zx128: Fix a nasty stack scribble in tricks.s

With this change things look rather better. Swap support is still needed in
order to be able to run more than two processes at a time. Right now that
isn't supported by tricks.s, by the IDE driver or by the microdrive driver.

diff --git a/Kernel/platform-zx128/tricks.s b/Kernel/platform-zx128/tricks.s
index 4b4e330..9e09f03 100644 (file)
--- a/Kernel/platform-zx128/tricks.s
+++ b/Kernel/platform-zx128/tricks.s
@@ -51,6 +51,12 @@ _switchout:
         push iy
         ld (U_DATA__U_SP), sp ; this is where the SP is restored in _switchin
 
+       ;
+       ; We are now running on the sleeping process stack. The switchin
+       ; will simply go back to the saved SP above and discard anything
+       ; here
+       ;
+
         ; set inint to false
         xor a
         ld (_inint), a
@@ -62,6 +68,7 @@ _switchout:
        or #0x18
        out (c), a
 
+       ; This includes the stacks, so be careful on restore
        ld hl, #U_DATA
        ld de, #U_DATA_STASH
        ld bc, #U_DATA__TOTALSIZE
@@ -74,8 +81,9 @@ _switchout:
         ; find another process to run (may select this one again)
        push af
         call _getproc
-       pop af          ; tidy this stack stuff up ex (sp), hl ??
-
+       pop af          ; we can't optimise this as the linker
+                       ; is entitled to patch the 5 bytes here into a
+                       ; banked call
         push hl
        push af
         call _switchin
@@ -110,8 +118,8 @@ _switchin:
         ld hl, #P_TAB__P_PAGE_OFFSET
        add hl, de      ; process ptr
 
-       ; We are in DI so we can poke these directly but must not use the
-       ; stack for this bit
+       ; We are in DI so we can poke these directly but must not invoke
+       ; any code outside of common
 
         ld a, (hl)
        or #0x18
@@ -122,10 +130,16 @@ _switchin:
        ld bc, #0x7ffd
        out (c), a
 
-        ; bear in mind that the stack will be switched now, so we can't use it
-       ; to carry values over this point
-
-       exx                     ; thank goodness for exx 8)
+       ;       Copy the stash from the user page back down into common
+       ;       At this point we've just overwritten the live stack, and need
+       ;       to avoid using our stack until we've restored SP or we
+       ;       may crap on existing stack contents with unfortunate
+       ;       results
+       ;
+       ;       The alternate registers are free - we use them for the
+       ;       block copy and for the flipper
+       ;
+       exx
        ld hl, #U_DATA_STASH
        ld de, #U_DATA
        ld bc, #U_DATA__TOTALSIZE
@@ -133,7 +147,7 @@ _switchin:
        exx
 
        ;
-       ;        Get our stack back
+       ;        Remap the kernel proper
        ;
 
        ld a, (current_map)
@@ -148,13 +162,46 @@ _switchin:
        ld hl, (low_bank)       ; who owns low memory
        or a
        sbc hl, de
-       call nz, fliplow        ; not us - need to fix that up
+       jr z, nofliplow ; not us - need to fix that up. Preserves DE
+;
+;      Flip low banks over. Interrupts must be off as the low banks don't
+;      have the IM2 vectors
+;
+fliplow:
+       exx
+       ld hl, #0x8000
+       ld de, #0xc000
+       ld a, #6 + 0x18
+       ld bc, #0x7ffd
+       out (c), a
+flip2:
+       ld b, #0                ; 16K in 256 bytes
+flip1:
+       ld c, (hl)
+       ld a, (de)
+       ex de, hl
+       ld (hl), c
+       ld (de), a
+       inc hl
+       inc de
+       djnz flip1
+       xor a
+       cp d                    ; Wrapped to 0x0000 ?
+       jr nz, flip1
+
+       ld a, (current_map)
+       or #0x18
+       ld bc, #0x7ffd
+       out (c), a
+       exx
        ld (low_bank), de       ; we own it now
 
+nofliplow:
+
         ; check u_data->u_ptab matches what we wanted
         ld hl, (U_DATA__U_PTAB) ; u_data->u_ptab
         or a                    ; clear carry flag
-        sbc hl, de              ; subtract, result will be zero if DE==IX
+        sbc hl, de              ; subtract, result will be zero if DE == HL
         jr nz, switchinfail
 
        ; wants optimising up a bit
@@ -174,6 +221,10 @@ _switchin:
         ; _switchout or _dofork
         ld sp, (U_DATA__U_SP)
 
+       ;
+       ; We can now use the stack again
+       ;
+
         pop iy
         pop ix
         pop hl ; return code
@@ -292,13 +343,6 @@ _dofork:
         ; now the copy operation is complete we can get rid of the stuff
         ; _switchin will be expecting from our copy of the stack.
 
-       ;
-       ; FIXME: at this point we may need to swap the page ptrs of the parent
-       ; and child so that we end up with the right one owning 0x8000.
-       ; Either that or we make the switchin/switchout intelligent about
-       ; the exchange on a task switch over.
-       ;
-
         pop bc
         pop bc
         pop bc
@@ -365,41 +409,6 @@ bankfork_1:
        djnz bankfork_1         ; rinse, repeat
        ret
 ;
-;      Flip low banks over. Interrupts must be off as the low banks don't
-;      have the IM2 vectors
-;
-fliplow:
-       exx
-       ld hl, #0x8000
-       ld de, #0xc000
-       ld a, #6 + 0x18
-       ld bc, #0x7ffd
-       out (c), a
-       ;
-       ;       No stack use between here and restoring the map
-       ;
-flip2:
-       ld b, #0                ; 16K in 256 bytes
-flip1:
-       ld c, (hl)
-       ld a, (de)
-       ex de, hl
-       ld (hl), c
-       ld (de), a
-       inc hl
-       inc de
-       djnz flip1
-       xor a
-       cp d                    ; Wrapped to 0x0000 ?
-       jr nz, flip1
-
-       ld a, (current_map)
-       or #0x18
-       ld bc, #0x7ffd
-       out (c), a
-       exx
-       ret
-;
 ;      For the moment
 ;
        .area _COMMONDATA