SixtyPical/README.markdown

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.

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.

This distribution will soon be placed under an open-source license.

Quick Start
-----------

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

    ./loadngo.sh eg/demo.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.
For example, the following is illegal:

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

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

*   the A register is declared to be a meaningful output of `update_score`
*   `update_score` was determined to not change the value of the A registers

The first must be done with an explicit declaration on `update_score` (NYI).
The second will be done using abstract interpretation of the code of
`update_score` (needs to be implemented again, now, and better).

### Structured Programming ###

You get an `if` and a `repeat` and instructions like `sei` work like `with`
where they are followed by a block and the `cli` instruction is implicitly
(and unavoidably) added at the end.

For more information, see the docs (which are written in the form of a
Falderal literate test suite.)

Concepts
--------

### Routines ###

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.

There is one top-level routine called `main` which represents the entire
program.

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.

If a register is not affected by a routine, then a caller of that routine may
assume that the value in that register is retained.

Of course, a routine may intentionally affect a register or memory location,
as an output.  It must declare this.  We're not there yet.

### Addresses ###

The body of a routine may not refer to an address literally.  It must use
a symbol that was declared previously.

An address may be declared with `reserve`, which is like `.data` or `.bss`
in an assembler.  This is an address into the program's data.  It is global
to all routines.

An address may be declared with `locate`, which is like `.alias` in an
assembler, with the understanding that the value will be treated "like an
address."  This is generally an address into the operating system or hardware
(e.g. kernal routine, I/O port, etc.)

Not there. yet:

> Inside a routine, an address may be declared with `temporary`.  This is like
> `static` in C, except the value at that address is not guaranteed to be
> retained between invokations of the routine.  Such addresses may only be used
> within the routine where they are declared.  If analysis indicates that two
> temporary addresses are never used simultaneously, they may be merged
> to the same address.

An address knows what kind of data is stored at the address:
    
*   `byte`: an 8-bit byte.  not part of a word.  not to be used as an address.
    (could be an index though.)
*   `word`: a 16-bit word.  not to be used as an address.
*   `vector`: a 16-bit address of a routine.  Only a handful of operations
    are supported on vectors:
    
    *   copying the contents of one vector to another
    *   copying the address of a routine into a vector
    *   jumping indirectly to a vector (i.e. to the code at the address
        contained in the vector (and this can only happen at the end of a
        routine (NYI))
    *   `jsr`'ing indirectly to a vector (which is done with a fun
        generated trick (NYI))
    
*   `byte table`: a series of `byte`s contiguous in memory starting from the
    address.  This is the only kind of address that can be used in
    indexed addressing.

### Blocks ###

Each routine is a block.  It may be composed of inner blocks, if those
inner blocks are attached to certain instructions.

SixtyPical does not have instructions that map literally to the 6502 branch
instructions.  Instead, it has an `if` construct, with two blocks (for the
"then" and `else` parts), and the branch instructions map to conditions for
this construct.

Similarly, there is a `repeat` construct.  The same branch instructions can
be used in the condition to this construct.  In this case, they branch back
to the top of the `repeat` loop.

The abstract states of the machine at each of the different block exits are
merged during analysis.  If any register or memory location is treated
inconsistently (e.g. updated in one branch of the test, but not the other,)
that register cannot subsequently be used without a declaration to the effect
that we know what's going on.  (This is all a bit fuzzy right now.)

There is also no `rts` instruction.  It is included at the end of a routine,
but only when the routine is used as a subroutine.  Also, if the routine
ends by `jsr`ing another routine, it reserves the right to do a tail-call
or even a fallthrough.

There are also _with_ instructions, which are associated with an opcode
that has a natural symmetrical opcode (e.g. `pha`, `sei`).  These instructions
take a block.  The natural symmetrical opcode is inserted at the end of the
block.

TODO
----

*   Initial values for reserved, incl. tables
*   give length for tables, must be there for reserved
*   Character tables ("strings" to everybody else)
*   Work out the analyses again and document them
*   Addressing modes; rename instructions to match
*   fix jmp (vector) syntax