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Work on the spec a bit.
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Chris Pressey
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Sixtypical
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SixtyPical
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==========
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Sixtypical is a simplified version of [Sixtypical][].
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This document describes the SixtyPical programming language version 0.2,
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both its execution aspect and its static analysis aspect (even though
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these are, technically speaking, separate concepts.)
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This is a complete reboot of the previous design and implementation, which
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was semantically a mess due to the way it was built.
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This aims to be a simpler design which gets the static semantics right first,
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and only then is extended to be more practical.
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This document is nominally normative, but the tests in the `tests` directory
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are even more normative.
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Types
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-----
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There are two TYPES in Sixtypical:
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There are two TYPES in SixtyPical:
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* bit (2 possible values)
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* byte (256 possible values)
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@ -19,17 +19,13 @@ There are two TYPES in Sixtypical:
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Memory locations
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----------------
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The primary concept in Sixtypical is the MEMORY LOCATION. At any given point
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A primary concept in SixtyPical is the MEMORY LOCATION. At any given point
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in time during execution, each memory location is either UNINITIALIZED or
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INITIALIZED. At any given point in the program text, too, each memory
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location is either uninitialized or initialized. Where-ever it is one or
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the other during execution, it is the same in the corresponding place in
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the program text; thus, it is a static property.
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(There is actually a third state, WRITTEN, which indicates that the memory
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location is not only initialized, but also that it has been written to in
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the current routine.)
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There are four general kinds of memory location. The first three are
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pre-defined and built-in.
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@ -53,9 +49,9 @@ Each of these hold a bit. They are initially uninitialized.
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### Constants ###
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It may be strange to think of constants as memory locations, but keep in mind
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that a memory location in Sixtypical need not map to a memory location in the
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underlying hardware. All constants are read-only. Each is
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initially initialized with the value that corresponds with its name.
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that a memory location in SixtyPical need not map to a memory location in the
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underlying hardware. All constants are read-only. Each is initially
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initialized with the value that corresponds with its name.
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They come in bit and byte types. There are two bit constants,
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@ -81,7 +77,8 @@ Routines
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Every routine must list all the memory locations it READS from, i.e. its
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INPUTS, and all the memory locations it WRITES to, whether they are OUTPUTS
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or merely TRASHED. Every memory location that is not written to is PRESERVED.
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or merely TRASHED. Every memory location that is not written to by the
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routine (or any routines that the routine calls) is PRESERVED by the routine.
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routine foo
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inputs a, score
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@ -93,8 +90,15 @@ or merely TRASHED. Every memory location that is not written to is PRESERVED.
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Routines may call only routines previously defined in the program source.
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Thus, recursive routines are not allowed.
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There must be one routine called `main`. This routine is executed when
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the program is run.
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For a SixtyPical program to be run, there must be one routine called `main`.
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This routine is executed when the program is run.
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The memory locations given given as inputs are considered to be initialized
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at the beginning of the routine. Various instructions cause memory locations
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to be initialized after they are executed. Calling a routine which trashes
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some memory locations causes those memory locations to be uninitialized after
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that routine is called. At the end of a routine, all memory locations listed
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as outputs must be initialised.
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Instructions
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------------
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@ -106,19 +110,14 @@ Instructions
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Reads from src and writes to dest.
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* It is illegal if dest is not a register.
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* It is illegal if dest does not occur in the WRITES list of the current
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* It is illegal if dest does not occur in the WRITES lists of the current
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routine.
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* It is illegal if src is not of same type as dest (i.e., is not a byte.)
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* It is illegal if src is uninitialized.
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* It is illegal if src does not either:
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* be a constant, or
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* occur in the READS list of the current routine, or
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* occur in the WRITES list of the current routine AND
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that location has previously been written inside this routine.
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After execution, dest is considered initialized. The flags `z` and `n` may be
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changed by this instruction, and they are considered initialized after it has
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executed.
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changed by this instruction; they must be named in the WRITES lists, and they
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are considered initialized after it has executed.
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Some combinations, such as `ld x, y`, are illegal because they do not map to
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underlying opcodes.
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@ -143,15 +142,10 @@ Notes:
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Reads from src and writes to dest.
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* It is illegal if dest is a register or if dest is read-only.
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* It is illegal if dest does not occur in the WRITES list of the current
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* It is illegal if dest does not occur in the WRITES lists of the current
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routine.
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* It is illegal if src is not of same type as dest.
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* It is illegal if src is uninitialized.
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* It is illegal if src does not either:
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* be a constant, or
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* occur in the READS list of the current routine, or
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* occur in the WRITES list of the current routine AND
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that location has previously been written inside this routine.
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After execution, dest is considered initialized. No flags are
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changed by this instruction (unless of course dest is a flag.)
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@ -172,17 +166,13 @@ Adds the contents of src to dest and stores the result in dest.
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* It is illegal if src OR dest OR c is uninitialized.
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* It is illegal if dest is read-only.
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* It is illegal if dest does not occur in the WRITES AND READS lists
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* It is illegal if dest does not occur in the WRITES lists
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of the current routine.
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* It is illegal if src does not either:
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* be a constant, or
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* occur in the READS list of the current routine, or
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* occur in the WRITES list of the current routine AND
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that location has previously been written inside this routine.
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Affects n, z, c, and v flags.
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Affects n, z, c, and v flags, requiring that they be in the WRITES lists,
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and initializing them afterwards.
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dest continues to be initialized afterwards.
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dest and src continue to be initialized afterwards.
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Notes:
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@ -191,7 +181,19 @@ Notes:
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### inc ###
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TODO: these do not honour carry!
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inc <dest-memory-location>
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Increments the value in dest. Does not honour carry.
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* It is illegal if dest is uninitialized.
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* It is illegal if dest is read-only.
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* It is illegal if dest does not occur in the WRITES lists
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of the current routine.
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Affects n and z flags, requiring that they be in the WRITES lists,
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and initializing them afterwards.
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Notes:
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inc x → INX
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inc y → INY
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@ -203,7 +205,15 @@ TODO: these do not honour carry!
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Subtracts the contents of src from dest and stores the result in dest.
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The constraints and effects are exactly the same as for `add`.
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* It is illegal if src OR dest OR c is uninitialized.
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* It is illegal if dest is read-only.
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* It is illegal if dest does not occur in the WRITES lists
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of the current routine.
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Affects n, z, c, and v flags, requiring that they be in the WRITES lists,
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and initializing them afterwards.
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dest and src continue to be initialized afterwards.
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Notes:
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@ -212,7 +222,19 @@ Notes:
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### dec ###
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TODO: these do not honour carry!
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inc <dest-memory-location>
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Decrements the value in dest. Does not honour carry.
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* It is illegal if dest is uninitialized.
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* It is illegal if dest is read-only.
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* It is illegal if dest does not occur in the WRITES lists
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of the current routine.
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Affects n and z flags, requiring that they be in the WRITES lists,
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and initializing them afterwards.
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Notes:
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dec x → DEX
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dec y → DEY
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@ -224,9 +246,10 @@ TODO: these do not honour carry!
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Subtracts the contents of src from dest, but does not store the result.
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The constraints and effects are the same as for `sub`, except that `c`
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need not be initialized before executing `cmp`, and the `v` flag is
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unaffected.
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* It is illegal if src OR dest is uninitialized.
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Affects n, z, and c flags, requiring that they be in the WRITES lists,
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and initializing them afterwards.
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Notes:
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@ -235,6 +258,8 @@ Notes:
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cmp x, 1 → CPX #1
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cmp y, 1 → CPY #1
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- - - -
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### and ###
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and <dest-memory-location>, <src-memory-location>
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