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Going over the spec for version 0.8-PRE.

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Chris Pressey 2017-11-17 16:56:52 +00:00
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doc/SixtyPical.md
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@ -14,20 +14,22 @@ the language.
Types
-----
There are five TYPES in SixtyPical:
There are seven *types* in SixtyPical:
* bit (2 possible values)
* byte (256 possible values)
* byte table (256 entries, each holding a byte)
* word (65536 possible values)
* word table (256 entries, each holding a word)
* routine (code stored somewhere in memory, read-only)
* vector (address of a routine)
Memory locations
----------------
A primary concept in SixtyPical is the MEMORY LOCATION. At any given point
in time during execution, each memory location is either UNINITIALIZED or
INITIALIZED. At any given point in the program text, too, each memory
A primary concept in SixtyPical is the *memory location*. At any given point
in time during execution, each memory location is either *uninitialized* or
*initialized*. At any given point in the program text, too, each memory
location is either uninitialized or initialized. Where-ever it is one or
the other during execution, it is the same in the corresponding place in
the program text; thus, it is a static property.
@ -64,17 +66,27 @@ They come in bit and byte types. There are two bit constants,
off
on
and two-hundred and fifty-six byte constants,
two hundred and fifty-six byte constants,
0
1
...
255
and sixty-five thousand five hundred and thirty-six word constants,
0
1
...
65535
Note that all byte constants serve double duty as word constants in that
context.
### User-defined ###
There may be any number of user-defined memory locations. They are defined
by giving the type, which must be `byte`, `byte table`, or `vector`, and the
by giving the type (which may be any type except `bit` and `routine`) and the
name.
byte pos
@ -88,10 +100,10 @@ case, an explicit address in memory cannot be given.
byte pos : 0
A user-defined vector memory location is decorated with READS and WRITES lists
like a routine (see below), and it may only hold addresses of routines which
are compatible. (Meaning, the routine's inputs (resp. outputs, trashes)
must be a subset of the vector's inputs (resp. outputs, trashes.))
A user-defined vector memory location is decorated with `inputs`, `outputs`
and `trashes` lists like a routine (see below), and it may only hold addresses
of routines which are compatible. (Meaning, the routine's inputs (resp. outputs,
trashes) must be a subset of the vector's inputs (resp. outputs, trashes.))
vector actor_logic
inputs a, score
@ -102,10 +114,16 @@ must be a subset of the vector's inputs (resp. outputs, trashes.))
Routines
--------
Every routine must list all the memory locations it READS from, i.e. its
INPUTS, and all the memory locations it WRITES to, whether they are OUTPUTS
or merely TRASHED. Every memory location that is not written to by the
routine (or any routines that the routine calls) is PRESERVED by the routine.
Every routine must list all the memory locations it *reads from*, which we
call its `inputs`, and all the memory locations it *writes to*. The latter
we divide into two groups: its `outputs` which it intentionally initializes,
and its `trashes`, which it does not care about, and leaves uninitialized.
For example, if it uses a register to temporarily store an intermediate
value used in a multiplication, that register has no meaning outside of
the multiplication, and is one of the routine's `trashes`.
It is common to say that the `trashes` are the memory locations that are
*not preserved* by the routine.
routine foo
inputs a, score
@ -114,6 +132,9 @@ routine (or any routines that the routine calls) is PRESERVED by the routine.
...
}
The `inputs` are sometimes called the routine's READS set, while the
`outputs` and `trashes` are collectively called the WRITES set.
Routines may call only routines previously defined in the program source.
Thus, directly recursive routines are not allowed. (However, routines may
also call routines via vectors, which are dynamically assigned. In this
@ -122,12 +143,12 @@ case, there is, for the time being, no check for recursive calls.)
For a SixtyPical program to be run, there must be one routine called `main`.
This routine is executed when the program is run.
The memory locations given given as inputs are considered to be initialized
The memory locations given as inputs to a routine are considered to be initialized
at the beginning of the routine. Various instructions cause memory locations
to be initialized after they are executed. Calling a routine which trashes
some memory locations causes those memory locations to be uninitialized after
that routine is called. At the end of a routine, all memory locations listed
as outputs must be initialised.
as outputs must be initialized.
A routine can also be declared as "external", in which case its body need
not be defined but an absolute address must be given for where the routine
@ -141,6 +162,13 @@ is located in memory.
Instructions
------------
Instructions are inspired by, and in many cases closely resemble, the 6502
instruction set. However, in many cases they do not map 1:1 to 6502 instructions.
If a SixtyPical instruction cannot be translated validly to one more more 6502
instructions while retaining all the stated constraints, that's a static error
in a SixtyPical program, and technically any implementation of SixtyPical, even
an interpreter, should flag it up.
### ld ###
ld <dest-memory-location>, <src-memory-location> [+ <index-memory-location>]
@ -148,13 +176,12 @@ Instructions
Reads from src and writes to dest.
* It is illegal if dest is not a register.
* It is illegal if dest does not occur in the WRITES lists of the current
routine.
* It is illegal if dest does not occur in the WRITES of the current routine.
* It is illegal if src is not of same type as dest (i.e., is not a byte.)
* It is illegal if src is uninitialized.
After execution, dest is considered initialized. The flags `z` and `n` may be
changed by this instruction; they must be named in the WRITES lists, and they
changed by this instruction; they must be named in the WRITES, and they
are considered initialized after it has executed.
