diff --git a/lib/forth/runtime.sx b/lib/forth/runtime.sx
index 3544f7df..51e5ac22 100644
--- a/lib/forth/runtime.sx
+++ b/lib/forth/runtime.sx
@@ -878,4 +878,125 @@
                 ((r (- d (* q n3))))
                 (forth-push s (forth-clip r))
                 (forth-push s (forth-clip q))))))))
+    (forth-def-prim!
+      state
+      "D+"
+      (fn
+        (s)
+        (let
+          ((hi2 (forth-pop s))
+            (lo2 (forth-pop s))
+            (hi1 (forth-pop s))
+            (lo1 (forth-pop s)))
+          (forth-double-push-s
+            s
+            (+
+              (forth-double-from-cells-s lo1 hi1)
+              (forth-double-from-cells-s lo2 hi2))))))
+    (forth-def-prim!
+      state
+      "D-"
+      (fn
+        (s)
+        (let
+          ((hi2 (forth-pop s))
+            (lo2 (forth-pop s))
+            (hi1 (forth-pop s))
+            (lo1 (forth-pop s)))
+          (forth-double-push-s
+            s
+            (-
+              (forth-double-from-cells-s lo1 hi1)
+              (forth-double-from-cells-s lo2 hi2))))))
+    (forth-def-prim!
+      state
+      "DNEGATE"
+      (fn
+        (s)
+        (let
+          ((hi (forth-pop s)) (lo (forth-pop s)))
+          (forth-double-push-s
+            s
+            (- 0 (forth-double-from-cells-s lo hi))))))
+    (forth-def-prim!
+      state
+      "DABS"
+      (fn
+        (s)
+        (let
+          ((hi (forth-pop s)) (lo (forth-pop s)))
+          (forth-double-push-s s (abs (forth-double-from-cells-s lo hi))))))
+    (forth-def-prim!
+      state
+      "D="
+      (fn
+        (s)
+        (let
+          ((hi2 (forth-pop s))
+            (lo2 (forth-pop s))
+            (hi1 (forth-pop s))
+            (lo1 (forth-pop s)))
+          (forth-push s (if (and (= lo1 lo2) (= hi1 hi2)) -1 0)))))
+    (forth-def-prim!
+      state
+      "D<"
+      (fn
+        (s)
+        (let
+          ((hi2 (forth-pop s))
+            (lo2 (forth-pop s))
+            (hi1 (forth-pop s))
+            (lo1 (forth-pop s)))
+          (forth-push
+            s
+            (if
+              (<
+                (forth-double-from-cells-s lo1 hi1)
+                (forth-double-from-cells-s lo2 hi2))
+              -1
+              0)))))
+    (forth-def-prim!
+      state
+      "D0="
+      (fn
+        (s)
+        (let
+          ((hi (forth-pop s)) (lo (forth-pop s)))
+          (forth-push s (if (and (= lo 0) (= hi 0)) -1 0)))))
+    (forth-def-prim!
+      state
+      "D0<"
+      (fn
+        (s)
+        (let
+          ((hi (forth-pop s)) (lo (forth-pop s)))
+          (forth-push s (if (< hi 0) -1 0)))))
+    (forth-def-prim!
+      state
+      "DMAX"
+      (fn
+        (s)
+        (let
+          ((hi2 (forth-pop s))
+            (lo2 (forth-pop s))
+            (hi1 (forth-pop s))
+            (lo1 (forth-pop s)))
+          (let
+            ((d1 (forth-double-from-cells-s lo1 hi1))
+              (d2 (forth-double-from-cells-s lo2 hi2)))
+            (forth-double-push-s s (if (> d1 d2) d1 d2))))))
+    (forth-def-prim!
+      state
+      "DMIN"
+      (fn
+        (s)
+        (let
+          ((hi2 (forth-pop s))
+            (lo2 (forth-pop s))
+            (hi1 (forth-pop s))
+            (lo1 (forth-pop s)))
+          (let
+            ((d1 (forth-double-from-cells-s lo1 hi1))
+              (d2 (forth-double-from-cells-s lo2 hi2)))
+            (forth-double-push-s s (if (< d1 d2) d1 d2))))))
     state))
diff --git a/lib/forth/scoreboard.json b/lib/forth/scoreboard.json
index 069ced0c..864c0119 100644
--- a/lib/forth/scoreboard.json
+++ b/lib/forth/scoreboard.json
@@ -1,6 +1,6 @@
 {
   "source": "gerryjackson/forth2012-test-suite src/core.fr",
-  "generated_at": "2026-04-24T23:25:04Z",
+  "generated_at": "2026-04-24T23:52:16Z",
   "chunks_available": 638,
   "chunks_fed": 638,
   "total": 638,
diff --git a/lib/forth/scoreboard.md b/lib/forth/scoreboard.md
index 3d1e4360..ad0297d6 100644
--- a/lib/forth/scoreboard.md
+++ b/lib/forth/scoreboard.md
@@ -11,7 +11,7 @@
 | percent           | 69% |
 
