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 and compatible systems.

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

Hardware Requirements

The minimum requirement for Applecorn is an Apple II system with 128KB of memory and a 65C02 processor. This includes the following:

• Apple //e Enhanced (but not the original ][e, which has a 6502.)
• Apple //c and //c+
• Apple IIgs

Note, for the Apple IIgs, with the latest code (as of September 26, 2021), you must NOT enable the "Alternate Text Mode" CDA!

How to Run the Software

Boot the diskette applecorn.po which is an 800KB 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. Alternatively, you can simply select APPLECORN in the ProDOS 2.4.2 Bitsy Bye file browser.

When first started, Applecorn will display a ROM selection menu. Choose the language ROM you wish to load by pressing the associated number key. Applecorn will then load the requested ROM file from the diskette. Each of these files is a dump of a 16KB BBC Micro language ROM.

Once the ROM has loaded, it will automatically be started and you will see the prompt. For BBC BASIC, the prompt character is >.

Most of the BBC Micro languages (including BBC BASIC) prefer upper case input. You may want to keep Caps Lock enabled most of the time!

32 Kilobytes of space is available for your programs and variables. PAGE is set to &0E00.

'Applecorn MOS' Features

Compatible Language ROMs

• In principle any Acorn language ROM should work.
• 'Sideways ROMs' are now emulated (see below). This allows languages which were supplied in more than one ROM to be supported.
• Currently I have verified operation with:
  • BBC BASIC
  • Acornsoft COMAL
  • Acornsoft Forth
  • Acornsoft Lisp
  • Acornsoft MicroProlog
  • Acornsoft BCPL
  • Acornsoft ISO-Pascal (supplied on two 16KB ROMs)

All of the language manuals are provided. They may be found in the Manuals subdirectory.

The Applecorn diskette includes all of the example programs which were provided on diskette with COMAL, Forth, Lisp and ISO Pascal. The main BCPL diskette, which contains tools that are essential for using the BCPL ROM, is also included on the Applecorn diskette.

Video Modes

Two text video modes and one graphics mode are currently supported:

• MODE 6 and MODE 7 - 40x24 text (in mode 7, chars $80 to $9F are converted to spaces)
• MODE 3 - 80x24 text.
• MODE 1 - Apple II high resolution mode. The physical resolution is 280x192 pixels. This is mapped onto the normal BBC Micro 1280x1024 virtual resolution, with the origin at the bottom left of the screen.

NOTE: You can set the startup video mode by holding down the appropriate number key while Applecorn starting (while it is loading the ROM file.)

Andy McFadden's FDraw library is used for efficient high resolution line and point plotting.

Escape Key

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.

Special VDU Features

• Ctrl-S will pause the screen and Ctrl-Q will resume scrolling.
• The BBC Micro 'Copy Editor' function is supported. Use the Apple II cursor keys to move the copy cursor and the Tab key to copy a character from the copy cursor to the insert cursor.
• BBC Micro function keys are supported. Use Open Apple with the number keys for the unshifted function keys. Use Closed Apple with the number keys for the function keys with Ctrl.

'Sideways ROM' Support

The BBC Micro allows multiple ROMs to be banked into the 16KB space between $8000 to $BFFF. Acorn referred to such ROMs as 'sideways ROMs'. The BBC Micro architecture allows up to 16 sideways ROMs to be connected to the system and paged into this address space (although the original BBC Micro only featured four physical sockets.)

A sideways ROM may be either a 'language ROM' or a 'service ROM'. Language ROMs include a user interface, and are typically programming languages. However, note that ROM-based applications such as Acorn's View word processor are also considered to be language ROMs. Service ROMs do not contain a user interface but instead provide additional star commands to the operating system. Utility and filing system ROMs fall into this category.

When a star command is entered, the operating system first offers it to the currently active language ROM. If the current language does not support the command, the system pages in the ROMs one at a time, starting from the highest slot, offering the command to each of them. If none of the ROMs services the command an error is displayed.

Sideways ROMs are used in a number of ways:

• Support for multiple programming languages. For example, a BBC Micro may have the COMAL ROM in addition to the BASIC ROM. The machine normally boots into the highest numbered ROM (BASIC typically). In this case, COMAL could be selected using the *COMAL command. In COMAL, one could return to BASIC using *BASIC.
• Support for languages which are too large to fit in a single ROM such as Acornsoft ISO-Pascal, which occupies two 16KB ROMs.
• Utility and filesystem ROMs.

Applecorn emulates sideways ROMs by simply loading the ROM images from disk into the $8000 to $BFFF space. Unlike switching physical ROMs in the BBC Micro, this is not instantaneous. While it is possible to configure up to 16 ROMs, fewer are recommended in order to keep response times down. This mechanism allows multi-ROM languages such as ISO-Pascal to be supported.

