uefix.s: add CHS mode

[sirocyl]

This should enable booting a sector payload from an FDD, and so enabling -D FLOPPY on the yasm line will switch to CHS drive logic.

[sirocyl]

Note: This also opens the path for supporting vintage 8088-and-up IBM PC-compatibles, as they do not have INT13h/AH=42 capability, only the old-standard AH=02 CHS.

While I'm submitting this for uefix, as its changes are low-impact and enable the single-sector Forth to load on such machines (albeit without a functioning LOAD keyword on those machines; and AUTOLOAD disabled), I probably will maintain the vintage-and-floppy-compatible miniforth as a fork, since it changes a lot with regards to the LOAD keyword, and the way --> block continuation works. I'll see how it develops though! I might be able to do it without mucking up the codebase too much.

[meithecatte]

Nice! Though, uefix is only necessary to work around BIOSes that insist on parsing the partition table portion of the bootsector. In practice, it's only the new UEFI firmware. Floppies commonly didn't have a partition table, so you shouldn't ever need to use uefix on a floppy.

As long as you patch the filesystem code, that is ;3

[meithecatte]

Perhaps check whether the drive number provided by BIOS is less than 0x80, instead of hardcoding this at compile time?

[sirocyl]

It was quick and dirty, but honestly that's probably a better approach. I'll hop on this and the 8 = HDD comment when I get back from breakfast.

Register DL contains the disk number on boot, so test 0x80, dl and jnz load.lba seems correct here, right?

[sirocyl]

Nice! Though, uefix is only necessary to work around BIOSes that insist on parsing the partition table portion of the bootsector. In practice, it's only the new UEFI firmware. Floppies commonly didn't have a partition table, so you shouldn't ever need to use uefix on a floppy.

I've dealt with funky vendorized vintage PC-compatible BIOSes that expect more than a 55 AA. Toshiba or Tandy acts like this, I believe.
To be clear, no modern BIOS is that weird - they all just see the 55AA signature, and load and run.

As long as you patch the filesystem code, that is ;3

I don't think I'll need to patch the filesystem code, if it's already LBA-aware.

I'll explain over in the #6 issue how I'll implement the load CHS compatibility.

Anyway, about to go in, fix and test the fixes here in a minute.