Site Tools

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

  .org $fbdd
  ; this code replaces the .1 second delay
  .byte $02  ; two byte NOP in 65C02
  .byte $01  ; should be ignored by CPU
  ; 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.

$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:

*300:02 01 02 01 02 01 60

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 DELETE, converts it to .
$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:

!300:jsr fd35
! nop
! nop
! jmp fdda

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 DELETE.

FF    (appears after pressing delete)

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 DELETE.

*303:02 06
88    (appears after pressing delete)

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

Try this sequence of instructions:

]CALL -151
*300:02 04 60

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.

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

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 utility for Davex can exploit this to determine which slot is configured for startup via the dispslot routine.