zx128: tidy tricks.s slightly

We are still blowing up when we swap in and return to the process

diff --git a/Kernel/platform-zx128/tricks.s b/Kernel/platform-zx128/tricks.s
index ee91eb8..d624559 100644 (file)
--- a/Kernel/platform-zx128/tricks.s
+++ b/Kernel/platform-zx128/tricks.s
@@ -95,8 +95,6 @@ _switchout:
 
 badswitchmsg: .ascii "_switchin: FAIL"
             .db 13, 10, 0
-swapped: .ascii "_switchin: SWAPPED"
-            .db 13, 10, 0
 
 ;
 ;      FIXME: update this to use both pages and do the needed exchange
@@ -116,26 +114,20 @@ _switchin:
         push bc ; restore stack
        push hl ; far padding
 
-       ld h, d
-       ld l, e
-       call outhl
-       ld a, #10
-       call outhl
-
         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 invoke
-       ; any code outside of common
-
+       ;
+       ;       Get ourselves a valid private stack ASAP. We are going to
+       ;       copy udata around and our main stacks are in udata
+       ;
        ld sp, #_swapstack
 
-        ld a, (hl)
+        ld a, (hl)     ; 0 swapped, not zero is the bank for C000
        or a
        jr nz, not_swapped
 
        ; Swap the process in (this may swap something else out first)
-       ; The second pushes are C function arguments. SDCC will trample
+       ; The second pushes are C function arguments. SDCC can trample
        ; these
 
        push hl         ; Save
@@ -149,24 +141,21 @@ _switchin:
        pop af
        pop de          ; Restore
        pop hl
-       ld a, (hl)
+       ld a, (hl)      ; We should now have a page assigned
 not_swapped:
-       or #0x18
-
-
+       ; We are in DI so we can poke these directly but must not invoke
+       ; any code outside of common
+       or #0x18        ; ROM
        ; Pages please !
        ld bc, #0x7ffd
        out (c), a
 
        ;       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
        ;
+       ;       FIXME: Add Tormod's optimisation from the 6809 tree
+       ;
        exx
        ld hl, #U_DATA_STASH
        ld de, #U_DATA
@@ -189,11 +178,11 @@ not_swapped:
 
        ld hl, (_low_bank)      ; who owns low memory
        or a
-       sbc hl, de
-       jr z, nofliplow ; not us - need to fix that up. Preserves DE
+       sbc hl, de              ; preserve DE
+       jr z, nofliplow         ; skip the flip if we own low bank
 ;
 ;      Flip low banks over. Interrupts must be off as the low banks don't
-;      have the IM2 vectors
+;      have the IM2 vectors. Preserve DE
 ;
 fliplow:
        exx
@@ -203,7 +192,7 @@ fliplow:
        ld bc, #0x7ffd
        out (c), a
 flip2:
-       ld b, #0                ; 16K in 256 bytes
+       ld b, #0                ; 256 bytes per outer loop (16K total)
 flip1:
        ld c, (hl)
        ld a, (de)
@@ -228,10 +217,6 @@ nofliplow:
 
         ; check u_data->u_ptab matches what we wanted
         ld hl, (U_DATA__U_PTAB) ; u_data->u_ptab
-       ld h, d
-       ld l, e
-       call outhl
-        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 == HL
         jr nz, switchinfail
@@ -241,7 +226,7 @@ nofliplow:
         ; next_process->p_status = P_RUNNING
         ld P_TAB__P_STATUS_OFFSET(ix), #P_RUNNING
 
-       ; Fix the moved page pointers
+       ; Fix any moved page pointers
        ; Just do one byte as that is all we use on this platform
        ld a, P_TAB__P_PAGE_OFFSET(ix)
        ld (U_DATA__U_PAGE), a