 machine code.

Quick Start

If you have the VICE emulator installed, from this directory, you can run

./loadngo.sh c64 eg/c64/hearts.60p

and it will compile the hearts.60p source code and automatically start it in the x64 emulator, and you should see:

You can try the loadngo.sh script on other sources in the eg directory tree, which contains more extensive examples, including an entire game(-like program); see eg/README.md for a listing.

Documentation

• Design Goals
• SixtyPical specification
• SixtyPical revision history
• Literate test suite for SixtyPical syntax
• Literate test suite for SixtyPical analysis
• Literate test suite for SixtyPical compilation
• Literate test suite for SixtyPical fallthru optimization
• 6502 Opcodes used/not used in SixtyPical

TODO

low and high address operators

To turn word type into byte.

Trying to remember if we have a compelling case for this or now. The best I can think of is for implementing 16-bit cmp in an efficient way. Maybe we should see if we can get by with 16-bit cmp instead though.

The problem is that once a byte is extracted, putting it back into a word is awkward. The address operators have to modify a destination in a special way. That is, when you say st a, >word, you are updating word to be word & $ff | a << 8, somelike. Is that consistent with st? Well, probably it is, but we have to explain it. It might make more sense, then, for it to be "part of the operation" instead of "part of the reference"; something like st.hi x, word; st.lo y, word. Dunno.

Save values

This preserves them, so that, semantically, they can be used later even though they are trashed (or otherwise alternately used) inside the block.

Inside the block, we set them as writeable (but not meaningful). When the block exits, we restore whatever status they had.

This act will trash a, both in the block, and outside it, unless the value being saved is a. One idiom would be something like

save a { save var {
    ...
} }

which would save all values. Maybe abbreviate this to

save a, var {
    ...
}

This can use the stack. But it need not use the stack.

Make all symbols forward-referencable

Basically, don't do symbol-table lookups when parsing, but do have a more formal "symbol resolution" (linking) phase right after parsing.

Associate each pointer with the buffer it points into

Check that the buffer being read or written to through pointer, appears in appropriate inputs or outputs set.

In the analysis, when we obtain a pointer, we need to record, in contect, what buffer that pointer came from.

When we write through that pointer, we need to set that buffer as written.

When we read through the pointer, we need to check that the buffer is readable.

Table overlays

They are uninitialized, but the twist is, the address is a buffer that is an input to and/or output of the routine. So, they are defined (insofar as the buffer is defined.)

They are therefore a "view" of a section of a buffer.

This is slightly dangerous since it does permit aliases: the buffer and the table refer to the same memory.

Although, if they are static, you could say, in the routine in which they are static, as soon as you've established one, you can no longer use the buffer; and the ones you establish must be disjoint.

(That seems to be the most compelling case for restricting them to static.)

An alternative would be static pointers, which are currently not possible because pointers must be zero-page, thus @, thus uninitialized.

Question "consistent initialization"

Question the value of the "consistent initialization" principle for if statement analysis.

Part of this is the trashes at the end; I think what it should be is that the trashes after the if is the union of the trashes in each of the branches; this would obviate the need to trash values explicitly, but if you tried to access them afterwards, it would still error.

Tail-call optimization

More generally, define a block as having zero or one gotos at the end. (and gotos cannot appear elsewhere.)

If a block ends in a call can that be converted to end in a goto? Why not? I think it can. The constraints should iron out the same both ways.

And - once we have this - why do we need goto to be in tail position, strictly? As long as the routine has consistent type context every place it exits, that should be fine.

"Include" directives

Search a searchlist of include paths. And use them to make libraries of routines.

One such library routine might be an interrupt routine type for various architectures. Since "the supervisor" has stored values on the stack, we should be able to trash them with impunity, in such a routine.