restructure keyboardhandler example due to X register bug, discussed in #94

examples/cx16/keyboardhandler.p8

@@ -10,56 +10,85 @@ main {
 
         sys.set_irqd()
         uword old_keyhdl = cx16.KEYHDL
-        cx16.KEYHDL = &main.keyboard_scancode_handler.asm_shim
+        cx16.KEYHDL = &keyboard_scancode_handler
         sys.clear_irqd()
 
         bool escape_pressed
         while not escape_pressed {
-            ; just sit here
+            handle_keyboard_event()
         }
         sys.set_irqd()
         cx16.KEYHDL = old_keyhdl
         sys.clear_irqd()
     }
 
-    sub keyboard_scancode_handler(ubyte prefix, ubyte scancode, bool updown) -> bool {
-        txt.print_ubhex(prefix, true)
+    ; Keyboard handler communication variables.
+    ; these need to be in block scope instead of in a subroutine,
+    ; so that they won't get overwritten with initialization values every time.
+    ; The assembly keyboard handler will set these, prog8 will read them.
+    bool @shared keyhdl_event       ; is there a keyboard event to handle?
+    ubyte @shared keyhdl_prefix
+    ubyte @shared keyhdl_scancode
+    ubyte @shared keyhdl_updown
+
+    sub handle_keyboard_event() {
+        ; Potentially handle keyboard event.
+        ; Note that we do this from the program's main loop instead of
+        ; the actual keyboard handler routine itself.
+        ; The reason for this is documented below in the handler assembly routine.
+        if not keyhdl_event
+            return
+        keyhdl_event = false
+        txt.print_ubhex(keyhdl_prefix, true)
         txt.chrout(':')
-        txt.print_ubhex(scancode, true)
+        txt.print_ubhex(keyhdl_scancode, true)
         txt.spc()
-        if updown
+        if keyhdl_updown
             txt.chrout('u')
         else
             txt.chrout('d')
         txt.nl()
-        if prefix==0 and scancode==119 and updown {
+        if keyhdl_prefix==0 and keyhdl_scancode==119 and keyhdl_updown {
             ; escape was pressed! exit back to basic
             main.start.escape_pressed = true
         }
-        return true     ; true = consume key event, false = continue processing it
+    }
+
+    asmsub keyboard_scancode_handler() {
+
+        ; NOTE that the keyboard handler is an asm subroutine.
+        ; Unfortunately is it not possible to use prog8 code or calls here,
+        ; because the X register gets overwritten here (to store the prefix byte)
+        ; and prog8 uses the X register internally (for the evaluation stack).
+        ; So it is unsafe to call prog8 code from here because the evaluation stack pointer
+        ; will be invalid which produces undefined results.
+        ; So, instead, we store the various keyboard event bytes and signal
+        ; the main prog8 program that a keyboard event has occurred.
+        ; It then processes it independently from the assembly code here.
+        ;
+        ; Unfortunately this also means you cannot decide from that prog8 code
+        ; if the keyboard press should be consumed/ignored or put into the keyboard queue
+        ; (this is controlled by returning 0 or 1 in register A here)
 
-asm_shim:
         %asm {{
             php
             pha
             phx
-            stz  updown
+            stz  keyhdl_updown
             bcc  +
-            inc  updown
-+           stx  prefix
-            sta  scancode
-            jsr  keyboard_scancode_handler
-            beq  +
+            inc  keyhdl_updown
++           stx  keyhdl_prefix
+            sta  keyhdl_scancode
+            lda  #1
+            sta  keyhdl_event
+            ; we can do additional stuff here and decide if we want to
+            ; consume the key event or not (A=0 or A!=0)
             plx
             pla
-            lda #0     ;By setting A=0 we will consume this key event
+            lda #0     ;By setting A=0 we will remove this key event for now
             tax
             plp
             rts
-+           plx
-            pla     ; leave A untouched, continue processing
-            plp
-            rts
         }}
     }
 }