Groundwork for BACKTRACK support

BACKTRACK support is (or rather, will be) a feature of the PCjs Debugger that makes it possible to track the sources of memory contents.

Part of the support for this feature comes from the Disk module, which is now able to read FAT volumes and create sector-to-file mappings. Next, the DMA component (part of the ChipSet module), which is involved in sector transfers from diskettes (and XT hard disks) to memory, creates byte-to-sector mappings. With these pieces in place, the Debugger will be able to "backtrack" the contents of any memory location containing file data to the original file name and position.

BACKTRACK support will include additional backtracking features (more than just file tracking) so stay tuned. For now, it's disabled in all compiled releases, as it currently serves no purpose in the non-DEBUGGER release, and even in the DEBUGGER release, there are no new Debugger commands (yet) to take advantage of it. The support is available only if you download/install/run the uncompiled PCjs code.

As you can imagine, this is a somewhat "expensive" feature, both in terms of increased memory requirements and operational overhead, so it will be a feature you must explicitly enable. On most machines, however, the overhead shouldn't be noticeable, and the simulations should still run fine at normal speed.