Applecorn

Applecorn is a ProDOS application for the Apple //e Enhanced which provides an environment for Acorn BBC Microcomputer language ROMs to run. This allows BBC BASIC and other Acorn languages to run on the Apple //e.

The language ROMs run as-is, without any modification required.

Hardware Requirements

Enhanced (65C02) Apple //e with 128KB of memory.

How to Run the Software

Boot the diskette applecorn.po which is a 143KB Disk ][ bootable ProDOS diskette. I use version 2.4.2 of ProDOS for my testing, but the software should run on other versions of ProDOS.

Run BASIC.SYSTEM and at the Applesoft BASIC prompt type:

BRUN APPLECORN

to start the software.

Applecorn will then load the content of the file BASIC.ROM from the diskette. This is a 16KB file containing BBC BASIC v2 from the BBC Micro. Once the ROM has loaded, it will automatically be started and you will see the > prompt of BBC BASIC.

32 Kilobytes of space is available for your programs and variables. PAGE is set to &0E0.

How to Build

Applecorn is built natively on the Apple //e using the Merlin 8 assembler v2.58.

Theory of Operation

BBC Micro

On the BBC Micro, language ROMs have a very clean interface to the Machine Operating System (MOS). Syscalls are used for all accesses to the hardware, rather than poking at memory mapped addresses directly, as is common in other 6502 systems. This was done partly to enable programs to run on a second processor connected to the main BBC Micro over an interprocessor interface called The Tube.

On the BBC Micro, the 64K address space looks like this:

                 +----------------------+ $ffff
                 |                      |
                 | MOS ROM (16KB)       |
                 |                      |
                 +----------------------+ $c000
                 |                      |
                 | Language ROM (16KB)  |
                 |                      |
                 +----------------------+ $8000
                 |                      |
                 | User RAM (32K)       |
                 |                      |
                 |                      |
                 |                      |
                 |                      |
                 |//////////////////////|
                 +----------------------+ $0000

The hatched area at the bottom represents reserved space such as zero page, the stack and pages two through seven which are used by the language ROM for various purposes.

Display memory on the BBC Micro is allocated at the top of user RAM, from $8000 down. Higher resolution modes use more memory, reducing the user RAM available for BASIC programs.

The BBC Micro uses a unique paging mechanism referred to as 'Sideways ROM', which allows up to 16 language and filing system ROMs to be banked into the 16KB space from $8000 to $bfff.

Apple //e

The Apple //e, with 128KB has two 64KB banks of memory. The main memory bank is as follows:

   +----------------------+ $ffff +----------------------+
   | BASIC/Monitor ROM    |       | Language Card        |
   |                      |       |                      | +-4K Bank Two----+
   |###I/O Space (4KB)####|       +----------------------+ +----------------+
   +----------------------+ $c000
   |                      |
   |                      |
   |                      |
   |                      |
   |                      |
   | User RAM (48K)       |
   |                      |
   |                      |
   |                      |
   |                      |
   |//////////////////////|
   +----------------------+ $0000

Here there is 48KB of RAM rather than 32KB, and the total ROM is just 12KB, starting at $d000. The 4KB from $c000 to $cfff is all memory mapped I/O.

An additional 16KB of memory is available, which is referred to as the Language Card (LC henceforth.) This memory can be banked into the space where the ROM usually resides from $d000 up. Note that this address space is only 12KB so the 16KB LC memory is itself banked with the bottom 4KB of LC space having two banks.

When an Extended 80 Column card is installed in the aux slot of the Apple //e, an additional 64KB of RAM is added, for a total of 128KB. The entire arrangement described above is duplicated, so there is 64KB of main memory (divided between the 'lower 48K' and 16KB of LC memory) and 64KB of auxiliary memory (divided in exactly the same manner.)

The Apple //e has softswitches to select whether to address the main or aux bank for the main portion of RAM (ie: the 48KB up to $bfff). Reading and writing may be switched separately so it is possible to execute code and read data from one bank while writing to the other. A separate softswitch controls whether zero page, the stack, and LC memory addresses will be passed to main or aux banks.

The ProDOS operating system resides primarily in the main bank Language Card memory, so this memory is not available to Applecorn if we wish to retain the facilties provided by ProDOS.

The Apple //e screen normally resides from $400 to $7ff in main memory (for 40 column mode) or at $400 to $7ff in both main and aux memory (for 80 column mode.) There is a softswitch to switch to text page two from $800 to $bff.