The *HELP command displays information about each of the available sideways ROMs. (Note that the BCPL ROM does not print any *HELP info. This is a bug in the ROM, not Applecorn!)

HostFS

Applecorn's HostFS uses the ProDOS MLI to service all Acorn MOS filesystem calls. This means that Applecorn works directly with ProDOS volumes, such as floppy disks, mass storage devices and even network connected drives using ADT's VEDrive.

HostFS Pathnames

Pathnames used within Applecorn are regular ProDOS paths. The directory separator is forward slash / and every ProDOS filesystem has a volume name which is used to access the top level ('volume') directory. For example, a volume with the name 'TEST' would be mounted under ProDOS as /TEST.

Applecorn adds a few extra features to ProDOS paths, as follows:

• A notation is provided for physical drive numbers, identifed by slot (1-7) and drive (0 or 1). This is very useful when you insert a floppy disk with an unknown volume name. The syntax for physical device specifiers is a colon followed by two digits - one for the slot number and the other for the drive number. So, for example, :61 would refer to slot 6, drive 1. Applecorn uses the ProDOS ON_LINE MLI call to find the volume associated with the physical device. If slot 6, drive 1, contains the volume 'FLOPPY', then a path :S61/TESTFILE will be converted to /FLOPPY/TESTFILE.
• It is possible to refer to the current working directory (current prefix in ProDOS terms) using . (like Linux or Windows) or @ (like BBC ADFS.) The current working directory notation is only supported at the beginning of pathnames.
• Support is provided for easily accessing the parent directory. This may be denoted using .. (like Linux or Windows) or ^ (like BBC ADFS.) The parent directory notation is only supported at the beginning of pathnames, but it may be applied multiple times to navigate further up the tree.
• Some examples:
  • /H1/APPLECORN - absolute path
  • APPLECORN - relative path
  • ./APPLECORN - relative path (explicit)
  • ^ - parent dir
  • .. - parent dir (alternate form)
  • ^/^ - up two levels
  • ../.. - up two levels (alternate form)
  • ^/MYSTUFF - file or directory in parent
  • ../MYSTUFF - alternative way to refer to sibling directory
• Since Acorn's DFS allows filenames beginning with a digit, while ProDOS requires names to begin with an alphabetic character, Applecorn prefixes any file or directory names beginning with a digit with the letter 'N'. An Applecorn path such as /FOO/0DIR/50DIR/FILE01 would be converted to /FOO/N0DIR/N50DIR/FILE01, for example, in order to make it a legal ProDOS path.

HostFS Wildcards

Applecorn HostFS provides support for wildcards. The following wildcard characters are used:

• # or ? - matches any single character
• * - matches zero or more characters

The HostFS tries to follow the same conventions as Acorn's ADFS, which allows wildcards to be used in some cases to abbreviate long pathnames and in others to specify a list of files to operate on at once.

Like ADFS, HostFS commands accept several different types of file argument. Following Acorn's convention, these may be described as follows:

• <obspec> is an 'object specification'. This is a pathname without any wildcard characters, as described in the previous section.
• <*obspec*> is an 'wildcard object specification'. This is a pathname which may include the wildcard characters. If the wildcard characters specified result in multiple matching objects, the first one is used.
• <listspec> is an 'list specification'. This is also a pathname which may include the wildcard characters. However, if the wildcard results in multiple matches, the command will operate on all of these files.
• <dry> is a drive number. (For example, :61).

Wildcards are expanded wherever they appear in the path with one exception. For non-leaf nodes, the first match will be always be substituted. In the case of <*objspec*> the first match for the leaf node will always be used. However for <listspec> all matches of the leaf node will be operated upon.

Examples: :71/T*DIR/TEST??.TXT, *A*/FILES/C*/TEST*.TXT

Wildcards may also be used for OSFILE and OSFIND system calls, so BASIC command like LOAD"", CHAIN"", OPENIN"" and OPENUP"" can use wildcards to specify the file to open.

The attentive reader will have noticed that I mention an exception to wildcard matching. Volume directory names are not currently subject to wildcard search. Either type them in full, or use the colon notation to specify physical drive as an abbreviation.

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 [topic] - 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.

• *HELP shows the version number and the language ROM in use.
• *HELP MOS shows the available MOS star commands.
• *HELP HOSTFS shows the available HostFS star commands.

*CAT [<*objspec*>] (or *. [*objspec*]) - Simple listing of the files in the specified directory, or the current working directory ('current prefix') if no directory argument is given.

