SixtyPical

Version 0.12. Work-in-progress, everything is subject to change.

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

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 write the address of something that was not a routine, to a jump vector

and suchlike. It also provides some convenient operations and abstractions based on common 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)
explicit tail calls
indirect subroutine calls

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

Documentation

TODO

Save registers on stack

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

Range checking in the abstract interpretation

If you copy the address of a buffer (say it is size N) to a pointer, it is valid. If you add a value from 0 to N-1 to the pointer, it is still valid. But if you add a value ≥ N to it, it becomes invalid. This should be tracked in the abstract interpretation. (If only because abstract interpretation is the major point of this project!)

Range-checking buffers might be too difficult. Range checking tables will be easier. If a value is ANDed with 15, its range must be 0-15, etc.

Re-order routines and optimize tail-calls to fallthroughs

Not because it saves 3 bytes, but because it's a neat trick. Doing it optimally is probably NP-complete. But doing it adeuqately is probably not that hard.

And at some point...

low and high address operators - to turn word type into byte.
consts that can be used in defining the size of tables, etc.
Tests, and implementation, ensuring a routine can be assigned to a vector of "wider" type
Related: can we simply view a (small) part of a buffer as a byte table? If not, why not?
Check that the buffer being read or written to through pointer, appears in approporiate inputs or outputs set. (Associate each pointer with the buffer it points into.)
static pointers -- currently not possible because pointers must be zero-page, thus @, thus uninitialized.
Question the value of the "consistent initialization" principle for if statement analysis.
interrupt routines -- to indicate that "the supervisor" has stored values on the stack, so we can trash them.
Error messages that include the line number of the source code.
Add absolute addressing in shl/shr, absolute-indexed for add, sub, etc.
Automatic tail-call optimization (could be tricky, w/constraints?)
Possibly ld x, [ptr] + y, possibly st x, [ptr] + y.
Maybe even copy [ptra] + y, [ptrb] + y, which can be compiled to indirect LDA then indirect STA!

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