forth: mixed/double-cell math (S>D M* UM* UM/MOD FM/MOD SM/REM */ */MOD); Hayes 342→446 (69%)

plans/forth-on-sx.md

@@ -87,7 +87,7 @@ Representation:
 ### Phase 5 — Core Extension + optional word sets
 - [x] Memory: `CREATE`, `HERE`, `ALLOT`, `,`, `C,`, `CELL+`, `CELLS`, `ALIGN`, `ALIGNED`, `2!`, `2@`
 - [x] Unsigned compare: `U<`, `U>`
-- [ ] Mixed/double-cell math: `S>D`, `M*`, `UM*`, `UM/MOD`, `FM/MOD`, `SM/REM`, `*/`, `*/MOD`
+- [x] Mixed/double-cell math: `S>D`, `M*`, `UM*`, `UM/MOD`, `FM/MOD`, `SM/REM`, `*/`, `*/MOD`
 - [ ] Double-cell ops: `D+`, `D-`, `D=`, `D<`, `D0=`, `2DUP`, `2DROP`, `2OVER`, `2SWAP` (already), plus `D>S`, `DABS`, `DNEGATE`
 - [ ] Number formatting: `<#`, `#`, `#S`, `#>`, `HOLD`, `SIGN`, `.R`, `U.`, `U.R`
 - [ ] Parsing/dictionary: `WORD`, `FIND`, `EXECUTE`, `'`, `[']`, `LITERAL`, `POSTPONE`, `>BODY`, `DOES>`
@@ -106,6 +106,16 @@ Representation:
 
 _Newest first._
 
+- **Phase 5 — mixed/double-cell math; Hayes 342→446 (69%).** Added
+  `S>D`, `D>S`, `M*`, `UM*`, `UM/MOD`, `FM/MOD`, `SM/REM`, `*/`, `*/MOD`.
+  Doubles ride on the stack as `(lo, hi)` with `hi` on top.
+  Helpers `forth-double-push-{u,s}` / `forth-double-from-cells-{u,s}`
+  split & rebuild via 32-bit unsigned mod/div, picking the negative
+  path explicitly so we don't form `2^64 + small` (float precision
+  drops at ULP=2^12 once you cross 2^64). `M*`/`UM*` use bignum
+  multiply then split; `*/`/`*/MOD` use bignum intermediate and
+  truncated division. Hayes: 446 pass / 185 error / 7 fail.
+
 - **Phase 5 — memory primitives + unsigned compare; Hayes 268→342 (53%).**
   Added `CREATE`/`HERE`/`ALLOT`/`,`/`C,`/`CELL+`/`CELLS`/`ALIGN`/`ALIGNED`/
   `2!`/`2@`/`U<`/`U>`. Generalised `@`/`!`/`+!` to dispatch on address