# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1435428100 -7200
# Node ID d7d91332b0aa73a7cffe57cfeeef04559042f13f
# Parent  4ffa83fa5fe119443ed7db2c4b57771ed2f1c6f9
Added the new_task subroutine, renamed task_index to task_offset, added aliases.

diff -r 4ffa83fa5fe1 -r d7d91332b0aa switcher.oph
--- a/switcher.oph	Sat Jun 27 19:21:03 2015 +0200
+++ b/switcher.oph	Sat Jun 27 20:01:40 2015 +0200
@@ -17,6 +17,8 @@
 
 .include "macros.oph"
 
+.alias IRQ1V $204
+
 .alias SP $70
 .alias SPH $71
 .alias NEXT $72
@@ -24,24 +26,34 @@
 .alias CURRENT $74
 .alias CURRENTH $75
 
+.alias TASK_TABLE_LENGTH 10
+
 .org $2000
 .text
 main:
     jsr install_handler
+    .invoke store16 first_task, $76
+    jsr new_task
+    .invoke store16 second_task, $76
+    jsr new_task
 test:
     jmp test
 
+
+
 ; install the interrupt handler address in IRQ1V
 ;
 ; affects: A
 
 install_handler:
     sei
-    .invoke mov16 $204, old_handler
-    .invoke store16 handler, $204
+    .invoke mov16 IRQ1V, old_handler
+    .invoke store16 handler, IRQ1V
     cli
     rts
 
+
+
 ; handle interrupts
 ;
 ; affects: (temporary stack usage)
@@ -91,7 +103,7 @@
 
     ; load next task
 
-    ldx task_index
+    ldx task_offset
     lda tasks, x
     sta NEXT
     inx
@@ -101,12 +113,12 @@
 
     ; reset the task index if necessary
 
-    cpx #10 ; length of tasks array
+    cpx #TASK_TABLE_LENGTH
     bne test_task
     ldx #0
 
 test_task:
-    stx task_index
+    stx task_offset
 
     ; exit if null task
 
@@ -161,6 +173,8 @@
     pla ; stack -> A
     jmp (old_handler)
 
+
+
 ; location of previous interrupt handler
 
 old_handler: .word 0
@@ -169,19 +183,62 @@
 
 current_task: .word 0
 
-; index of current task
+; offset of current task in table (in multiples of 2)
 
-task_index: .byte 0
+task_offset: .byte 0
 
 ; task table containing locations of each task
 
 tasks:
-    .word first_task
-    .word second_task
+    .word 0
+    .word 0
     .word 0
     .word 0
     .word 0
 
+
+
+; add a new task to the table
+;
+; $76, $77: location of task structure
+; affects: A, Y
+; returns: A = 0 (success); A = 1 (failure)
+
+new_task:
+
+    ; check the MSB of each task table entry
+
+    ldy #1
+    lda tasks, y
+    cmp #0
+    beq add_new_task
+    iny
+    iny
+
+    ; if no space is found by the end of the table, exit
+
+    cpy #TASK_TABLE_LENGTH
+    bpl no_new_task
+
+add_new_task:
+
+    ; copy the task structure location to the table
+
+    lda $77
+    sta tasks, y
+    dey
+    lda $76
+    sta tasks, y
+
+    lda #0
+    rts
+
+no_new_task:
+    lda #1
+    rts
+
+
+
 ; example tasks
 
 first_task:
@@ -194,6 +251,8 @@
     .invoke add16 1, $7000
     jmp first_task_start
 
+
+
 second_task:
     .byte 0 ; currently unused
     .byte 0 ; currently unused