# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1439816126 -7200
# Node ID 1cbf83f55fcc5ff39fbc16c798fcced15912ad08
# Parent  8460f118146c1654f5b95b96643cfa32ba83edce
Access the original value of A in the handler, not the version corrupted by
Acorn's interrupt handler mechanism.
Made the first and second test tasks more illustrative.
Simplified the third task slightly.

diff -r 8460f118146c -r 1cbf83f55fcc switcher.oph
--- a/switcher.oph	Mon Aug 17 01:03:14 2015 +0200
+++ b/switcher.oph	Mon Aug 17 14:55:26 2015 +0200
@@ -142,6 +142,7 @@
 ; affects: (temporary stack usage)
 
 handler:
+    lda $fc ; obtain the original value of A
     pha ; A -> stack
     txa
     pha ; X -> stack
@@ -171,9 +172,9 @@
     ; obtain the stack location of the stored PC MSB
 
     ldy #7 ; offset of MSB (_, _, Y, X, A, F, LSB, MSB)
-    lda (SP), y
 
     ; reference the stack location and compute PC MSB & $80
+    lda (SP), y
     and #$80
     cmp #$80
 
@@ -281,6 +282,7 @@
     pla
     tax ; stack -> X
     pla ; stack -> A
+    sta $fc ; override the default handler's restoration of A
     jmp (old_handler)
 
 
@@ -384,8 +386,16 @@
 first_task_start:
     lda #1
     sta $7000
+    lda #0
+    sta $7001
 first_task_continue:
-    inc $7000
+    clc
+    lda $7000
+    adc #1
+    sta $7000
+    lda $7001
+    adc #0
+    sta $7001
     jmp first_task_continue
 
 
@@ -400,8 +410,16 @@
 second_task_start:
     lda #1
     sta $7010
+    lda #0
+    sta $7011
 second_task_continue:
-    inc $7010
+    clc
+    lda $7010
+    adc #1
+    sta $7010
+    lda $7011
+    adc #0
+    sta $7011
     jmp second_task_continue
 
 
@@ -418,11 +436,8 @@
 *   stx $7028
     .invoke call third_task_delay, +
 *   txa
-    .invoke pushA
     .invoke call remove_task, +
-*   .invoke pullA
-    tax
-    inx ; move to next offset
+*   inx ; move to next offset
     inx ; which is 2 locations away
     cpx #TASK_TABLE_LENGTH
     beq +