trash indicates a value is no longer considered meaningful.

HISTORY.md

@@ -10,6 +10,7 @@ History of SixtyPical
 *   Initialized `word` memory locations.
 *   Can `copy` a literal word to a word table.
 *   Subtract word (constant or memory location) from word memory location.
+*   `trash` instruction explicitly indicates a value is no longer considered meaningful.
 *   Fixed bug which was preventing `if` branches to diverge in what they initialized,
     if it was already initialized when going into the `if`.
 *   Fixed a bug which was making it crash when trying to analyze `repeat forever` loops.

README.md

@@ -74,7 +74,6 @@ This should be tracked in the abstract interpretation.
 *   `byte table` and `word table` of sizes other than 256
 *   initialized `byte table` memory locations
 *   always analyze before executing or compiling, unless told not to
-*   `trash` instruction.
 *   `interrupt` routines -- to indicate that "the supervisor" has stored values on the stack, so we can trash them.
 *   error messages that include the line number of the source code
 *   have `copy` instruction able to copy a byte to a user-def mem loc, etc.

src/sixtypical/analyzer.py

@@ -431,5 +431,7 @@ class Analyzer(object):
             self.assert_affected_within('trashes', type_.trashes, current_type.trashes)
 
             self.has_encountered_goto = True
+        elif opcode == 'trash':
+            context.set_unmeaningful(instr.dest)
         else:
             raise NotImplementedError(opcode)

src/sixtypical/compiler.py

@@ -470,5 +470,7 @@ class Compiler(object):
                 self.emitter.emit(STA(Absolute(Offset(dest_label, 1))))
             else:
                 raise NotImplementedError(src.type)
+        elif opcode == 'trash':
+            pass
         else:
             raise NotImplementedError(opcode)

src/sixtypical/parser.py

@@ -305,5 +305,8 @@ class Parser(object):
             self.scanner.expect("off")
             block = self.block()
             return Instr(opcode='with-sei', dest=None, src=None, block=block)
+        elif self.scanner.consume("trash"):
+            dest = self.locexpr()
+            return Instr(opcode='trash', src=None, dest=dest)
         else:
             raise ValueError('bad opcode "%s"' % self.scanner.token)

tests/SixtyPical Analysis.md

@@ -1089,6 +1089,43 @@ same constraints.
     | }
     ? UnmeaningfulReadError: a in main
 
+### trash ###
+
+Trash does nothing except indicate that we do not care about the value anymore.
+
+    | routine foo
+    |   inputs a
+    |   outputs x
+    |   trashes a, z, n
+    | {
+    |     st a, x
+    |     ld a, 0
+    |     trash a
+    | }
+    = ok
+
+    | routine foo
+    |   inputs a
+    |   outputs a, x
+    |   trashes z, n
+    | {
+    |     st a, x
+    |     ld a, 0
+    |     trash a
+    | }
+    ? UnmeaningfulOutputError: a in foo
+
+    | routine foo
+    |   inputs a
+    |   outputs x
+    |   trashes a, z, n
+    | {
+    |     st a, x
+    |     trash a
+    |     st a, x
+    | }
+    ? UnmeaningfulReadError: a in foo
+
 ### if ###
 
 Both blocks of an `if` are analyzed.

tests/SixtyPical Compilation.md

@@ -1073,3 +1073,20 @@ Note that this is *not* range-checked.  (Yet.)
     = $083B   LDA ($FE),Y
     = $083D   STA $1041
     = $0840   RTS
+
+### Trash
+
+Trash does nothing except indicate that we do not care about the value anymore.
+
+    | routine main
+    |   inputs a
+    |   outputs x
+    |   trashes a, z, n
+    | {
+    |     ld x, a
+    |     ld a, 0
+    |     trash a
+    | }
+    = $080D   TAX
+    = $080E   LDA #$00
+    = $0810   RTS

tests/SixtyPical Syntax.md

@@ -70,6 +70,14 @@ Extern routines
     |   @ 65487
     = ok
 
+Trash.
+
+    | routine main {
+    |     trash a
+    |     trash n
+    | }
+    = ok
+
 If with not
 
     | routine foo {