6 ms·

A: drive is 3.5; B: drive is 5.25; C: drive is hard disk

dspillett 3d ago
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.
- cesarb 3d ago
  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?)
  - Sharlin 3d ago
    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!)
- brewmarche 3d ago
  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.
  - jasomill 3d ago
    It does. https://github.com/microsoft/MS-DOS/blob/2d04cacc5322951f187bb17e017c12920ac8ebe2/v4.0-ozzie/bin/DISK2/BIOS/IBMBIO.ASM#L1428 https://github.com/microsoft/MS-DOS/blob/2d04cacc5322951f187...
  - dspillett 3d ago
    Yes, it did, or something did (maybe the individual commands rather than it being through the OS). I'd completely forgotten about that…
    - jasomill 3d ago
      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.
compiler-guy 3d ago
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.
- raldi 3d ago
  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)
- gumby 3d ago
  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!
  - LukeShu 3d ago
    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?
    - adrian_b 3d ago
      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.
alightsoul 3d ago
Yeah it's a relic from when computers booted off floppy and hard disks were rare and expensive

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.

cesarb 3d ago
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?)
- Sharlin 3d ago
  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!)
brewmarche 3d ago
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.
- jasomill 3d ago
  It does. https://github.com/microsoft/MS-DOS/blob/2d04cacc5322951f187bb17e017c12920ac8ebe2/v4.0-ozzie/bin/DISK2/BIOS/IBMBIO.ASM#L1428 https://github.com/microsoft/MS-DOS/blob/2d04cacc5322951f187...
- dspillett 3d ago
  Yes, it did, or something did (maybe the individual commands rather than it being through the OS). I'd completely forgotten about that…
  - jasomill 3d ago
    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.

compiler-guy 3d ago

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.

raldi 3d ago
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)
gumby 3d ago
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!
- LukeShu 3d ago
  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?
  - adrian_b 3d ago
    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.

alightsoul 3d ago

Yeah it's a relic from when computers booted off floppy and hard disks were rare and expensive