====== LC //e Card - Weird Stuff: Opcode $02 ======

===== Weird Beep =====

The %%//e%% Card is able to replace the standard Apple II beep with a Macintosh sound.  The means by which this is accomplished is quite interesting.

The routine at $fbdd (BELL1) is

<code>
  .org $fbdd
  ; this code replaces the .1 second delay
  .byte $02  ; two byte NOP in 65C02
  .byte $01  ; should be ignored by CPU
  rts
  nop
  nop
  ; remaining code matches the original //e code and calling $fbe2
  ; will produce a beep, at the wrong pitch if the card is in fast mode.
</code>

$02 is a two-byte NOP on the 65C02 (if it was an '802 or '816 it'd be a COP).  Interestingly enough, when the processor on the Card executes the sequence $02 $01, it produces the configured beep sound.

Try this in the monitor:

<code>
*300:02 01 02 01 02 01 60
*300G
</code>

Three beep sounds!

So this must be part of the magic that interfaces the card to the host Macintosh.  How very very interesting.

===== Other Weird Instructions =====

The preceding find led me to search through the monitor ROM to look for other unusual instruction sequences.

Here is what I found:

^ In Routine ^ Address ^ Code    ^ Function ^
| PWRUP   | $FAB4   | $02 $02 | Loads A reg with $Cn+1 where n = startup slot or $C8 if scan. |
| PWRUP   | $FAC0   | $02 $03 | Displays "UNABLE TO BOOT FROM STARTUP SLOT" if A reg = $Cn-1 where n = startup slot or $c0 if scan.  Disappears if screen scrolls. |
| APPLEII | $FB63   | $02 $04 | Display copyright message on screen, disappears if screen scrolls. |
| BELL1   | $FBDD   | $02 $01 | Play system bell sound. |
| GETLN1  | $FD78   | $02 $06 | Key translation called right after rdchar. If A reg has <key>DELETE</key>, converts it to <key><-</key>. |
|         |         | $02 $05 | Not found in firmware, yet, but presumably this exists. |

==== The Key Translation and the A register ====

Get to the monitor in your %%//%%e Card and try this:

<code>
*!
!300:jsr fd35
! nop
! nop
! jmp fdda
!
*300G
</code>

FD35 is the RDCHAR routine, FDDA is the print byte routine.  This routine reads a keypress and outputs its hex code.  Run it a few times to convince yourself there is no funny business.  Run it a final time and press <key>DELETE</key>.

<code>
*300G
FF    (appears after pressing delete)
*
</code>

FF is exactly what we expect to see with the Apple II delete key.

Now want to see something interesting?  Change the NOPs to $02 $06 and run it again.  Try a few keys, then try it with <key>DELETE</key>.

<code>
*303:02 06
*300G
88    (appears after pressing delete)
*
</code>

88 is the code for the left arrow key.  That's some serious magic, and in two bytes the Card converts <key>DELETE</key> to <key><-</key>.

==== The Two-Byte Copyright ====

Try this sequence of instructions:

<code>
]HOME
]CALL -151
*300:02 04 60
*300G
</code>

Hit the left arrow a bunch of times until the display scrolls. **POOF!**

==== Slot Scan Scam ====

The %%//%%e Card lets the user pick the startup slot in the control panel or "Scan" which is the behavior of a standard %%//%%e.

This is implemented by the sequences $02 $02 which replaces the LDA #$C8 at the start of the slot scan loop, and $02 $03 which replaces the CMP #$C0 instruction that decides loop termination.

The $02 $02 sequence loads the accumulator with $C8 if scan is selected, or $Cn+1 if a specific slot is selected.

<code>
*300:02 02 4C DA FD
*300G
C8   (if scan or slot 7 selected, "Cx" if another slot is selected)
*
</code>

The $02 $03 sequence behaves as if CMP #$C0 or CMP #$Cn-1 has been executed and if it has, displays "UNABLE TO BOOT FROM STARTUP SLOT" in the center of the screen in a similar manner to the copyright message.  The message is not in Apple II memory.  It returns with the flags set as executing the CMP instruction would have.

My ''iie.card'' [[https://github.com/mgcaret/davex-mg-utils/blob/master/iie.card.s|utility]] for Davex can exploit this to determine which slot is configured for startup via the [[https://github.com/mgcaret/davex-mg-utils/blob/master/iie.card.s#L118|dispslot routine]].