MiSTer PC XT BIOS

[n0p]

Here's my small contribution to this project:
MiSTer PC XT BIOS
It's a fork of well known 8088_bios (https://github.com/skiselev/8088_bios)

MiSTer PC XT related enhancements:

• Memory size set to 704KB. So if A000h UMB enabled, 704KB memory will be available to system without any additional steps.
• Read-only RTC functions implemented in BIOS INT 1Ah, using service port 2C0h in MiSTer PC XT.
  DOS 3.31 (Compaq OEM) version that i use, detects and uses this functionality.

General:

• BIOS setup support. I use MBR sector (sector 0) on first IDE drive for that. Press F1 on boot to enter setup menu. Floppy drive types, memory test, extension scan and boot sound can be set there.
• PWM floppy sound on OS boot (can be disabled via setup)

Known issues:

• Setup menu can be entered only on cold boot.
• Version without XT-IDE Universal BIOS boots floppy only on cold boot as well.

Archive contents:

• bios-mister-m.rom: BIOS contains XT-IDE Universal BIOS (XUB) configured to use only master IDE drive.
• bios-mister-s.rom: XUB configured to use master and slave IDE drive.
• bios-mister-e.rom: Only BIOS, no XUB injected.

I consider this beta, but should work stable enough.

[Bas]

Can I ask where in sector 0 you store BIOS data? How does this work, and is this idea also present in actual hardware from the 1980's?

[n0p]

Can I ask where in sector 0 you store BIOS data? How does this work, and is this idea also present in actual hardware from the 1980's?

Offset 190h, four bytes - id, two config bytes and checksum. It's checked to be clear (zeroes) or correct id/checksum before any operations, so if that area occupied, nothing will be written.
As for original hardware - i directly interface IDE device for that, so if XT-IDE (v1) if present there, it will work. As my code was originally designed to work on Book8088, code can be changed to work with XT-CF card.