SixtyPical

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 execute, analyze, and compile SixtyPical programs to 6502 machine code.

SixtyPical is a work in progress. The current released version of SixtyPical is 0.10 (not released yet).

Documentation

TODO

Demo game

Finish the little demo "game" where you can move a block around the screen with the joystick (i.e. bring it up to par with the original demo game that was written for SixtyPical)

`call` routines that are defined further down in the source code

We might have a graph of states that refer to each other and that want to goto each other. Thus we need this. We have it for vectors, but we need it for call.

Allow branches to diverge in what they touch

For example, if the routine inputs and outputs foo, and one branch of an if sets foo and the other does not touch it, that should be OK.

`vector table` type

`low` and `high` address operators

To turn word type into byte.

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!)

And at some point...

Compare word (constant or memory location) with memory location or pointer. (Maybe?)
copy x, [ptr] + y
Maybe even copy [ptra] + y, [ptrb] + y, which can be compiled to indirect LDA then indirect STA!
Check that the buffer being read or written to through pointer, appears in approporiate inputs or outputs set.
byte table and word table of sizes other than 256
initialized byte table memory locations
always analyze before executing or compiling, unless told not to
trash instruction.
interrupt routines -- to indicate that "the supervisor" has stored values on the stack, so we can trash them.
pre-initialized word variables
error messages that include the line number of the source code
have copy instruction able to copy a byte to a user-def mem loc, etc.
add absolute addressing in shl/shr, absolute-indexed for add, sub, etc.
check and disallow recursion.
automatic tail-call optimization (could be tricky, w/constraints?)
re-order routines and optimize tail-calls to fallthroughs

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