If and only if src is a byte table, the index-memory-location must be given.
@ -169,8 +196,7 @@ underlying opcodes.
Reads from src and writes to dest.
* It is illegal if dest is a register or if dest is read-only.
* It is illegal if dest does not occur in the WRITES lists of the current
routine.
* It is illegal if dest does not occur in the WRITES of the current routine.
* It is illegal if src is not of same type as dest.
* It is illegal if src is uninitialized.
@ -187,10 +213,9 @@ Adds the contents of src to dest and stores the result in dest.
* It is illegal if src OR dest OR c is uninitialized.
* It is illegal if dest is read-only.
* It is illegal if dest does not occur in the WRITES lists
of the current routine.
* It is illegal if dest does not occur in the WRITES of the current routine.
Affects n, z, c, and v flags, requiring that they be in the WRITES lists,
Affects n, z, c, and v flags, requiring that they be in the WRITES,
and initializing them afterwards.
dest and src continue to be initialized afterwards.
@ -203,10 +228,9 @@ Increments the value in dest. Does not honour carry.
* It is illegal if dest is uninitialized.
* It is illegal if dest is read-only.
* It is illegal if dest does not occur in the WRITES lists
of the current routine.
* It is illegal if dest does not occur in the WRITES of the current routine.
Affects n and z flags, requiring that they be in the WRITES lists,
Affects n and z flags, requiring that they be in the WRITES,
and initializing them afterwards.
### sub ###
@ -217,10 +241,9 @@ Subtracts the contents of src from dest and stores the result in dest.
* It is illegal if src OR dest OR c is uninitialized.
* It is illegal if dest is read-only.
* It is illegal if dest does not occur in the WRITES lists
of the current routine.
* It is illegal if dest does not occur in the WRITES of the current routine.
Affects n, z, c, and v flags, requiring that they be in the WRITES lists,
Affects n, z, c, and v flags, requiring that they be in the WRITES,
and initializing them afterwards.
dest and src continue to be initialized afterwards.
@ -233,10 +256,9 @@ Decrements the value in dest. Does not honour carry.
* It is illegal if dest is uninitialized.
* It is illegal if dest is read-only.
* It is illegal if dest does not occur in the WRITES lists
of the current routine.
* It is illegal if dest does not occur in the WRITES of the current routine.
Affects n and z flags, requiring that they be in the WRITES lists,
Affects n and z flags, requiring that they be in the WRITES,
and initializing them afterwards.
### cmp ###
@ -248,7 +270,7 @@ does not store the result anywhere, only sets the resulting flags.
* It is illegal if src OR dest is uninitialized.
Affects n, z, and c flags, requiring that they be in the WRITES lists,
Affects n, z, and c flags, requiring that they be in the WRITES,
and initializing them afterwards.
### and, or, xor ###
@ -262,10 +284,9 @@ the result in dest.
* It is illegal if src OR dest OR is uninitialized.
* It is illegal if dest is read-only.
* It is illegal if dest does not occur in the WRITES lists
of the current routine.
* It is illegal if dest does not occur in the WRITES of the current routine.
Affects n and z flags, requiring that they be in the WRITES lists of the
Affects n and z flags, requiring that they be in the WRITES of the
current routine, and sets them as initialized afterwards.
dest and src continue to be initialized afterwards.
@ -284,10 +305,9 @@ and `c` becomes the bit that was shifted off the right.
* It is illegal if dest is a register besides `a`.
* It is illegal if dest is read-only.
* It is illegal if dest OR c is uninitialized.
* It is illegal if dest does not occur in the WRITES lists
of the current routine.
* It is illegal if dest does not occur in the WRITES of the current routine.
Affects the c flag, requiring that it be in the WRITES lists of the
Affects the c flag, requiring that it be in the WRITES of the
current routine, and it continues to be initialized afterwards.
### call ###
@ -299,17 +319,15 @@ defined routine, or a vector location which contains the address of a routine
which will be called indirectly. Execution will be transferred back to the
current routine, when execution of the executable is finished.
Just before the call,
* It is illegal if any of the memory locations in the target executable's
READS list is uninitialized.
* It is illegal if any of the memory locations listed in the called routine's
`inputs` are uninitialized immediately before the call.
Just after the call,
* All memory locations listed as TRASHED in the called routine's WRITES
list are considered uninitialized.
* All memory locations listed as TRASHED in the called routine's OUTPUTS
list are considered initialized.
* All memory locations listed in the called routine's `trashes` are considered
to now be uninitialized.
* All memory locations listed in the called routine's `outputs` are considered
to not be initialized.
### goto ###
@ -330,8 +348,8 @@ Just before the goto,
In addition,
* The target executable's WRITES lists must not include any locations
that are not already included in the current routine's WRITES lists.
* The target executable's WRITES must not include any locations
that are not already included in the current routine's WRITES.
### if ###
@ -383,8 +401,7 @@ copy more general types of data (for example, vectors,) and it trashes the
`z` and `n` flags and the `a` register.
* It is illegal if dest is read-only.
* It is illegal if dest does not occur in the WRITES lists of the current
routine.
* It is illegal if dest does not occur in the WRITES of the current routine.
* It is illegal if src is not of same type as dest.
* It is illegal if src is uninitialized.