It's been a few weeks since I've updated here as I've not had as much time to spend on the project as I'd like, but some progress has been made on the software side - I'm now using a proper toolchain for the firmware, consisting of GCC and VASM, so no more Easy68k weirdness to trip me up!
I've also fleshed out the Serial output via the MC68901 such that it is now completely interrupt driven. This replaces the polling it was doing previously. Whenever the software writes to the serial output, the data is placed into a ring buffer and the MC68901's vectored interrupts drive the actual output from there.
The serial driver is implemented partly in a mixture of C and assembly - the C code is here and the assembly is here. The top level directory for the firmware is here and contains all the code, the Make build, and bits and bobs like the linker script that ties everything together.
This was all done in a bit of a rush as I was giving a talk on the computer (and demoing it) at work so I wanted to have some level of sophistication. It's all since been cleaned up and is now pretty nice (in my opinion). The next step in the plan is to enable the RX serial line, and then put in a simple protocol (probably xmodem initially) to allow software to be uploaded via serial link, freeing me from the 16K ROM limit and allowing me to develop the OS without pulling the ROMS for programming every time I iterate :)