Additional ROM patches manually set the memTop variable to a given
memory size, and the memory test routine is skipped. This allows
the Plus ROM to cope with a Mac 256K configuration, but also non-
power-of-two sizes. That's useful for an MCU, making a Mac 208K,
Mac 192K, etc. possible.
Memory addresses are clamped such that wrapping within the RAM
region works, and any emulated address can be used without
overflowing the host RAM allocation. For a power-of-two memory
size this boils down (after compiler optimisation) to a plain
tmask; for non-Po2, this involves a divide/mod in the weird ROM
probing "off the end" cases, and a simple compare in all other
runtime cases (so shouldn't be too slow).
Instead of relying on the disc data being directly mapped, and
accessing blocks by memcpy'ing from it, provide op_read/op_write
callbacks so that the host can do disc ops itself (e.g. access SD).
The disc_descr_t describes the callbacks (and an opaque pointer
passed as an argument when they are invoked), per-disc.
...slightly. This shows the sync/hunt status, and TxUnderrun, which
are expected by system 7's startup. (It enables SDLC mode and probes
stuff.)
With this (and increasing memory, e.g. 4MB), umac boots 7.5.5 :)
The unix_main gets another option, -w, that enables the disc
mapping to reflect writes back to the backing file.
The disc emulation code now handles writes, which are copied
back to the disc data.
This still doesn't abstract read/write ops to disc, which will
be the next step (to support, for example, block-based storage).