Applecorn Architecture

MAIN BANK:

   +----------------------+ $ffff +----------------------+
   | BASIC/Monitor ROM    |       | Language Card        |
   |                      |       | ProDOS               | +-4K Bank Two----+
   |###I/O Space (4KB)####|       +----------------------+ +-Unused---------+
   +----------------------+ $c000
   |                      |
   |                      |
   |                      |
   |                      |
   | Applecorn loader &   |
   | Applecorn code to    |
   | interface with       |
   | ProDOS               |
   |                      |
   |                      |
   |//////////////////////|
   +----------------------+ $0000

AUX BANK:

   +----------------------+ $ffff 
   | Language Card        |
   | Applecorn MOS        |       +-4K Bank Two----+
   |###I/O Space (4KB)####|       +-Unused---------+
   +----------------------+ $c000
   |                      |
   | Acorn Language ROM   |
   |                      |
   +----------------------+
   |                      |
   | Acorn language       |
   | user code/data       |
   | space                |
   |                      |
   |                      |
   |//////////////////////|
   +----------------------+ $0000

• Applecorn maintains a 'BBC Micro virtual machine' in the Apple //e auxiliary memory. In particular, the 'BBC Micro' has its own zero page and stack in auxiliary memory, so there is no contention with ProDOS or with Applecorn.
• Applecorn primarily uses the main memory for servicing ProDOS file system requests for the 'BBC Micro virtual machine'.
• The Acorn language ROM is loaded to $8000 in aux memory.
• The Language Card memory is enabled and used to store the 'Applecorn MOS' from $d000 up in aux memory. (The main bank LC memory contains ProDOS.)
• Applecorn copies its own 'Applecorn MOS' code to $d000 in aux memory and relocates the MOS entry vectors to high memory.
• An 80 column screen is configured using PAGE2 memory from $800 to $bfff in both main and aux memory. This conveniently just fits in above page 7, which is the highest page used as Acorn language ROM workspace.
• The only real difference between the Apple //e aux memory map and that of the BBC Micro is the Apple //e has a 'hole' from $c000 to $cfff where memory mapped I/O resides. Fortunately this does not really matter because the language only uses well-defined entry points to call into the MOS, so we can simply avoid this address range.
• The memory map for the main and aux banks is illustrated in the diagram above. For the aux bank, the LC is always banked in since no Apple monitor or BASIC ROM routines are called, so this is omitted from the diagram.

'Applecorn MOS' Features

• In principle any Acorn language ROM should work.
• Currently I have verified operation with:
  • BBC BASIC
  • Acornsoft COMAL
  • Acornsoft FORTH

Limitations

Applecorn currently has the following limitations:

• A number of MOS calls relating to file I/O are not yet implemented including OSFIND, OSGBPB. OSBPUT, OSBGET, OSARGS. This means that file operations such as the BBC BASIC OPENIN and OPENOUT do not work in the Acorn languages. OSFILE is (partially) implemented however, so BASIC and COMAL can LOAD and SAVE programs.
• The VDU driver is quite primitive at present. In particular it only supports 80 column text mode. There is currently no graphics support.
• Only a few OSBYTE calls are implemented. More will be added in due course.
• The only implemented OSWORD call is A=&00, to read a line of input from the console. More will be added as needs arise.
• Special BBC Micro functions such as sound, A/D interfaces, programmable function keys and so on are currently not supported.
• The Applecorn MOS command line is currently quite limited. More commands will be added in due course (for example *FX, *LOAD, *SAVE etc.).

Escape

The BBC Micro uses interrupts extensively and the Escape key is handled asynchronously. The language ROM code simply checks an 'escape flag' in zero page ($FF) from time to time to detect if Escape has been pressed.

The Apple //e does not use interrupts in its keyboard handling and the basic machine include no sources of interrupts at all (there is no system timer.) This prevents Escape from being handled in the same manner.

As a partial workaround, Applecorn checks whether the Escape key is pressed from time to time when it has control, but there are cases where a program can run forever without ever making a MOS call. In these cases the only way to interrupt the program is to press Ctrl-Reset.

Ctrl-Reset

The Ctrl-Reset key combination is the only asynchronously handled keyboard event on the Apple //e. Applecorn sets up a reset handler which will restart the ROM after Ctrl-Reset. Any user program in aux memory will be untouched.

For ROMs such as BASIC or COMAL, the OLD command can be used to recover the program in memory.

Star Commands

*QUIT - Terminate Applecorn and quit to ProDOS. Because the 'BBC Micro' lives in auxiliary memory, you can usually restart Applecorn by running it again and recover your program with OLD.

*HELP - Prints out information similar to the same command on the BBC micro. Specifically it lists the version of Applecorn MOS and the name of the current language ROM.

*CAT (or *.) - Simple listing of the files in the current directory.

*DIR pathname - Allows the current directory to be changed to any ProDOS path. For example *DIR /H1/APPLECORN.