A: and B: were not specific to the type of drive. It was common to have two drives before fixed storage became common, often you would have the application disk in one and your data disk in the other though there were other common use patterns for two drives also (with the OS, or at least the core of it, resident in memory you can copy and otherwise manage data over two data disks, and so on).
The first hard-drive in a system was made C: to reserve A: and B: in part because there was software out there that assumed A: and B: were floppy drives and could cause problems if something else was allocated to those signifiers. There are many things that are due to long forgotten compatibility issues like this (try naming file LPT1 under Windows to see another). Another reason is that the BIOS on many PCs was just hard-wired to assume two floppy drives so DOS would see that even if there were no drives really there.
From what I recall from the time I still used MS-DOS, even if you only have one floppy disk drive, it's accessible as both A: and B: and it prompts you to insert the other floppy disk when you change the drive letter. That is, it's "virtualizing" two floppy disk drives using a single physical one. That explains why the first hard disk drive was always C: even when you only had one floppy disk drive (and of course you had at least one floppy disk drive, how could you use a computer without one?)
> and of course you had at least one floppy disk drive, how could you use a computer without one?
They were relatively rare, but there absolutely were DOS-era machines without a floppy drive – found in schools, corporations, government agencies.
The most common implementation used Novell NetWare's RPL protocol to boot using an Ethernet card with an installed RPL BOOT ROM. The machine didn't need any disk drives (hard disk or floppy) at all, it loaded MS-DOS off a NetWare server, which was also used for all files or data.
You could also install a hard disk for local storage, without installing a floppy drive, but the more common configuration was no disks at all.
Also, the original IBM PC supported external floppy drives; as the 80s went on, external floppy connectors became increasingly rare, but if you specifically wanted an ISA FDC card with an external connector, they were still making new ones into the 1990s. (These didn't use anything like USB; they took the signals of the internal floppy ribbon cable, and surfaced them on a port on the rear of the card.) So, you could have a machine with a hard disk but without a floppy most of the time, and you'd plug in an external drive when you needed it, and you could keep it locked in a cupboard out of the hands of the plebian users the rest of the time. I doubt many people did this – diskless workstations were a lot less work – but probably somebody did at some point.
> of course you had at least one floppy disk drive, how could you use a computer without one?
IBM PCs, even into the PS/2 days, would boot into Cassette BASIC if you didn't have a floppy drive or boot disk inserted: https://en.wikipedia.org/wiki/IBM_BASIC
My high school had the PS/2 Model 25 SX, introduced in 1992, without hard drives. All of the software ran off of a Novell NetWare file server, but required a bootable floppy disk. If you removed the floppy disk, they would boot into the built-in BASIC from the machine's ROM.
Which is very convenient (and necessary for backwards compatibility) because programs could be (and were) written to assume they're in B: (and the DOS disk is in A:), or that they're in A: and the user's data is in B:, or a two-disk program is in both. All of that just worked (tm) even with a single drive, as long as the program didn't try to interleave accesses to both drives (I think it would still work because DOS was single-process, single-thread, it would just annoy the user!)
Both of these things are true: commands like diskcopy have special handling to allow the same drive to be used as both source and destination, and the DOS I/O has special handling to request disk changes from A: to B: on single-drive systems for programs that lack specific support.
Even further back into time, before 3.5" disks, both A: and B: were 5.25" disks. And, although my memory is hazy, 3.5's were commonly slotted into B: at first, because no one had 3.5" boot disks until the drives became somewhat common.
You are forgetting 8” floppies. IBM used only the soft sector ones (one hole punched close to the spindle to mark sector 0) but there were also hard sector ones (a ring of 32 holes close to the spindle hole).
There was a lot of experimentation back in those days. The Wikipedia’s page on floppy disks is surprisingly long!
8" floppy drives were not compatible mechanically with IBM PC cases, but there were 8" floppy drives with their own enclosures and power supplies, which could be put on a desktop along the PC case.
