* Import unmodified USE!UMBS source.
From https://forum.vcfed.org/index.php?threads/loading-dos-high-on-a-xt.32320/
* Customize the UMB Driver prompts.
* Refactor memory management. Allow activation of EMS/UMB on demand.
* Update the XTEMM and XTUMBS drivers to map their ranges.
* Add instructions for all the drivers.
* Revert "Some more refactor of the Teensy code."
This reverts commit e03594b514b7d1e41af70f44ef220a1571d88deb.
* Fix issue booting from other fixed disk.
* Revert "Uploaded_2_9_2025"
This reverts commit ec79bbdbc9052617461a3c1b4952290a05a51adb.
* Fixing DEBUG_IO builds.
* Make the IO port base a define.
* Fix CPU test to properly handle 808x vs V20.
* Improve timeout management logic.
* Improve SD Card initialization logic.
* Implement INT18h bootstrapping.
* Implementing a different test for processor type.
* Allow use with another fixed drive (eg: ESDI drive).
* Allow swapping driver ID between the BIOS disk and XTMax.
* Fix address of the ROM segment.
* Fixing missing STI at the top of the handler.
* Major rework of IO to use MOVSW.
* More code reorg and auto-detecting second drive.
* Update XTSD to use MOVSW.
* Add diagnostics program.
* Workaround for MS-DOS strange handling of interrupts.
* Use a lookup table for the memory map.
* Make the BootROM relocatable.
* Some more refactor of the Teensy code.
