SixtyPical
==========

_Version 0.13.  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
-------------

*   [Design Goals](doc/Design%20Goals.md)
*   [SixtyPical specification](doc/SixtyPical.md)
*   [SixtyPical revision history](HISTORY.md)
*   [Literate test suite for SixtyPical syntax](tests/SixtyPical%20Syntax.md)
*   [Literate test suite for SixtyPical execution](tests/SixtyPical%20Execution.md)
*   [Literate test suite for SixtyPical analysis](tests/SixtyPical%20Analysis.md)
*   [Literate test suite for SixtyPical compilation](tests/SixtyPical%20Compilation.md)
*   [6502 Opcodes used/not used in SixtyPical](doc/6502%20Opcodes.md)

TODO
----

### `for`-like loop

We have range-checking in the abstract analysis now, but we lack practical ways
to use it.

We can `and` a value to ensure it is within a certain range.  However, in the 6502
ISA the only register you can `and` is `A`, while loops are done with `X` or `Y`.
Insisting this as the way to do it would result in a lot of `TXA`s and `TAX`s.

What would be better is a dedicated `for` loop, like

    for x in 0 to 15 {
        // in here, we know the range of x is exactly 0-15 inclusive
        // also in here: we are disallowed from changing x
    }

However, this is slightly restrictive, and hides a lot.

However however, options which do not hide a lot, require a lot of looking at
(to ensure: did you increment the loop variable? only once? etc.)

The leading compromise so far is an "open-faced for loop", like

    ld x, 15
    for x downto 0 {
        // same as above
    }

This makes it a little more explicit, at least, even though the loop
decrementation is still hidden.

### Save registers on stack

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

### 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...

*   Confirm that `and` can be used to restrict the range of table reads/writes.
*   `low` and `high` address operators - to turn `word` type into `byte`.
*   `const`s 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.
*   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!