IBM PCs have never supported 8" floppy drives, but it was easy to make an adapter between IBM PC floppy cables and 8" floppy drives, so there have existed IBM PC clones from countries where 5¼" floppies were scarce (e.g. Eastern Europe), which supported the attachment of 8" floppy drives.
The original 5¼" floppies had only the size advantage, but they had both a lower capacity and a lower speed than 8" floppies, so attaching 8" floppy drives would have been a higher performance option in the beginning.
Only after the IBM PC/AT introduced the high-density 1.2 Mbyte floppy disks, the 5¼" format matched (actually very slightly exceeded) the performance of the old 8" floppies.
But in the context of drive lettering, although CP/M supported 8" floppies, MS-DOS never did. MS-DOS adopted the naming scheme, but not the boot style.
Floppies, superfloppies, removable hard disks (platter-only, as opposed to modern external hard drives that incorporate the entire drive mechanism), magneto-optical disks, rewritable CD/DVD, even today magnetic tape is still used as removable storage in applications where large capacity is required and shelf life is more important than fast random access.
The floppies got A and B because hard drives were expensive and for a time a lot of people got by without them, though everybody had at least one floppy.
Even if you only had one floppy, it was both A and B, so you could say "copy a:DOC.TXT b:" and it would read the doc and then ask you to insert the floppy you're considering B (and depending on file size and available memory, sometimes switch back and forth a few more times)
I have vague memories of something like this from my childhood, where my family's IBM 486SX had the 3.5" drive on A and the 5.25" drive on B but my grandparents' Epson 286 had the opposite. Also learning about disk densities when none of the disks from the 486 worked on the 286.
A: drive is 3.5; B: drive is 5.25; C: drive is hard disk
A: and B: were not specific to the type of drive. It was common to have two drives before fixed storage became common, often you would have the application disk in one and your data disk in the other though there were other common use patterns for two drives also (with the OS, or at least the core of it, resident in memory you can copy and otherwise manage data over two data disks, and so on).
The first hard-drive in a system was made C: to reserve A: and B: in part because there was software out there that assumed A: and B: were floppy drives and could cause problems if something else was allocated to those signifiers. There are many things that are due to long forgotten compatibility issues like this (try naming file LPT1 under Windows to see another). Another reason is that the BIOS on many PCs was just hard-wired to assume two floppy drives so DOS would see that even if there were no drives really there.
From what I recall from the time I still used MS-DOS, even if you only have one floppy disk drive, it's accessible as both A: and B: and it prompts you to insert the other floppy disk when you change the drive letter. That is, it's "virtualizing" two floppy disk drives using a single physical one. That explains why the first hard disk drive was always C: even when you only had one floppy disk drive (and of course you had at least one floppy disk drive, how could you use a computer without one?)
> and of course you had at least one floppy disk drive, how could you use a computer without one?
They were relatively rare, but there absolutely were DOS-era machines without a floppy drive – found in schools, corporations, government agencies.
The most common implementation used Novell NetWare's RPL protocol to boot using an Ethernet card with an installed RPL BOOT ROM. The machine didn't need any disk drives (hard disk or floppy) at all, it loaded MS-DOS off a NetWare server, which was also used for all files or data.
You could also install a hard disk for local storage, without installing a floppy drive, but the more common configuration was no disks at all.
Also, the original IBM PC supported external floppy drives; as the 80s went on, external floppy connectors became increasingly rare, but if you specifically wanted an ISA FDC card with an external connector, they were still making new ones into the 1990s. (These didn't use anything like USB; they took the signals of the internal floppy ribbon cable, and surfaced them on a port on the rear of the card.) So, you could have a machine with a hard disk but without a floppy most of the time, and you'd plug in an external drive when you needed it, and you could keep it locked in a cupboard out of the hands of the plebian users the rest of the time. I doubt many people did this – diskless workstations were a lot less work – but probably somebody did at some point.
> of course you had at least one floppy disk drive, how could you use a computer without one?
