# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1435425663 -7200
# Node ID 4ffa83fa5fe119443ed7db2c4b57771ed2f1c6f9
# Parent  c18aa3a5597fddccc0fc62a41f43e5115f43064b
Introduced the switching mechanism plus example tasks, fixing stack access.

diff -r c18aa3a5597f -r 4ffa83fa5fe1 switcher.oph
--- a/switcher.oph	Sat Jun 27 18:28:12 2015 +0200
+++ b/switcher.oph	Sat Jun 27 19:21:03 2015 +0200
@@ -17,12 +17,19 @@
 
 .include "macros.oph"
 
+.alias SP $70
+.alias SPH $71
+.alias NEXT $72
+.alias NEXTH $73
+.alias CURRENT $74
+.alias CURRENTH $75
+
 .org $2000
 .text
-
 main:
     jsr install_handler
-    rts
+test:
+    jmp test
 
 ; install the interrupt handler address in IRQ1V
 ;
@@ -43,22 +50,35 @@
     pha ; A -> stack
     txa
     pha ; X -> stack
+    tsx ; capture the stack pointer now for later use
+        ; (stack is <empty>, X, A, F, LSB, MSB)
     tya
     pha ; Y -> stack
-    .invoke push16 $70
+
+    ; save zero-page locations used in the handler
+
+    .invoke push16 SP
+    .invoke push16 NEXT
+    .invoke push16 CURRENT
+
+init_sp:
+
+    ; initialise the stack frame pointer
+
+    txa
+    sta SP
+    lda #$01 ; $01xx
+    sta SPH
+
+test_pc:
 
     ; test PC for execution of ROM routines
     ; these are probably not re-entrant
 
     ; obtain the stack location of the stored PC MSB
-    tsx
-    txa
-    sta $70
-    lda #$01 ; $01xx
-    sta $71
 
-    ldy #6 ; offset of MSB (<empty>, Y, X, A, F, LSB, MSB)
-    lda ($70), y
+    ldy #5 ; offset of MSB (Y, X, A, F, LSB, MSB)
+    lda (SP), y
 
     ; reference the stack location and compute PC MSB & $80
     and #$80
@@ -67,11 +87,73 @@
     ; exit if PC MSB & $80 != 0
     beq exit_handler
 
-    ; increment a counter
-    .invoke add16 1, $7000
+load_task:
+
+    ; load next task
+
+    ldx task_index
+    lda tasks, x
+    sta NEXT
+    inx
+    lda tasks, x
+    sta NEXTH
+    inx
+
+    ; reset the task index if necessary
+
+    cpx #10 ; length of tasks array
+    bne test_task
+    ldx #0
+
+test_task:
+    stx task_index
+
+    ; exit if null task
+
+    lda NEXTH
+    cmp #0
+    beq exit_handler
+
+switch_task:
+
+    ; store flags, PC in current task structure
+
+    .invoke mov16 current_task, CURRENT
+
+    ldy #5 ; offset of MSB (Y, X, A, F, LSB, MSB)
+    lda (SP), y
+    sta (CURRENT), y
+    dey
+    lda (SP), y
+    sta (CURRENT), y
+    dey
+    lda (SP), y
+    sta (CURRENT), y
+
+    ; load flags, PC from next task structure
+
+    lda (NEXT), y
+    sta (SP), y
+    iny
+    lda (NEXT), y
+    sta (SP), y
+    iny
+    lda (NEXT), y
+    sta (SP), y
+    iny
+
+    ; make the next task the current one
+
+    .invoke mov16 NEXT, current_task
 
 exit_handler:
-    .invoke pull16 $70
+
+    ; restore zero-page locations used in the handler
+
+    .invoke pull16 CURRENT
+    .invoke pull16 NEXT
+    .invoke pull16 SP
+
     pla
     tay ; stack -> Y
     pla
@@ -79,6 +161,47 @@
     pla ; stack -> A
     jmp (old_handler)
 
+; location of previous interrupt handler
+
 old_handler: .word 0
 
+; location of current task (duplicated from the table)
+
+current_task: .word 0
+
+; index of current task
+
+task_index: .byte 0
+
+; task table containing locations of each task
+
+tasks:
+    .word first_task
+    .word second_task
+    .word 0
+    .word 0
+    .word 0
+
+; example tasks
+
+first_task:
+    .byte 0 ; currently unused
+    .byte 0 ; currently unused
+    .byte 0 ; currently unused
+    .byte 0 ; saved flags
+    .word first_task_start ; saved PC
+first_task_start:
+    .invoke add16 1, $7000
+    jmp first_task_start
+
+second_task:
+    .byte 0 ; currently unused
+    .byte 0 ; currently unused
+    .byte 0 ; currently unused
+    .byte 0 ; saved flags
+    .word second_task_start ; saved PC
+second_task_start:
+    .invoke add16 1, $7008
+    jmp second_task_start
+
 ; vim: tabstop=4 expandtab shiftwidth=4