Commit 8ccabdf202231da35dc11f69f080670b2012c21a - SixtyPical

Work on the spec a bit. Chris Pressey 6 years ago

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0		Sixtypical
	0	SixtyPical
1	1	==========
2	2
3		Sixtypical is a simplified version of [Sixtypical][].
4
5		This is a complete reboot of the previous design and implementation, which
6		was semantically a mess due to the way it was built.
7		This aims to be a simpler design which gets the static semantics right first,
8		and only then is extended to be more practical.
	3	This document describes the SixtyPical programming language version 0.2,
	4	both its execution aspect and its static analysis aspect (even though
	5	these are, technically speaking, separate concepts.)
	6
	7	This document is nominally normative, but the tests in the `tests` directory
	8	are even more normative.
9	9
10	10	Types
11	11	-----
12	12
13		There are two TYPES in Sixtypical:
	13	There are two TYPES in SixtyPical:
14	14
15	15	* bit (2 possible values)
16	16	* byte (256 possible values)

18	18	Memory locations
19	19	----------------
20	20
21		The primary concept in Sixtypical is the MEMORY LOCATION. At any given point
	21	A primary concept in SixtyPical is the MEMORY LOCATION. At any given point
22	22	in time during execution, each memory location is either UNINITIALIZED or
23	23	INITIALIZED. At any given point in the program text, too, each memory
24	24	location is either uninitialized or initialized. Where-ever it is one or
25	25	the other during execution, it is the same in the corresponding place in
26	26	the program text; thus, it is a static property.
27	27
28		(There is actually a third state, WRITTEN, which indicates that the memory
29		location is not only initialized, but also that it has been written to in
30		the current routine.)
31
32	28	There are four general kinds of memory location. The first three are
33	29	pre-defined and built-in.
34	30

52	48	### Constants ###
53	49
54	50	It may be strange to think of constants as memory locations, but keep in mind
55		that a memory location in Sixtypical need not map to a memory location in the
56		underlying hardware. All constants are read-only. Each is
57		initially initialized with the value that corresponds with its name.
	51	that a memory location in SixtyPical need not map to a memory location in the
	52	underlying hardware. All constants are read-only. Each is initially
	53	initialized with the value that corresponds with its name.
58	54
59	55	They come in bit and byte types. There are two bit constants,
60	56

80	76
81	77	Every routine must list all the memory locations it READS from, i.e. its
82	78	INPUTS, and all the memory locations it WRITES to, whether they are OUTPUTS
83		or merely TRASHED. Every memory location that is not written to is PRESERVED.
	79	or merely TRASHED. Every memory location that is not written to by the
	80	routine (or any routines that the routine calls) is PRESERVED by the routine.
84	81
85	82	routine foo
86	83	inputs a, score

92	89	Routines may call only routines previously defined in the program source.
93	90	Thus, recursive routines are not allowed.
94	91
95		There must be one routine called `main`. This routine is executed when
96		the program is run.
	92	For a SixtyPical program to be run, there must be one routine called `main`.
	93	This routine is executed when the program is run.
	94
	95	The memory locations given given as inputs are considered to be initialized
	96	at the beginning of the routine. Various instructions cause memory locations
	97	to be initialized after they are executed. Calling a routine which trashes
	98	some memory locations causes those memory locations to be uninitialized after
	99	that routine is called. At the end of a routine, all memory locations listed
	100	as outputs must be initialised.
97	101
98	102	Instructions
99	103	------------

105	109	Reads from src and writes to dest.
106	110
107	111	* It is illegal if dest is not a register.
108		* It is illegal if dest does not occur in the WRITES list of the current
	112	* It is illegal if dest does not occur in the WRITES lists of the current
109	113	routine.
110	114	* It is illegal if src is not of same type as dest (i.e., is not a byte.)
111	115	* It is illegal if src is uninitialized.
112		* It is illegal if src does not either:
113		* be a constant, or
114		* occur in the READS list of the current routine, or
115		* occur in the WRITES list of the current routine AND
116		that location has previously been written inside this routine.
117	116
118	117	After execution, dest is considered initialized. The flags `z` and `n` may be
119		changed by this instruction, and they are considered initialized after it has
120		executed.
	118	changed by this instruction; they must be named in the WRITES lists, and they
	119	are considered initialized after it has executed.
121	120
122	121	Some combinations, such as `ld x, y`, are illegal because they do not map to
123	122	underlying opcodes.