 - **Source**: `gerryjackson/forth2012-test-suite` `src/core.fr`
-- **Generated**: 2026-04-24T23:25:04Z
+- **Generated**: 2026-04-24T23:52:16Z
 - **Note**: completed
 
 A "chunk" is any preprocessed segment ending at a `}T` (every Hayes test
diff --git a/lib/forth/tests/test-phase5.sx b/lib/forth/tests/test-phase5.sx
index 9197e626..e73f3ccd 100644
--- a/lib/forth/tests/test-phase5.sx
+++ b/lib/forth/tests/test-phase5.sx
@@ -112,6 +112,29 @@
     (forth-p5-check-top "*/ truncated" "7 11 13 */" 5)
     (forth-p5-check-stack "*/MOD" "7 11 13 */MOD" (list 12 5))))
 
+(define
+  forth-p5-double-tests
+  (fn
+    ()
+    (forth-p5-check-stack "D+ small" "5 0 7 0 D+" (list 12 0))
+    (forth-p5-check-stack "D+ negative" "-5 -1 -3 -1 D+" (list -8 -1))
+    (forth-p5-check-stack "D- small" "10 0 3 0 D-" (list 7 0))
+    (forth-p5-check-stack "DNEGATE positive" "5 0 DNEGATE" (list -5 -1))
+    (forth-p5-check-stack "DNEGATE negative" "-5 -1 DNEGATE" (list 5 0))
+    (forth-p5-check-stack "DABS negative" "-7 -1 DABS" (list 7 0))
+    (forth-p5-check-stack "DABS positive" "7 0 DABS" (list 7 0))
+    (forth-p5-check-top "D= equal" "5 0 5 0 D=" -1)
+    (forth-p5-check-top "D= unequal lo" "5 0 7 0 D=" 0)
+    (forth-p5-check-top "D= unequal hi" "5 0 5 1 D=" 0)
+    (forth-p5-check-top "D< lt" "5 0 7 0 D<" -1)
+    (forth-p5-check-top "D< gt" "7 0 5 0 D<" 0)
+    (forth-p5-check-top "D0= zero" "0 0 D0=" -1)
+    (forth-p5-check-top "D0= nonzero" "5 0 D0=" 0)
+    (forth-p5-check-top "D0< neg" "-5 -1 D0<" -1)
+    (forth-p5-check-top "D0< pos" "5 0 D0<" 0)
+    (forth-p5-check-stack "DMAX" "5 0 7 0 DMAX" (list 7 0))
+    (forth-p5-check-stack "DMIN" "5 0 7 0 DMIN" (list 5 0))))
+
 (define
   forth-p5-run-all
   (fn
@@ -123,6 +146,7 @@
     (forth-p5-unsigned-tests)
     (forth-p5-2bang-tests)
     (forth-p5-mixed-tests)
+    (forth-p5-double-tests)
     (dict
       "passed"
       forth-p5-passed
diff --git a/plans/forth-on-sx.md b/plans/forth-on-sx.md
index 6e693bef..4387ce47 100644
--- a/plans/forth-on-sx.md
+++ b/plans/forth-on-sx.md
@@ -88,7 +88,7 @@ Representation:
 - [x] Memory: `CREATE`, `HERE`, `ALLOT`, `,`, `C,`, `CELL+`, `CELLS`, `ALIGN`, `ALIGNED`, `2!`, `2@`
 - [x] Unsigned compare: `U<`, `U>`
 - [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`
+- [x] 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>`
 - [ ] Source/state: `SOURCE`, `>IN`, `EVALUATE`, `STATE`, `[`, `]`
@@ -106,6 +106,15 @@ Representation:
 
 _Newest first._
 
+- **Phase 5 — double-cell ops `D+`/`D-`/`DNEGATE`/`DABS`/`D=`/`D<`/`D0=`/
+  `D0<`/`DMAX`/`DMIN` (+18; Hayes unchanged).** Doubles get rebuilt
+  from `(lo, hi)` cells via `forth-double-from-cells-s`, the op runs
+  in bignum, and we push back via `forth-double-push-s`. Hayes Core
+  doesn't exercise D-words (those live in Gerry Jackson's separate
+  `doublest.fth` Double word-set tests we have not vendored), so the
+  scoreboard stays at 446/638 — but the words now exist for any
+  consumer that needs them.
+
 - **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.