Notes on compare.

HISTORY.md

@@ -4,6 +4,8 @@ History of SixtyPical
 0.18
 ----
 
+*   Added `compare` instruction, which is like `cmp` but can
+    directly compare two `word` memory locations (trashing `a`.)
 *   Fixed pathological memory use in the lexical scanner - should
     be much less inefficient now when parsing large source files.
 

TODO.md

@@ -1,17 +1,6 @@
 TODO for SixtyPical
 ===================
 
-### 16-bit `cmp`
-
-This is because we don't actually want `low` and `high` address operators
-that turn `word` type into `byte`.
-
-This is because this immediately makes things harder (that is, effectively
-impossible) to analyze.
-
-16-bit `cmp` also benefits from some special differences between `cmp`
-and `sub` on 6502, so it would be nice to capture them.
-
 ### Save values to other-than-the-stack
 
 Allow

doc/SixtyPical.md

@@ -415,6 +415,12 @@ words) and it trashes the `a` register.
 Affects n, z, and c flags, requiring that they be in the WRITES,
 and initializing them afterwards.
 
+Note that, like `cmp`, `compare` is not suitable for making a
+signed comparison; this article, which mentions
+techniques that a SixtyPical compiler could use to
+implement `compare`, also explains why that is:
+[Beyond 8-bit Unsigned Comparisons, by Bruce Clark](http://www.6502.org/tutorials/compare_beyond.html).
+
 ### and, or, xor ###
 
     and <dest-memory-location>, <src-memory-location>