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README.md
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README.md
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# Applecorn
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Allows Acorn BBC Microcomputer language ROMs to run on Apple //e enhanced
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Applecorn is a ProDOS application for the Apple //e Enhanced which provides
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an environment for Acorn BBC Microcomputer language ROMs to run. This
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allows BBC BASIC and other Acorn languages to run on the Apple //e.
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## Hardware Requirements
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Enhanced (65C02) Apple //e with 128KB of memory.
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## How to Run the Software
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Boot the diskette `applecorn.po` which is a 143KB Disk ][ bootable ProDOS
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diskette. I use version 2.4.2 of ProDOS for my testing, but the software
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should run on other versions of ProDOS.
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Run `BASIC.SYSTEM` and at the Applesoft BASIC prompt type:
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```
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BRUN APPLECORN
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```
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to start the software.
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Applecorn will then load the content of the file `BASIC.ROM` from the
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diskette. This is a 16KB file containing BBC BASIC v2 from the BBC Micro.
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Once the ROM has loaded, it will automatically be started and you will
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see the `>` prompt of BBC BASIC.
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32 Kilobytes of space is available for your programs and variables. `PAGE`
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is set to `&0E0`.
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## Theory of Operation
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### BBC Micro
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On the BBC Micro, language ROMs have a very clean interface to the Machine
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Operating System (MOS). Syscalls are used for all accesses to the hardware,
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rather than poking at memory mapped addresses directly, as is common in
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other 6502 systems. This was done partly to enable programs to run on a
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second processor connected to the main BBC Micro over an interprocessor
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interface called The Tube.
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On the BBC Micro, the 64K address space looks like this:
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```
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+----------------------+ $ffff
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| MOS ROM (16KB) |
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+----------------------+ $c000
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| Language ROM (16KB) |
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+----------------------+ $8000
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| User RAM (32K) |
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|//////////////////////|
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+----------------------+ $0000
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```
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The hatched area at the bottom represents reserved space such as zero page,
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the stack and pages 2 through 7 which are used by the language ROM for
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various purposes.
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Display memory on the BBC Micro is allocated at the top of user RAM, from
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$8000 down. Higher resolution modes use more memory, reducing the user RAM
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available for BASIC programs.
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The BBC Micro uses a unique paging mechanism referred to as 'Sideways ROM',
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which allows up to 16 language and filing system ROMs to be banked into the
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16KB space from $8000 to $bfff.
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### Apple //e
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The Apple //e, with 128KB has two 64KB banks of memory. The main memory bank
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is as follows:
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```
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+----------------------+ $ffff +----------------------+
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| BASIC/Monitor ROM | | Language Card |
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| | | | +-4K Bank Two----+
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|###I/O Space (4KB)####| +----------------------+ +----------------+
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+----------------------+ $c000
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| User RAM (48K) |
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|//////////////////////|
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+----------------------+ $0000
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```
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Here there is 48KB of RAM rather than 32KB, and the total ROM is just 12KB,
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starting at $d000. The 4KB from $c000 to $cfff is all memory mapped I/O.
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An additional 16KB of memory is available, which is referred to as the
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Language Card. This memory can be banked into the space where the ROM usually
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resides from $d000 up. Note that this space is only 12KB so the 16KB LC
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memory is itself banked with the bottom 4KB of LC space having two banks.
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When an Extended 80 Column card is installed in the aux slot of the Apple
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//e, an additional 64KB of RAM is added, for a total of 128KB. The entire
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arrangement described above is duplicated, so there is 64KB of main memory
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(divided between the 'lower 48K' and 16KB of LC memory) and 64KB of
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auxiliary memory (divided in exactly the same manner.)
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The Apple //e has softswitches to select whether to address the main or aux
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bank for the main portion of RAM (ie: the 48KB up to $bfff). Reading and
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writing may be switched separately so it is possible to execute code and
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read data from one bank while writing to the other. A separate softswitch
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controls whether zero page, the stack, and LC memory addresses will be passed
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to main or aux banks.
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The ProDOS operating system resides primarily in the main bank Language
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Card memory, so this memory is not available to Applecorn if we wish to
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retain the facilties provided by ProDOS.
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The Apple //e screen normally resides from $400 to $7ff in main memory (for
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40 column mode) or at $400 to $7ff in both main and aux memory (for 80
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column mode.) There is a softswitch to switch to text page two from $800
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to $bff.
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### Applecorn Architecture
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- Applecorn maintains a 'BBC Micro virtual machine' in the Apple //e auxiliary
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memory. In particular, the 'BBC Micro' has its own zero page and stack in
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auxiliary memory, so there is no contention with ProDOS or with Applecorn.
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- Applecorn primarily uses the main memory for servicing ProDOS file system
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requests for the 'BBC Micro virtual machine'.
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- An 80 column screen is configured using PAGE2 memory from $800 to $bfff
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in both main and aux memory. This conveniently just fits in above page 7,
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which is the highest page used as Acorn language ROM workspace.
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- The Acorn language ROM is loaded to $8000 in aux memory.
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- The Language Card memory is enabled and used to store the 'Applecorn MOS'
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from $d000 up in aux memory. (The main bank LC memory contains ProDOS.)
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- Applecorn copies its own 'Applecorn MOS' code to $d000 in aux memory and
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relocates the MOS entry vectors to high memory.
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- The only real difference between the Apple //e aux memory map and that of
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the BBC Micro is the Apple //e has a 'hole' from $c000 to $cfff where memory
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mapped I/O resides. Fortunately this does not really matter because the
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language only uses well-defined entry points to call into the MOS, so we
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can simply avoid this address range.
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## Limitations
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...
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## 'Applecorn MOS' Features
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### Escape
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...
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### Reset
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...
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### Star Commands
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`*QUIT` - Terminate Applecorn and quit to ProDOS. Because the 'BBC Micro'
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lives in auxiliary memory, you can usually restart Applecorn by running it
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again and recover your program with `OLD`.
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`*HELP` - Prints out information similar to the same command on the BBC micro.
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Specifically it lists the version of Applecorn MOS and the name of the current
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language ROM.
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`*CAT` (or `*.`) - Simple listing of the files in the current directory.
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`*DIR pathname` - Allows the current directory to be changed to any ProDOS
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path. For example `*DIR /H1/APPLECORN`.
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