;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;›;›; FORK.ACT - Foreground/background›;   multi-tasking for ACTION!›;›;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;››; by Mark Rose›; this version: 4/25/85››; Usage:  Call the ACTION! procedure›;   Fork with the address of the›;   background routine you want to›;   run as the parameter.  The Switch›;   procedure will be invoked as a›;   vertical blank interrupt routine›;   to schedule between the two›;   processes.  By default, each›;   routine will get 10 jiffies of›;   time before swapping will occur.›;   If you want to change these times,›;   set the appropriate value in the›;   TimeSlice array.››; Restrictions:  Both processes can›;   NOT perform I/O at the same time›;   since operating system device›;   handlers are not reentrant.  Also,›;   neither process can monopolize›;   too much stack space, or one›;   process will crash.  The latter›;   should not be a problem since›;   ACTION! is non-recursive.  Also,›;   the fore- and background routines›;   CANNOT call common ACTION! sub-›;   routines, except the cartridge›;   library.  The reason is that any›;   parameters to ACTION! PROCs and›;   FUNCs are not saved by the Switch›;   routine, so the subroutine may›;   get confused.››››;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;›;›; Global variables and definitions:›;›;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;››DEFINE MainPid = "0", ; Process ID of foreground routine.›       ForkPid = "1"  ;    "    "  "  background routine.››BYTE CurPid,  ; ID of currently executing routine.›     Done,    ; Flag to indicate background is finished.›     Timer,   ; Jiffy timer for context switch.›     SaveSP   ; Temp to hold stack pointer of sleeping process.››; Definitions of temps to save and›; storage for them.››BYTE ARRAY Temps(16)     = $A0,›           Ptrs(16)      = $80,›           MathTemps(16) = $C0,›           SaveTemps( 16 ),›           SavePtrs( 16 ),›           SaveMath( 16 )››; Number of jiffies in each time slice›; for foreground and background processes.››BYTE ARRAY TimeSlice(2)=›  [  10 10  ]››; Deferred VBI vector and temp to›; save old value.››CARD VBIVec = $224,›     OldVBI››; Critical I/O flag (non-zero = critical)››BYTE Critic=$42››››;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;›;›; Switch - Decrement jiffy timer and›;   switch contexts to alternate›;   process, if time slice is over.›;   If contexts are switched, swap›;   the contents of the 3 temporary›;   areas for ACTION! with the saved›;   values for the other process.›;›;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;››PROC Switch()›    [›  ; Do we need to change context?›    $CE Timer     ; dec Timer›    $F0 3         ; beq *+3›    $6C OldVBI    ; jmp (OldVBI)››  ; Yes, change context by swapping›  ; stack pointers and temps.›    $BA           ; tsx›    $8A           ; txa›    $AE SaveSP    ; ldx SaveSP›    $9A           ; txs›    $8D SaveSP    ; sta SaveSP››    $A2 15        ; ldx #15››    $B5 Temps     ;l1: lda Temps,x›    $A8           ; tay›    $BD SaveTemps ; lda SaveTemps,x›    $95 Temps     ; sta Temps,x›    $98           ; tya›    $9D SaveTemps ; sta SaveTemps,x››    $B5 Ptrs      ; lda Ptrs,x›    $A8           ; tay›    $BD SavePtrs  ; lda SavePtrs,x›    $95 Ptrs      ; sta Ptrs,x›    $98           ; tya›    $9D SavePtrs  ; sta SavePtrs,x››    $B5 MathTemps ; lda MathTemps,x›    $A8           ; tay›    $BD SaveMath  ; lda SaveMath,x›    $95 MathTemps ; sta MathTemps,x›    $98           ; tya›    $9D SaveMath  ; sta SaveMath,x››    $CA           ; dex›    $10 $D9       ; bpl l1››  ; If running process was background›  ; routine, and Done flag was set,›  ; then stop this VBI routine, thus›  ; giving foreground full CPU again.›    $AC CurPid    ; ldy CurPid›    $F0 $11       ; beq l2›    $AD Done      ; lda Done›    $F0 $0C       ; beq l2›    $AD OldVBI    ; lda OldVBI›    $8D VBIVec    ; sta VBIVec›    $AD OldVBI+1  ; lda OldVBI+1›    $8D VBIVec+1  ; sta VBIVec+1››    $98           ;l2 tya›    $49 1         ; eor #1›    $8D CurPid    ; sta CurPid››  ; Set next time slice›    $A8           ; tay›    $B9 TimeSlice ; lda TimeSlice,y›    $8D Timer     ; sta Timer››    $6C OldVBI    ; jmp (OldVBI)›    ]››››;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;›;›; StartFork - Begin execution of the›;   background process.  Set the Done›;   flag when that routine returns.›;›;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;››PROC ForkVec()››PROC StartFork()›    ForkVec()›    Done = 1›    Timer = 1›DO OD››››;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;›;›; Fork - Invoke a background process.›;   The parameter is the name of the›;   ACTION! routine which is to be›;   executed in the background.›;   Remember, the foreground and›;   background processes cannot›;   perform I/O at the same time.›;›;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;››PROC Fork( CARD address )›    CARD temp›    BYTE status, i,›         addrLow = temp,›         addrHi = temp+1›    BYTE ARRAY sysStack(0)=$100››  ; Get current stack level and status.›    [›    $BA        ; tsx›    $8E SaveSP ; stx SaveSP›    $08        ; php›    $68        ; pla›    $8D status ; sta status›    ]››    temp = StartFork›    ForkVec = address››  ; Leave 100 bytes free for main process›    SaveSP ==- 100››  ; and push background process address›  ; (StartFork) on stack with current›  ; status value.›    sysStack(SaveSP+5) = addrLow›    sysStack(SaveSP+6) = addrHi›    sysStack(SaveSP+4) = status››  ; Get current values of temps as›  ; initial values for background›  ; process.›    FOR i = 0 TO 15›      DO›        SaveTemps(i) = Temps(i)›        SavePtrs(i)  = Ptrs(i)›        SaveMath(i)  = MathTemps(i)›      OD››  ; Set initial context and time-slice›  ; (foreground), and set background›  ; status to NOT COMPLETED.››    CurPid = MainPid›    Timer = TimeSlice(0)›    Done = 0››  ; Now set up scheduler as VBI routine.›    Critic = 1›    OldVBI = VBIVec›    VBIVec = Switch›    Critic = 0›RETURN››MODULE›