*EX [<*objspec*>] - Detailed listing of files in the current directory showing the load address, length and permissions. *EX expects a directory as an argument, for example *EX :71/MYDIR.

*INFO [<listspec>] - Displays the same detailed file listing as *EX, but accepting a <listspec>. *INFO expects a list of objects as an argument, so *INFO :71/MYDIR/* would display the same info as the *EX example above.

*DIR <*objspec*> - Allows the current directory to be changed to any ProDOS path. *CD and *CHDIR are synonyms for *DIR. The argument is expected to be a directory.

*LOAD <*objspec*> SSSS - Load file <*objspec*> into memory at hex address SSSS. If the address SSSS is omitted then the file is loaded to the address stored in its aux filetype.

*SAVE <objspec> SSSS EEEE - Save memory from hex address SSSS to hex address EEEE into file <objspec>. The start address SSSS is recorded in the aux filetype. (Note, no wildcards when saving.)

*RUN <*objspec*> - Load file filepath into memory at the address stored in its aux filetype and jump to to it. This is used for loading and starting machine code programs. You can also simply do *<*objspec*> or */<*objspec*> (the latter form is useful if the name is the same as that of a ROM routine.)

*DELETE <objspec> - Delete file <objspec> from disk. This command can also delete directories, provided they are empty. No wildcards are allowed.

*DESTROY <listspec> - Deletes multiple files as specified by <listspec>. For example, *DESTROY PROJECT/*.ASM.

*RENAME <objspec1> <objspec2> - Rename file or directory <objspec1> to <objspec2>. No wildcards are allowed.

*DRIVE <dry> - Switch to the specified physical drive. This is equivalent to using *DIR but does not allow subdirectories to be specified. The working directory will be set to the volume directory corresponding to the physical device specified.

*FREE [<dry>] - Shows blocks used and blocks free on the specified physical device. If no drive is specified, the free space on the drive corresponding to the current ProDOS prefix is returned.

*CDIR <objspec> - create directory dirname. *MKDIR is a synonym.

*ACCESS <listspec> attribs - change file permisions. The string attribs can contain any of the following:

• R - File is readable
• W - File is writeable
• L - File is locked against deletion, renaming and writing. For example: *ACCESS *.ASM WR

*COPY <listspec> <*objspec*> - Copy file(s). There are two forms of the *COPY command:

• *COPY <objspec> <*objspec*> - Copy a single file. The first argument must refer to a file and the second can be a file or a directory. If the target file exists and is writeable it will be overwritten. If a directory is specified as the destination then the file will be copied into the directory using the same filename. No wildcards are allowed in the source filename in this case. An example of this type of usage is *COPY TEXT/ABC.TXT ../BACKUPS/ABC.BACKUP.TXT
• *COPY <listspec> <*objspec*> - Copy multiple files. The first argument refers to a list of files, specified using wildcards. The second argument must refer to a directory. All the files included in the wildcard pattern will be copied into the destination directory. For example of copying multiple files is *COPY :71/DOCS/*.TXT :72/TEXTDIR
• Recall that @ or . may be used to specify the current working directory, while ^ or .. may be used to specify the parent directory.

*FX a[,x,y] - invokes OSBYTE MOS calls.

*OPT - sets file system options. *OPT 255,x may be used to enable or disable debugging output.

How to Build

Applecorn is built natively on the Apple //e using the Merlin 16 assembler v3.53 (requires a 65816 though.) It may also be built using Merlin-32 on Windows, Linux or Mac if preferred. The code should also assemble on Merlin-8 2.58 provided some of the longer comments are trimmed (Merlin-16 allows longer lines.)

In Merlin-16 (Merlin-8 in parenthesis, where different):

• Press D for disk commands and enter the prefix of the build directory: PFX /APPLECORN
• Press L to load a file and enter the filename APPLECORN.
• Merlin will enter the editor automatically (or press E). Open Apple-A starts assembly. (Merlin-8: Issue the following command a the editor's : prompt: asm
• Once assembly is complete, enter the command q to quit the editor.
• Press Q to quit Merlin.

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 $bff 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.

Limitations

Applecorn currently has the following limitations:

• Most MOS calls are implemented.
• There is file I/O support for file-orientated (OSFILE) file operations. This allows LOAD and SAVE to work in languages such as BASIC or COMAL.
• There is also support for the character orientated operations (OSFIND, OSBGET OSBPUT) which allows all the disk file operations in the Acorn languages to work correctly. For example in BBC BASIC the following commands work: OPENIN, OPENOUT, OPENUP, BGET# BPUT# PTR#=, EOF#, EXT#.
• Many OSBYTE and OSWORD calls are implemented.
• Special BBC Micro functions such as sound, A/D interfaces, programmable function keys and so on are currently not supported.