Acorn 6809 Monitor

Acorn have provided a 6809 Monitor program for the 6809 CPU Board which is listed in the 6809 User Manual, this is intended to be used with the 40-column Teletext VDU and ASCII keyboard, it is also capable of booting the 6809 FLEX Operating System from a floppy disk using the Floppy Disk Controller.

It provides the following single character commands:

• 'G'    Go to program

• 'M'   Memory examine

• 'R'    Examine registers

• 'P'    Proceed after break

• 'T'    Set trace number

• 'S'    Save on cassette

• 'L'    Load from cassette

• 'V'    Set break address

• 'D'    Disc bootstrap

• 'C'    Copy to printer

• '.'      Do trace operation

• 'F'    Finish file load

Refer to the 6809 User Manual for more details

Memory Configuration

The original, as listed in the 'Acorn 6809 User's Manual' is intended to be used with the 40-column Teletext VDU board and has the I/O and RAM memory arranged as in the normal System Computer set-up:

• the VIA, VDU and optional FDC are located in low memory from $0800

• the Monitor itself occupies 2K bytes at address $F000

• RAM in page zero is the Monitors workspace

• RAM from $2000+ is avaliable for user programs

 For FLEX a different arrangement is required:

• The VIA, VDU and optional FDC are moved up to $E800

• the Monitor's workspace moves to $E000

• FLEX is then usually loaded from disk to RAM at $C000

• RAM at $0000 onwards is avaliable for user programs

Here are a couple of .pdf diagrams which show the two configurations:

• Acorn 6809 System Memory Map

• Acorn 6809 FLEX Memory Map

Versions

The original Acorn Monitor listing is identified as version V49 (dated Feb 1980), I assume this is actually V4.9.

I also have what must be a later version with various small modifications throughout, though this doesn't seem to work ?  I have called this version 4.95 because there is also a version 5.2 from Control Universal.
This must be from after they took over the Eurocard business from Acorn so I'm assuming they may have started at version 5.0 !

The Control Universal Version is quite different because it works with their versions of FDC and VDU boards which are not compatible with the original Acorn boards so for the moment at least I have not incorporated it into my source code.

Assembler Files

I have formatted the original Monitor source code for the SBASM Assembler, this single source file can be configured to produce builds for the 40 or 80-column VDUs and for either memory mapping.

• Monitor.asm

and here are some builds, the first are for the standard memory map and the second for the FLEX memory map:

• monitor40.bin (for 40-column Teletext VDU)

• monitor40.list

• monitor41.bin (for 40-column Teletext VDU with alternative screen configuration to match System COS/DOS)

• monitor41.list

• monitor80.bin (for 80-column VDU)

• monitor80.list

• flex40.bin (for 40-column Teletext VDU)

• flex40.list

• flex41.bin (for 40-column Teletext VDU with alternative screen configuration to match System COS/DOS)

• flex41.list

• flex80.bin (for 80-column VDU)

• flex80.list

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