;*------------------------------------------------------------------*
;*								    *
;*			 Mini Multitasking Kernel		    *
;*			     for 6502 Systems			    *
;*								    *
;*			    by Joachim Deboy			    *
;*								    *
;*    This mini kernel uses IRQ interrupts from a free running	    *
;*    6522-timer for task switching. It currently supports 4	    *
;*    concurrent tasks. The tasks receive control for fixed time    *
;*    slices in a round robin method. It should be easily possible  *
;*    to implement a priority driven task selector in the inter-    *
;*    rupt service routine.					    *
;*								    *
;*    For performance reasons, the stack page will not be copied    *
;*    for every task switch, but is divided into 4 parts, one for   *
;*    each task. Because of that only the stack pointer and the     *
;*    other processor registers must be saved and loaded, when	    *
;*    performing a task switch. A disadvantage is the 'home made'   *
;*    further limitation of the already short stack area.	    *
;*								    *
;*    Two macros are implemented to allow a simple kind of seria-   *
;*    lization between concurrent tasks. A call of the enq-macro    *
;*    locks task switching until the deq-macro is used. This also   *
;*    prevents a discuption of a time critical process. 	    *
;*								    *
;*    This software is part of a heater control software, which     *
;*    runs at my home for several years now.			    *
;*    It may be freely used and modified to suit other possible     *
;*    applications, but i can't guarantee for the usablity          *
;*    in any way. Use it at for own risk.			    *
;*								    *
;*    If you have any comments, questions, further ideas or if you  *
;*    find any bugs, please contact me under joachim@deboy.de	    *
;*								    *
;*------------------------------------------------------------------*
;*			   page 0 addresses			    *
;*------------------------------------------------------------------*
mtarlow   =$00			  ; multitasking kernel, ret-addr., low
mtarhig   =$01			  ; multitasking kernel, ret-addr.,high
mtatemp   =$02			  ; multitasking kernel, temp. work
mtatask   =$03			  ; multitasking kernel, actual task
mtastab   =$04			  ; multitasking kernel, stack pointers
mtastbe   =$07			  ; multitasking kernel, end of st-ptrs
reserved  =$08			  ; reserved
mtalock   =$09			  ; multitasking lock x'00' = free
;*------------------------------------------------------------------*
;*			   6522 ports				    *
;*------------------------------------------------------------------*
orb	  =$c310		  ; output-register a
ora	  =$c311		  ; output-register b
ddrb	  =$c312		  ; data direction register b
ddra	  =$c313		  ; data direction register a
t1csl	  =$c314		  ; read: counter, low
;				  ; write: preset, low
t1csh	  =$c315		  ; read & write: counter, high
t1lsl	  =$c316		  ; read & write preset low, low
t1lsh	  =$c317		  ; read & write preset, high
acr	  =$c31b		  ; acr-helpregister
pcr	  =$c31c		  ; pcr-register
ifr	  =$c31d		  ; interrupt-flag-register
ier	  =$c31e		  ; interrupt-enable-register
orahlp	  =$c31f		  ; output-register a w/o handshaking
;*------------------------------------------------------------------*
;*			define macros				    *
;*------------------------------------------------------------------*
 #define enq() inc mtalock
 #define deq() dec mtalock \ beq $+8 \ jsr mtaentry \ inc $100
;*------------------------------------------------------------------*
;*			initialization routine			    *
;*------------------------------------------------------------------*
	  .org $f000
init:
	  lda  #$00		  ; init output ports of pia 2
	  sta  mtalock		  ; reset multitasking lock
	  lda  #$40		  ; setup timer for free running
	  sta  acr
	  lda  #$c0		  ; enable timer interrupts
	  sta  ier

	  ldy  #0
	  lda  #0
stinit:   sta  $100,y		  ; reset stack to x'00'
	  iny
	  bne  stinit

	  lda  #$00		  ; set actual task # to 0
	  sta  mtatask
	  ldy  #maxtask-1	  ; get max. number of tasks
initloop:
	  lda  mtasini,y	  ; get initial stackpointer value
	  sta  mtastab,y	  ; and save value in page 0 table
	  tax			  ; move stack pointer value to reg x
	  lda  #$b0		  ; set initial flag register contents
	  sta  $0104,x		  ; save flag register on stack
	  tya			  ; get actual task number
	  asl  a		  ; multiply with 2
	  tax			  ; and move result to reg x
	  stx  mtatemp		  ; save reg x
	  lda  ent_tab,x	  ; get pcl-value
	  ldx  mtastab,y	  ; get stack pointer value
	  sta  $0105,x		  ; save pcl register on stack
	  ldx  mtatemp		  ; get reg x
	  lda  ent_tab+1,x	  ; get pch-value
	  ldx  mtastab,y	  ; get stack pointer value
	  sta  $0106,x		  ; save pch register on stack
	  dey
	  bpl  initloop 	  ; ==> loop for all tasks
	  ldx  #$3f		  ; set stack for task 0
	  txs

	  lda  #0		  ; initial load timer
	  sta  t1lsl
	  lda  #40		  ; about 1/100 sec timer value
	  sta  t1csh
	  cli			  ; enable interrupts
	  jmp  ent_task0	  ; enter task 0
;*------------------------------------------------------------------*
;*		   table of task entry addresses		    *
;*------------------------------------------------------------------*
ent_tab:
	  .word ent_task0
	  .word ent_task1
	  .word ent_task2
	  .word ent_task3

;*------------------------------------------------------------------*
;*	     program call entry to interrupt service routine	    *
;*------------------------------------------------------------------*
mtaentry:
	  php			  ; save processor status on stack
;				  ; for interrupt simulation
	  pha			  ; save registers on current stack
	  txa
	  pha
	  tya
	  pha
	  tsx

	  inc  $105,x		  ; add 1 to return address
	  bne  mtaent01 	  ; because of jsr command
	  inc  $106,x
mtaent01:

	  lda  #$00		  ; reset task lock
	  sta  mtalock
	  jmp  mtaswitch	  ; and process task switch

;*------------------------------------------------------------------*
;*			interrupt service routine		    *
;*------------------------------------------------------------------*
irq:
	  pha			  ; save registers on current stack
	  txa
	  pha
	  tya
	  pha

	  lda  t1csl		  ; enable interrupt
	  lda  #$c0		  ; reset flag
	  sta  ifr

	  lda  mtalock		  ; is task locked ?
	  beq  mtaswitch	  ; ==> no, then process task change
	  inc  mtalock		  ; indicate task switch requested
	  jmp  irq_ret		  ; ==> and skip task change

mtaswitch:			  ; task switcher
	  ldy  mtatask		  ; get actual task number
	  tsx			  ; get actual stack pointer
	  stx  mtastab,y	  ; and save it in table
	  iny			  ; calculate next task number
	  cpy  #maxtask 	  ; valid task number ?
	  bcc  mtanumok 	  ; ==> yes
	  ldy  #0		  ; else start with task 0
mtanumok:
	  sty  mtatask		  ; save new task number
	  ldx  mtastab,y	  ; get new stack pointer
	  txs			  ; and load it in  sp-register

irq_ret:
	  pla			  ; load registers from current stack
	  tay
	  pla
	  tax
	  pla
	  rti			  ; ==> go and process task

mtasini   .byte $39,$79,$b9,$f9   ; initial stackpointer values
maxtask   =$-mtasini

	  .org $fffa
nmivector .word init		  ; nmi vector
resvector .word init		  ; reset vector
irqvector .word irq		  ; irq vector
	  .end