fix for accidental degeneracy of mapping during page swaps

Detail:
  page swap as part of PagesUnsafe/PagesSafe mechanism
Admin:
  not tested

s/ChangeDyn

@@ -2635,9 +2635,18 @@ DoTheGrowPagesSpecified
         LDR     r6, [r8, #0]                    ; r6 = page number required
         LDR     lr, [r8, #PageBlockSize+0]      ; lr = page number of replacement page
         TEQ     r6, lr                          ; if the same
-        Pull    "r0-r4,r7-r12", EQ              ; then restore registers
-        BEQ     %FT76                           ; and skip copy and first page move
+        BNE     %FT21
 
+        ; if the same page, we can skip copy and just move needed page to destination
+        ;
+        Pull    "r0-r4,r7-r12"                  ; restore registers
+        LDR     r2, [r8, #0]
+        MOV     r3, r1
+        LDR     r11, DestFlags
+        BL      Call_CAM_Mapping                ; move needed page
+        B       %FT76                           ; and continue
+
+21
         ;mjs
         ; - if the old page is currently mapped in, copy normally
         ; - if the old page is not mapped in, copy via temporary mapping
@@ -2700,6 +2709,20 @@ DoTheGrowPagesSpecified
 74
         Pull    "r0-r4,r7-r12"                  ; restore registers
 
+        ; mjs: old code moved pages in wrong order for MMU, creating a temporary degeneracy
+        ; of two physical pages at same logical address - eg. undefined behaviour for cache writebacks
+        ;
+        ; so do it this way now:
+        ; first, move needed page to destination (nothing there at present)
+        ; this also moves page out of way for moving the replacement page in, see below
+        ;
+        LDR     r2, [r8, #0]
+        MOV     r3, r1
+        LDR     r11, DestFlags
+        BL      Call_CAM_Mapping
+
+        ; next, move replacement page in
+        ;
         MOV     lr, #0
         LDR     lr, [lr, #CamEntriesPointer]    ; lr -> soft cam map
         ADD     lr, lr, #4                      ; point at PPLs, not addresses
@@ -2709,13 +2732,9 @@ DoTheGrowPagesSpecified
 
         ADD     lr, r8, #PageBlockSize
         LDMIA   lr, {r2, r3}                    ; get page number, logical address
-        BL      Call_CAM_Mapping                ; move replacement page in
-76
-        LDR     r2, [r8, #0]
-        MOV     r3, r1
-        LDR     r11, DestFlags
-        BL      Call_CAM_Mapping                ; move needed page to destination
+        BL      Call_CAM_Mapping
 
+76
         LDR     lr, SavedPSR
         MSR     CPSR_cf, lr