How did MS-DOS assign drive letters in the case of more than two floppy disk drives?

Question

The IBM PC BIOS had provisions for up to four floppy disk drives, by virtue of returning the number of floppy disk drives in a two-bit field plus a none/some flag (allowing for reporting values of 0, 1, 2, 3 or 4 floppy disk drives being installed). (See BIOS interrupt 11h, GET EQUIPMENT LIST, bits 0 and 7-6 of register AX as returned.)

Of course, most common configurations had one or two floppy disk drives installed. These were, intuitively enough, labelled A: and B:.

• What happened if you installed a third floppy disk drive in such a system?

• What happened if you did the same in a system that, in addition to the two floppy disk drives, also sported a hard disk? (For simplicity, you can assume that the hard disk held a single partition.)

Specifically, how were drive letters assigned by MS-DOS in each of these cases? Did the hard disk (which would ordinarily be C:) get pushed further into the alphabet (making it D: in the example above), or was the hard disk partition still referred to as C: and the third floppy disk drive became D:? Or some other combination?

Answer 1

As you might imagine, the answer depends on the version of DOS.

The Wikipedia article on the topic matches my memories.

Before MS-DOS 5.0

Before MS-DOS 5.0, the basic assignments are as follows:

• A: goes to the first floppy drive, if any, or to a “phantom” drive;
• B: goes to the second floppy drive, if any, or to a “virtual” floppy drive (the virtual floppy drive was mapped to A:, but DOS remembered whether the floppy in the drive was A: or B:, prompting the user to switch when necessary).

Then DOS assigns drive letters to any remaining floppy drives (up to four in total), then to the first primary partitions of the first two hard drives (if any), then to the logical partitions in the first extended partition of the hard drives, then to device drivers, then to TSRs. (Before MS-DOS 5.0, other hard drives and other primary partitions were ignored by default.)

Applying this to your two examples:

• adding a third floppy drive to a two-drive machine, with no hard drive, would result in floppies on A:, B: and C:;
• adding a third floppy drive to a two-drive machine with a hard drive would result in floppies on A:, B: and C:, and the hard drive on D:.

MS-DOS 5.0 and later

With MS-DOS 5.0 and later, the basic assignments are as follows:

• A: goes to the first floppy drive, if any;
• B: goes to the second floppy drive, if any, or to the “virtual” floppy drive if there is only one floppy drive, or to nothing if there is no floppy drive;
• C: always goes to the first primary partition on the first hard drive.

Then DOS assigns drive letters to the first primary partitions on the remaining hard drives, then to logical partitions, then to other primary partitions, then to other floppy drives (up to four altogether in practice), then to device drivers, then to TSRs (MSCDEX and network redirectors typically).

So a machine with one primary partition and four floppy drives would end up with floppies on A:, B:, D: and E:, and the hard drive on C:.

Applying this to your two examples:

• adding a third floppy drive to a two-drive machine, with no hard drive, would result in floppies on A:, B: and C:;
• adding a third floppy drive to a two-drive machine with a hard drive would result in floppies on A:, B: and D:, and the hard drive on C:.

Other combinations were of course possible. For a while I had a single-floppy-drive PC (no hard drive) with A: and B: mapped to the floppy drive, and C: a RAM drive. OEM versions of DOS could behave differently, and device drivers can pretty much do what they want (including replace existing drives).

Lots of software fails if the boot drive isn’t A: or C:. Drive assignments would typically get messed up by installing an additional hard drive (which would shift the logical partitions of the first hard drive by one); you could also run into trouble with network drives which might replace a local drive assignment.

Answer 2

While Int 11h may have been able to return up to four drives available, in practice I think you'll find it extremely difficult to find a system with more than two drives. Not impossible, just very difficult.

The reason is that for the overwhelming majority of DOS systems, a floppy cable would only support two drives, see this super-user answer for the details of why: https://superuser.com/questions/849079/what-is-the-little-twist-in-this-floppy-ribbon-cable-for

This means that in this situation, with only a single 34 pin header, you're stuck with a maximum of two floppy drives.

I'd make a guess that the ability of BIOS to return up to four was due to the heritage of DOS, being basically a clean room implementation of a CP/M like operating system for the 8086. CP/M did correctly support four drives on a single cable, since it didn't do the twist, but instead relied on setting jumpers on the drives. And there are four drive select pins on the standard 34 pin floppy cable: https://allpinouts.org/pinouts/connectors/data_storage/5-1-4-inch-floppy-34-pin/

The decision to support four drives in the BIOS was quite possibly made before IBM made the decision to avoid the jumper issue, configure all drives as B:, and then throw a twist in the cable to get the far end drive to respond to drive select 0 (i.e. A: )

So to get four drives on a DOS system, you'd need either a system that used a non-standard cable, i.e. straight through without the twist, and some fiddling with jumpers on the drives, or a motherboard with two floppy headers.

Granted, I came fairly late to the MS-DOS party, getting my first DOS PC in 1991, by which time hard drives were commonplace. However the fact that two floppies places the first hard drive partition at C: and the fact that this has been so for at least the last 30 years, and still is the de facto standard today, suggests very strongly to me that anything more than two floppy drives was extremely rare, if not non-existent.

Certainly possible, DOS was incredibly good at dealing with changing setups, notice how it progressed with no trouble at all from DSDD 360K drives up through 1.2mb, 1.44Mb and later 2.88Mb floppies.

Might be another interesting retrocomputing question to ask. "Did anyone ever make a DOS machine with more than two floppy drives?"