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100 lines
4.1 KiB
Markdown
100 lines
4.1 KiB
Markdown
SixtyPical
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==========
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_Version 0.13. Work-in-progress, everything is subject to change._
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SixtyPical is a very low-level programming language, similar to 6502 assembly,
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with static analysis through abstract interpretation.
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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 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 and abstractions
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based on common 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)
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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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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 execution](tests/SixtyPical%20Execution.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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* [6502 Opcodes used/not used in SixtyPical](doc/6502%20Opcodes.md)
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TODO
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----
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### `for`-like loop
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We have range-checking in the abstract analysis now, but we lack practical ways
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to use it.
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We can `and` a value to ensure it is within a certain range. However, in the 6502
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ISA the only register you can `and` is `A`, while loops are done with `X` or `Y`.
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Insisting this as the way to do it would result in a lot of `TXA`s and `TAX`s.
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What would be better is a dedicated `for` loop, like
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for x in 0 to 15 {
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// in here, we know the range of x is exactly 0-15 inclusive
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// also in here: we are disallowed from changing x
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}
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However, this is slightly restrictive, and hides a lot.
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However however, options which do not hide a lot, require a lot of looking at
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(to ensure: did you increment the loop variable? only once? etc.)
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The leading compromise so far is an "open-faced for loop", like
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ld x, 15
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for x downto 0 {
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// same as above
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}
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This makes it a little more explicit, at least, even though the loop
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decrementation is still hidden.
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### Save registers on stack
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This preserves them, so that, semantically, they can be used later even though they
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are trashed inside the block.
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### Re-order routines and optimize tail-calls to fallthroughs
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Not because it saves 3 bytes, but because it's a neat trick. Doing it optimally
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is probably NP-complete. But doing it adeuqately is probably not that hard.
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### And at some point...
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* Confirm that `and` can be used to restrict the range of table reads/writes.
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* `low` and `high` address operators - to turn `word` type into `byte`.
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* `const`s that can be used in defining the size of tables, etc.
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* Tests, and implementation, ensuring a routine can be assigned to a vector of "wider" type
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* Related: can we simply view a (small) part of a buffer as a byte table? If not, why not?
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* Check that the buffer being read or written to through pointer, appears in approporiate inputs or outputs set.
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(Associate each pointer with the buffer it points into.)
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* `static` pointers -- currently not possible because pointers must be zero-page, thus `@`, thus uninitialized.
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* Question the value of the "consistent initialization" principle for `if` statement analysis.
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* `interrupt` routines -- to indicate that "the supervisor" has stored values on the stack, so we can trash them.
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* Error messages that include the line number of the source code.
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* Add absolute addressing in shl/shr, absolute-indexed for add, sub, etc.
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* Automatic tail-call optimization (could be tricky, w/constraints?)
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* Possibly `ld x, [ptr] + y`, possibly `st x, [ptr] + y`.
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* Maybe even `copy [ptra] + y, [ptrb] + y`, which can be compiled to indirect LDA then indirect STA!
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