Revert "update cx16 keyhandler example to r43 keyboard changes"

This reverts commit 1e6d7673bc6b93747db7b64be003ca9adfeb0b65.

examples/cx16/keyboardhandler.p8

@@ -2,57 +2,99 @@
 %zeropage basicsafe
 %option no_sysinit
 
-; The documentation for a custom key handler can be found here:
+; The documentation for custom PS2 key handlers can be found here:
 ; https://github.com/X16Community/x16-docs/blob/master/X16%20Reference%20-%2002%20-%20Editor.md#custom-keyboard-scancode-handler
 
 main {
 
     sub start() {
 
-        txt.print("custom key handler test - press keys! esc to quit!\n")
+        txt.print("ps2 custom key handler test - press keys! esc to quit!\n")
 
         sys.set_irqd()
         uword old_keyhdl = cx16.KEYHDL
-        cx16.KEYHDL = &main.key_handler.asm_hook
+        cx16.KEYHDL = &keyboard_scancode_handler
         sys.clear_irqd()
 
         bool escape_pressed
         while not escape_pressed {
-            ; do nothing here, just wait until ESC is pressed
+            handle_keyboard_event()
         }
-
         sys.set_irqd()
         cx16.KEYHDL = old_keyhdl
         sys.clear_irqd()
     }
 
-    sub key_handler(ubyte keycode) -> bool {
+    ; Keyboard handler communication variables.
-        txt.print_ubhex(keycode, true)
+    ; 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(keyhdl_scancode, true)
         txt.spc()
-        if keycode & $80
+        if keyhdl_updown
             txt.chrout('u')
         else
             txt.chrout('d')
         txt.nl()
-        if keycode==$6e {
+        if keyhdl_prefix==0 and keyhdl_scancode==119 and keyhdl_updown {
-            main.start.escape_pressed=true
+            ; escape was pressed! exit back to basic
-        }
+            main.start.escape_pressed = true
-        return false        ; this eats the keypress - return true if you want to pass it through
-
-        asmsub asm_hook(ubyte keycode @A) -> ubyte @A {
-            %asm {{
-                sta  key_handler.keycode
-                pha
-                jsr  key_handler
-                tay
-                pla
-                cpy  #0
-                beq  +
-                rts
-+               tya
-                rts
-            }}
         }
     }
 
+    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)
+        ;
+        ; see:
+        ; https://github.com/X16Community/x16-docs/blob/master/X16%20Reference%20-%2002%20-%20Editor.md#custom-keyboard-scancode-handler
+
+        %asm {{
+            php
+            pha
+            phx
+            stz  keyhdl_updown
+            bcc  +
+            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 remove this key event for now
+            tax
+            plp
+            rts
+        }}
+    }
 }