IBM PCs, even into the PS/2 days, would boot into Cassette BASIC if you didn't have a floppy drive or boot disk inserted: https://en.wikipedia.org/wiki/IBM_BASIC
My high school had the PS/2 Model 25 SX, introduced in 1992, without hard drives. All of the software ran off of a Novell NetWare file server, but required a bootable floppy disk. If you removed the floppy disk, they would boot into the built-in BASIC from the machine's ROM.
Which is very convenient (and necessary for backwards compatibility) because programs could be (and were) written to assume they're in B: (and the DOS disk is in A:), or that they're in A: and the user's data is in B:, or a two-disk program is in both. All of that just worked (tm) even with a single drive, as long as the program didn't try to interleave accesses to both drives (I think it would still work because DOS was single-process, single-thread, it would just annoy the user!)
I thought MS-DOS had special handling for A: and B: since it allowed you to copy from A: to B: even with just one floppy drive.
It does.
https://github.com/microsoft/MS-DOS/blob/2d04cacc5322951f187...
Yes, it did, or something did (maybe the individual commands rather than it being through the OS). I'd completely forgotten about that…
Both of these things are true: commands like diskcopy have special handling to allow the same drive to be used as both source and destination, and the DOS I/O has special handling to request disk changes from A: to B: on single-drive systems for programs that lack specific support.
Even further back into time, before 3.5" disks, both A: and B: were 5.25" disks. And, although my memory is hazy, 3.5's were commonly slotted into B: at first, because no one had 3.5" boot disks until the drives became somewhat common.
You are forgetting 8” floppies. IBM used only the soft sector ones (one hole punched close to the spindle to mark sector 0) but there were also hard sector ones (a ring of 32 holes close to the spindle hole).
There was a lot of experimentation back in those days. The Wikipedia’s page on floppy disks is surprisingly long!
But those weren't on "IBM PC"-compatibles, just older micros? The 5150 had 5¼" drives. That is: (Q|MS|PC)-DOS never supported 8" floppies, right?
8" floppy drives were not compatible mechanically with IBM PC cases, but there were 8" floppy drives with their own enclosures and power supplies, which could be put on a desktop along the PC case.
IBM PCs have never supported 8" floppy drives, but it was easy to make an adapter between IBM PC floppy cables and 8" floppy drives, so there have existed IBM PC clones from countries where 5¼" floppies were scarce (e.g. Eastern Europe), which supported the attachment of 8" floppy drives.
The original 5¼" floppies had only the size advantage, but they had both a lower capacity and a lower speed than 8" floppies, so attaching 8" floppy drives would have been a higher performance option in the beginning.
Only after the IBM PC/AT introduced the high-density 1.2 Mbyte floppy disks, the 5¼" format matched (actually very slightly exceeded) the performance of the old 8" floppies.
MS-DOS 1.25 supported 8" disks. I think 2.0 still did.
Trust me, I will never forget 8" floppies.
But in the context of drive lettering, although CP/M supported 8" floppies, MS-DOS never did. MS-DOS adopted the naming scheme, but not the boot style.
MS-DOS 1.25 supported 8" disks. I think 2.0 still did.
Floppies, superfloppies, removable hard disks (platter-only, as opposed to modern external hard drives that incorporate the entire drive mechanism), magneto-optical disks, rewritable CD/DVD, even today magnetic tape is still used as removable storage in applications where large capacity is required and shelf life is more important than fast random access.
The floppies got A and B because hard drives were expensive and for a time a lot of people got by without them, though everybody had at least one floppy.
Even if you only had one floppy, it was both A and B, so you could say "copy a:DOC.TXT b:" and it would read the doc and then ask you to insert the floppy you're considering B (and depending on file size and available memory, sometimes switch back and forth a few more times)
I have vague memories of something like this from my childhood, where my family's IBM 486SX had the 3.5" drive on A and the 5.25" drive on B but my grandparents' Epson 286 had the opposite. Also learning about disk densities when none of the disks from the 486 worked on the 286.
Yeah it's a relic from when computers booted off floppy and hard disks were rare and expensive
MS-DOS didn't have built in support for 3.5" drives until version 3.2.