So! In Parts 1 and 2 I covered getting this old thing cleaned up, getting it booting, hacking together a boot floppy disc and getting a working file transfer onto a work floppy disc.
Today I'm going to cover what it took to get it going off of a "hard disk", which in 2020 can look quite a bit different than 1990.
First up - what's the hard disk? Well, in the 90s we could still get plenty of MFM and early IDE hard disks to throw into systems like this. In 2020, well, we can get Very Large IDE disks from NOS (like multi hundred gigabyte parallel ATA interface devices), but BIOSes tend to not like them. The MFM disks are .. well, kinda dead. It's understandable - they didn't exactly build them for a 40 year shelf life.
The IBM PC, like most computers at the time, allow for peripherals to include software support in ROM for the hardware you're trying to use. For example, my PC/AT BIOS doesn't know anything about IDE hardware - it only knows about ye olde ST-412/ST-506 Winchester/MFM drive controllers. But contemporary IDE hardware would include the driver code for the BIOS in an on-board ROM, which the BIOS would enumerate and use. Other drive interconnects such as SCSI did the same thing.
By the time the 80386's were out, basic dumb IDE was pretty well supported in BIOSes as, well, IDE is really code for "let's expose some of the 16 bit ISA bus on a 40 pin ribbon cable to the drives". But, more about that later.
Luckily some electronics minded folk have gone and implemented alternatives that we can use. Notably:
- There's now an open-source "Universe IDE BIOS" available for computers that don't have IDE support in their BIOS - notably PC/XT and PC/AT, and
- There are plenty of projects out there which break out the IDE bus on an XT or AT ISA bus - I'm using XT-IDE.