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176 lines
6.7 KiB
Markdown
176 lines
6.7 KiB
Markdown
SixtyPical
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==========
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_Version 0.16. Work-in-progress, everything is subject to change._
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**SixtyPical** is a 6502-like programming language with advanced
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static analysis.
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"6502-like" means that it has similar restrictions as programming
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in 6502 assembly (e.g. the programmer must choose the registers that
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values will be stored in) and is concomitantly easy for a compiler to
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translate it to 6502 machine language code.
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"Advanced static analysis" includes _abstract interpretation_, where we
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go through the program step by step, tracking not just the changes that
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happen during a _specific_ execution of the program, but _sets_ of changes
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that could _possibly_ happen in any run of the program. This lets us
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determine that certain things can never happen, which we can then formulate
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as safety checks.
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In practice, this means it catches things like
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* you forgot to clear carry before adding something to the accumulator
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* a subroutine that you call trashes a register you thought was preserved
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* you tried to read or write a byte beyond the end of a byte array
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* you tried to write the address of something that was not a routine, to
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a jump vector
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and suchlike. It also provides some convenient operations based on
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machine-language programming idioms, such as
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* copying values from one register to another (via a third register when
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there are no underlying instructions that directly support it); this
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includes 16-bit values, which are copied in two steps
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* explicit tail calls
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* indirect subroutine calls
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The reference implementation can analyze and compile SixtyPical programs to
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6502 machine code.
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Quick Start
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-----------
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If you have the [VICE][] emulator installed, from this directory, you can run
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./loadngo.sh c64 eg/c64/hearts.60p
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and it will compile the [hearts.60p source code](eg/c64/hearts.60p) and
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automatically start it in the `x64` emulator, and you should see:
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You can try the `loadngo.sh` script on other sources in the `eg` directory
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tree, which contains more extensive examples, including an entire
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game(-like program); see [eg/README.md](eg/README.md) for a listing.
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[VICE]: http://vice-emu.sourceforge.net/
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Documentation
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-------------
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* [Design Goals](doc/Design%20Goals.md)
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* [SixtyPical specification](doc/SixtyPical.md)
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* [SixtyPical revision history](HISTORY.md)
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* [Literate test suite for SixtyPical syntax](tests/SixtyPical%20Syntax.md)
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* [Literate test suite for SixtyPical analysis](tests/SixtyPical%20Analysis.md)
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* [Literate test suite for SixtyPical compilation](tests/SixtyPical%20Compilation.md)
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* [Literate test suite for SixtyPical fallthru optimization](tests/SixtyPical%20Fallthru.md)
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* [6502 Opcodes used/not used in SixtyPical](doc/6502%20Opcodes.md)
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TODO
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----
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### `low` and `high` address operators
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To turn `word` type into `byte`.
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Trying to remember if we have a compelling case for this or now. The best I can think
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of is for implementing 16-bit `cmp` in an efficient way. Maybe we should see if we
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can get by with 16-bit `cmp` instead though.
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The problem is that once a byte is extracted, putting it back into a word is awkward.
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The address operators have to modify a destination in a special way. That is, when
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you say `st a, >word`, you are updating `word` to be `word & $ff | a << 8`, somelike.
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Is that consistent with `st`? Well, probably it is, but we have to explain it.
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It might make more sense, then, for it to be "part of the operation" instead of "part of
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the reference"; something like `st.hi x, word`; `st.lo y, word`. Dunno.
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### Save values
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This preserves them, so that, semantically, they can be used later even though they
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are trashed (or otherwise alternately used) inside the block.
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Inside the block, we set them as writeable (but not meaningful). When the block
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exits, we restore whatever status they had.
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This act will trash `a`, both in the block, and outside it, unless the value being
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saved is `a`. One idiom would be something like
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save a { save var {
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...
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} }
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which would save all values. Maybe abbreviate this to
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save a, var {
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...
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}
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This can use the stack. But it need not use the stack.
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### Make all symbols forward-referencable
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Basically, don't do symbol-table lookups when parsing, but do have a more formal
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"symbol resolution" (linking) phase right after parsing.
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### Associate each pointer with the buffer it points into
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Check that the buffer being read or written to through pointer, appears in appropriate
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inputs or outputs set.
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In the analysis, when we obtain a pointer, we need to record, in contect, what buffer
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that pointer came from.
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When we write through that pointer, we need to set that buffer as written.
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When we read through the pointer, we need to check that the buffer is readable.
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### Table overlays
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They are uninitialized, but the twist is, the address is a buffer that is
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an input to and/or output of the routine. So, they are defined (insofar
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as the buffer is defined.)
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They are therefore a "view" of a section of a buffer.
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This is slightly dangerous since it does permit aliases: the buffer and the
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table refer to the same memory.
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Although, if they are `static`, you could say, in the routine in which they
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are `static`, as soon as you've established one, you can no longer use the
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buffer; and the ones you establish must be disjoint.
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(That seems to be the most compelling case for restricting them to `static`.)
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An alternative would be `static` pointers, which are currently not possible because
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pointers must be zero-page, thus `@`, thus uninitialized.
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### Question "consistent initialization"
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Question the value of the "consistent initialization" principle for `if` statement analysis.
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Part of this is the trashes at the end; I think what it should be is that the trashes
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after the `if` is the union of the trashes in each of the branches; this would obviate the
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need to `trash` values explicitly, but if you tried to access them afterwards, it would still
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error.
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### Tail-call optimization
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More generally, define a block as having zero or one `goto`s at the end. (and `goto`s cannot
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appear elsewhere.)
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If a block ends in a `call` can that be converted to end in a `goto`? Why not? I think it can.
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The constraints should iron out the same both ways.
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And - once we have this - why do we need `goto` to be in tail position, strictly?
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As long as the routine has consistent type context every place it exits, that should be fine.
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### "Include" directives
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Search a searchlist of include paths. And use them to make libraries of routines.
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One such library routine might be an `interrupt routine` type for various architectures.
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Since "the supervisor" has stored values on the stack, we should be able to trash them
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with impunity, in such a routine.
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