An Operating System for the "Model C Micro" and "C20K" and the 65816 which allows existing BBC Micro / Master programs and ROMs to coexist with full native mode 65816 executables

The model-c-mos is a proof of concept operating system that presents a new clean API using the 65816's COP instruction to perform system calls. The calls are loosely based on those of the MOS developed for the Acorn Communicator in the prototype phase, though that is likely to change if a proper full-blown OS is developed.

The driver for the the model-c-mos is twofold:

• to allow new user applications to break the 64K limit of the 6502 and grow into the 16MB address space of the 65186.
• to allow existing filing system ROMS, applications and games to run in a pseudo-virtual environment

Aims:

• existing games, applications and ROMs to be able to run
• new applications can access 8-bit filing systems
• 8-bit applications can access 16-bit filing systems

16-bit applications in general will use the COP call mechanism to access system resources, where these are provided by an 8-bit ROM extension, for example an existing filing system the 16-bit OS will transform the call into one or more 8-bit system calls and use the 8-bit vectors and ROM switching to pass the call to the 8-bit ROM

Conversely 8-bit applications can call the 8-bit API and if the services i.e. filing system is provided as a 16-bit module the call will be passed up to the 16-bit filing system.

One of the main challenges is to handle interrupts in a way which is efficient and allows interrupts to occur in either 8-bit (emulation) or 16-bit (native) mode seamlessly. Interrupts must also be passed round the 16-bit modules and 8-bit ROMS where relevant.

The current PoC code manages this, but with a fair amount of overhead.

The 65186 allows for larger program stacks (up to 64K) and for the Direct Page to be located at different locations within bank 0. This provides a good deal of flexibility and allows for different processes to have different stacks which is a boon for writing multi-tasking operating systems

One of the challenges in future will be in handling the switching of stacks and stack contents between the 8 and 16-bit arenas, currently some of the stack contents must be copied on switches from 8- to 16-bit modes which is costly.

• 8-bit filing systems supports
• 8-bit ROMs and service calls
• nascent 16-bit modules (keyboard, VDU)
• COP call dispatching some services implemented (VDU, memory management)
• 8-bit OSBYTE/OSWORD emulation (pass back to 16-bit APIs)

• 8-bit filing systems load to 16MB memory space not supported, WINDOW!
• 16-bit filing systems not yet defined
• executable load format (relocatable o65 or a.out format?)
• eveything else!