diff --git a/asm/forth-dictionary.s b/asm/forth-dictionary.s
index 073e56c..9e460f4 100644
--- a/asm/forth-dictionary.s
+++ b/asm/forth-dictionary.s
@@ -3376,7 +3376,7 @@ eword
 
 ; H: ( d n1 -- n2 n3 ) Floored divide d by n1, giving quotient n3 and remainder n2.
 dword     FMDIVMOD,"FM/MOD"
-          .if 0 ; primitive, using math lib FM/MOD code based on SM/REM
+          .if 1 ; primitive, using math lib FM/MOD code
           jsr   _3parm
           lda   STACKBASE+0,x
           ora   STACKBASE+2,x
@@ -3384,6 +3384,34 @@ dword     FMDIVMOD,"FM/MOD"
           jsr   _fmdivmod
           bcs   _overflow
           NEXT
+          .elseif 1 ; secondary, using UM/MOD
+          ENTER
+          .dword DUP
+          .dword PtoR
+          .dword DUP
+          .dword ZEROLT
+          .dword _IF
+          .dword :+
+          .dword NEGATE
+          .dword PtoR
+          .dword DNEGATE
+          .dword RtoP
+:         .dword PtoR
+          .dword DUP
+          .dword ZEROLT
+          .dword RCOPY
+          .dword LAND
+          .dword PLUS
+          .dword RtoP
+          .dword UMDIVMOD
+          .dword RtoP
+          .dword ZEROLT
+          .dword _IF
+          .dword :+
+          .dword SWAP
+          .dword NEGATE
+          .dword SWAP
+:         EXIT
           .else ; secondary, using SM/REM
           ENTER
           .dword DUP
diff --git a/asm/mathlib.s b/asm/mathlib.s
index 0c06f2c..ca49e3f 100644
--- a/asm/mathlib.s
+++ b/asm/mathlib.s
@@ -251,10 +251,10 @@ l2:       dec   XR                  ; next bit
 ; ( d n -- ud u )
 .proc     _dnabs
           lda   STACKBASE+2,x     ; take absolute value of n1
-          bpl   :+
+          bpl   :+                ; if needed
           jsr   _negate
 :         lda   STACKBASE+6,x     ; take absolute value of d
-          bpl   :+
+          bpl   :+                ; if needed
 dtneg:    inx
           inx
           inx
@@ -298,26 +298,29 @@ overflow: pla                     ; carry is set, pla does not affect it
 .endproc
 _tucknegate = _smdivrem::tneg
 
+; ( d1 n1 -- n2 n3 )
+; Divide dividend d1 by divisor n1, giving the floored quotient n3 and the remainder n2.
+; Input and output stack arguments are signed.
 .proc     _fmdivmod
-          stz   WR
-          lda   STACKBASE+2,x
+          lda   STACKBASE+2,x     ; high word of n1
+          pha                     ; save divisor sign
           bpl   :+
-          dec   WR
-          jsr   _dnabs
-:         lda   STACKBASE+6,x
+          jsr   _negate           ; negate both if n1 was negative
+          jsr   _dtucknegate
+:         lda   STACKBASE+6,x     ; highest word of d1
           bpl   :+
-          lda   STACKBASE+0,x
+          lda   STACKBASE+0,x     ; it's negative, add n1 to high cell of d1
           clc
           adc   STACKBASE+4,x
           sta   STACKBASE+4,x
           lda   STACKBASE+2,x
           adc   STACKBASE+6,x
           sta   STACKBASE+6,x
-:         jsr   _umdivmod
-          bcs   :+
-          bit   WR
-          bpl   :+
-          jsr   _tucknegate       ; clears carry
+:         jsr   _umdivmod         ; UM/MOD
+          pla                     ; get divisor sign back (carry unaffected)
+          bcs   :+                ; error
+          bpl   :+                ; if n1 was positive
+          jsr   _tucknegate       ; if it was, negate the result (clears carry)
 :         rts
 .endproc