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. The language has a type system which distinguishes addresses from non-addresses (16-bit values for which it does not make sense to treat them as addresses.) Some 6502 opcodes have no SixtyPical equivalent. Some SixtyPical instructions are named after 6502 opcodes, but generate slightly different (safer, but intuitively related) sequences of opcodes. Et cetera.

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.

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 bytes 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 jsring 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.

Unsupported Opcodes

6502 opcodes with no language-level equivalent instructions in SixtyPical are brk, cli, pla, plp, rti, and rts. These may be inserted into the output program as a SixtyPical → 6502 compiler sees fit, however.

Note to self, the pl opcodes do change flags.

Instruction Support so far

A X indicates unsupported.

Funny syntax indicates use of a special form.

In these, absolute must be a reserved or located address. immediate must be a literal decimal or hexadecimal number (or in future, a declared constant.)

.
  adc #immediate
  adc absolute

  and #immediate
  and absolute

X asl
X asl absolute

  if bcc { block } else { block }

  if bcs { block } else { block }

  if beq { block } else { block }

X bit absolute

  if bmi { block } else { block }

  if bne { block } else { block }

  if bpl { block } else { block }

  if bvc { block } else { block }

  if bvs { block } else { block }

  clc

  cld

  clv

  cmp #immediate
  cmp absolute
  
  cpx #immediate
  cpx absolute

  cpy #immediate
  cpy absolute

  dec absolute

  dex

  dey

X eor #immediate
X eor absolute

  inc absolute

  inx

  iny

* jsr routine
X jsr vector

X jmp routine
* jmp vector

  lda #immediate
  lda absolute
  lda absolute, x
  lda absolute, y
  lda (absolute), y

  ldx #immediate
  ldx absolute

  ldy #immediate
  ldy absolute

X lsr
X lsr absolute

  nop
  
  ora #immediate
  ora absolute

X pha { block }

X php { block }

X rol
X rol absolute

X ror
X ror absolute

  sbc #immediate
  sbc absolute

  sec

  sed

  sei { block }

  sta absolute
  sta absolute, x
  sta absolute, y
  sta (absolute), y
  
  stx absolute
  
  sty absolute

  tax
  
  tay
  
X tsx

  txa

X txs

  tya

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

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