142	141	Reads from src and writes to dest.
143	142
144	143	* It is illegal if dest is a register or if dest is read-only.
145		* It is illegal if dest does not occur in the WRITES list of the current
	144	* It is illegal if dest does not occur in the WRITES lists of the current
146	145	routine.
147	146	* It is illegal if src is not of same type as dest.
148	147	* It is illegal if src is uninitialized.
149		* It is illegal if src does not either:
150		* be a constant, or
151		* occur in the READS list of the current routine, or
152		* occur in the WRITES list of the current routine AND
153		that location has previously been written inside this routine.
154	148
155	149	After execution, dest is considered initialized. No flags are
156	150	changed by this instruction (unless of course dest is a flag.)

171	165
172	166	* It is illegal if src OR dest OR c is uninitialized.
173	167	* It is illegal if dest is read-only.
174		* It is illegal if dest does not occur in the WRITES AND READS lists
	168	* It is illegal if dest does not occur in the WRITES lists
175	169	of the current routine.
176		* It is illegal if src does not either:
177		* be a constant, or
178		* occur in the READS list of the current routine, or
179		* occur in the WRITES list of the current routine AND
180		that location has previously been written inside this routine.
181
182		Affects n, z, c, and v flags.
183
184		dest continues to be initialized afterwards.
	170
	171	Affects n, z, c, and v flags, requiring that they be in the WRITES lists,
	172	and initializing them afterwards.
	173
	174	dest and src continue to be initialized afterwards.
185	175
186	176	Notes:
187	177

190	180
191	181	### inc ###
192	182
193		TODO: these do not honour carry!
	183	inc <dest-memory-location>
	184
	185	Increments the value in dest. Does not honour carry.
	186
	187	* It is illegal if dest is uninitialized.
	188	* It is illegal if dest is read-only.
	189	* It is illegal if dest does not occur in the WRITES lists
	190	of the current routine.
	191
	192	Affects n and z flags, requiring that they be in the WRITES lists,
	193	and initializing them afterwards.
	194
	195	Notes:
194	196
195	197	inc x → INX
196	198	inc y → INY

202	204
203	205	Subtracts the contents of src from dest and stores the result in dest.
204	206
205		The constraints and effects are exactly the same as for `add`.
	207	* It is illegal if src OR dest OR c is uninitialized.
	208	* It is illegal if dest is read-only.
	209	* It is illegal if dest does not occur in the WRITES lists
	210	of the current routine.
	211
	212	Affects n, z, c, and v flags, requiring that they be in the WRITES lists,
	213	and initializing them afterwards.
	214
	215	dest and src continue to be initialized afterwards.
206	216
207	217	Notes:
208	218

211	221
212	222	### dec ###
213	223
214		TODO: these do not honour carry!
	224	inc <dest-memory-location>
	225
	226	Decrements the value in dest. Does not honour carry.
	227
	228	* It is illegal if dest is uninitialized.
	229	* It is illegal if dest is read-only.
	230	* It is illegal if dest does not occur in the WRITES lists
	231	of the current routine.
	232
	233	Affects n and z flags, requiring that they be in the WRITES lists,
	234	and initializing them afterwards.
	235
	236	Notes:
215	237
216	238	dec x → DEX
217	239	dec y → DEY

223	245
224	246	Subtracts the contents of src from dest, but does not store the result.
225	247
226		The constraints and effects are the same as for `sub`, except that `c`
227		need not be initialized before executing `cmp`, and the `v` flag is
228		unaffected.
	248	* It is illegal if src OR dest is uninitialized.
	249
	250	Affects n, z, and c flags, requiring that they be in the WRITES lists,
	251	and initializing them afterwards.
229	252
230	253	Notes:
231	254

233	256	cmp a, 1 → CMP #1
234	257	cmp x, 1 → CPX #1
235	258	cmp y, 1 → CPY #1
	259
	260	- - - -
236	261
237	262	### and ###
238	263