This project builds custom ROMs for the Apple /// - for the main system ROM (the ROM in position "B9").

Currently, ROMs with the following functionality are available:

• A custom ROM supporting bootstrapping volumes directly from the DAN II Controller card. No more floppy disks required.

• A custom ROM extended the Apple /// debug monitor - adding a disassembler.

  The Apple /// ROM inherited many routines from the Apple II, however, the disassembler had to be stripped, since Apple only installed a tiny 4KB ROM in the Apple ///. The machine does support 8KB ROMs though. So, by installing a larger ROM, we have the necessary space to copy missing disassembler routine from the Apple II and add it to the Apple /// ROM.

If you need an adapter to plug a standard (E)EPROM on to the Apple /// mainboard, here's my PCB design which fits into the tight space of the Apple ///:

https://github.com/ThorstenBr/Apple_III_ROM_Adapter.

The /bin folder contains the following ROM variants for the Apple ///:

• A3ROM_ORIGINAL_4KB.bin

  Original 4KB ROM for the Apple ///. In case you just want to replace a defective ROM with an original.

• A3ROM_ORIGINAL_EXTMONITOR_8KB.bin

  8KB ROM which contains the original (unmodified) Apple /// ROM in the default ROM bank (bank 1). The alternate ROM bank 0 contains an adapted ROM variant with improved "debug monitor" (see below).

• A3ROM_DANII_4KB.bin

  Custom 4KB Apple /// ROM variant. Adapted to support direct bootstrapping of volumes from the DAN II Controller card.

• A3ROM_DANII_EXTMONITOR_8KB.bin

  Custom 8KB Apple /// ROM variant. Default ROM bank 1 contains the adapted ROM to support direct bootstrapping of volumes from the DAN II Controller card. The alternate ROM bank 0 contains an adapted ROM variant with improved "debug monitor" (see below).

The Apple /// has two ROM banks, each is 4KB. Apple /// were orignally only shipped with a 4KB ROM, which is mapped to ROM bank 1. ROM bank 0 could be activated, but was empty (would show random data).

The ROM banks are switched through the Apple ///'s "system environment register" at $FFDF, bit 1 (value of 0x2). When you installed one of the 8KB ROM variants, you can activate the alternate ROM bank when using the monitor:

• Press "OpenApple-Ctrl-RESET" to enter the debug monitor (then release RESET, but keep the OpenApple-key pressed for a moment).

• Enter "FFDF:75" to enable alternate ROM bank 0 (clears bit 1 = value 0x02).

  

• Now the alternate ROM is active. If you programmed a ROM variant with extended debug monitor, e.g. you can now use the monitor command "L" for a debug listing:

  

• Enter "FFDF:77" to switch to default ROM bank 1 again (stock ROM, no extended debug monitor).

The debug monitor in the Apple /// ROM is very similar to the Apple ][. It was implemented based on the same code base. However, due to the limited space of the Apple ///'s 4KB ROM, a number of options were removed. The bank 0 ROM "extended Apple /// Debug Monitor" (see above) uses the normally unused second Apple /// ROM bank to add some of missing Apple ][ monitor options back to the Apple ///. The following commands/options are added:

• L command to trigger a disassembler listing.

  Example: Trigger a disassembler listing starting at address $300.

     300L
  

• <CTRL-E>+RETURN to print register values (A, X, Y, Processor status flags, Stack).

• : command edits register values, when called immediately after a <CTRL-E>+RETURN command.

  Example: Show all register values, then set registers A,X,Y to values 11, 22 and 33 respectively.

     ->(CTRL-E)+(RETURN)
     A=FF X=00 Y=00 P=00 S=FF
     ->:11 22 33
     ->(CTRL-E)+(RETURN)
     A=11 X=22 Y=33 P=00 S=FF
  

• J jumps to the given address, after restoring the registers. The J command is also supported by the default Apple /// ROM, however, the default ROM does not restore registers (no debugging/stepping was supported, so it did not make sense for the original ROM to save/restore registers). The extended debug monitor restores this feature, exactly as it was supported on the Apple ][.

• The BRK instruction is caught by the debug monitor in the extended monitor (just as on the Apple ][). The monitor also saves registers when entering the monitor via a BRK instruction.