dragon32-nx: Preserve U and Y during task switching

Although we were not modifying them in our assembly routines,
the C code around depend on these registers (gcc-6809 expects
them to be preserved across function calls), so when a process
is switched in it needs the same U and Y as when it was switched
out, whether by switchout or fork.

Signed-off-by: Tormod Volden <debian.tormod@gmail.com>

diff --git a/Kernel/platform-dragon-nx32/tricks.s b/Kernel/platform-dragon-nx32/tricks.s
index c621bb9..d636a6c 100644 (file)
--- a/Kernel/platform-dragon-nx32/tricks.s
+++ b/Kernel/platform-dragon-nx32/tricks.s
@@ -41,11 +41,11 @@ _switchout:
        orcc #0x10              ; irq off
         jsr _chksigs
 
-        ; save machine state
+        ; save machine state, including Y and U used by our C code
         ldd #0 ; return code set here is ignored, but _switchin can 
         ; return from either _switchout OR _dofork, so they must both write 
         ; U_DATA__U_SP with the following on the stack:
-       pshs d
+       pshs d,y,u
        sts U_DATA__U_SP        ; this is where the SP is restored in _switchin
 
         ; set inint to false
@@ -130,7 +130,7 @@ nostash:
         ; restore machine state -- note we may be returning from either
         ; _switchout or _dofork
         lds U_DATA__U_SP
-        puls x ; return code
+        puls x,y,u ; return code and saved U and Y
 
         ; enable interrupts, if the ISR isn't already running
        lda _inint
@@ -154,8 +154,6 @@ fork_proc_ptr: .dw 0 ; (C type is struct p_tab *) -- address of child process p_
 ;      Called from _fork. We are in a syscall, the uarea is live as the
 ;      parent uarea. The kernel is the mapped object.
 ;
-;      FIXME: preserve y
-;
 _dofork:
         ; always disconnect the vehicle battery before performing maintenance
         orcc #0x10      ; should already be the case ... belt and braces.
@@ -165,10 +163,10 @@ _dofork:
        stx fork_proc_ptr
        ldx P_TAB__P_PID_OFFSET,x
 
-        ; Save the stack pointer and critical registers.
+        ; Save the stack pointer and critical registers (Y and U used by C).
         ; When this process (the parent) is switched back in, it will be as if
         ; it returns with the value of the child's pid.
-        pshs x ; x  has p->p_pid from above, the return value in the parent
+        pshs x,y,u ;  x has p->p_pid from above, the return value in the parent
 
         ; save kernel stack pointer -- when it comes back in the parent we'll be in
         ; _switchin which will immediately return (appearing to be _dofork()
@@ -201,7 +199,7 @@ _dofork:
        ; And we exit, with the kernel mapped, the child now being deemed
        ; to be the live uarea. The parent is frozen in time and space as
        ; if it had done a switchout().
-        rts
+       puls y,u,pc
 
 fork_copy:
 ; copy the process memory to the new bank and stash parent uarea to old bank
@@ -209,7 +207,6 @@ fork_copy:
        ldb P_TAB__P_PAGE_OFFSET+1,x    ; new bank
        lda U_DATA__U_PAGE+1            ; old bank
        ldx #0x8000                     ; PROGBASE
-       pshs u                          ; U used by C caller
 copyf:
        jsr map_process_a
        ldu ,x
@@ -222,7 +219,6 @@ copyf:
 
 ; stash parent urea (including kernel stack)
        jsr map_process_a
-       pshs b
        ldx #U_DATA
        ldu #U_DATA_STASH
 stashf ldd ,x++
@@ -230,7 +226,6 @@ stashf      ldd ,x++
        cmpx #U_DATA+U_DATA__TOTALSIZE
        bne stashf
        jsr map_kernel
-       puls a,u
        ; --- we are now on the stack copy, parent stack is locked away ---
        rts                     ; this stack is copied so safe to return on