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The early CP/M and IBM PC style computers had no hard drive. You had one floppy drive, and that was it. Unless you spent another $1k or so on a second floppy drive, then your system was smokin'! If you only had one drive it was common to boot from one disk, put in the other disk with your programs and data, then run the program. Once the program finished, the computer would request that you reinsert the boot disk so you could use the command line again. Copying data from one disk to the other was a series of "Please insert source disk into drive A:... Please insert destination disk into drive A:... Please insert source disk into drive A:... . . ." By the time hard drives became cheap, the "expensive" computers typically had two floppy drives (one to boot and run common programs, one to save data and run specific programs). And so it was common for the motherboard hardware to support two floppy drives at fixed system addresses. Since it was built into the hardware, it was thought that building the same requirement into the OS was acceptable, and any hard drives added to the machine would start with disk C: and so forth. During the transition from 5.25" disks (which were actually, physically floppy) to 3.5" disks (which were encased in a harder plastic shell) it was common to have both drives in one system, and again it was supported on the motherboard with hardware, and in the OS at fixed addresses. As very few systems ran out of drive letters, it was not thought to be important to consider making those drives re-assignable in the OS until much later when drives were abstracted along with addresses due to the plug'n'play standard. A lot of software was developed since that time, and unfortunately much of it expected to see long-term storage on the C: drive. This includes the BIOS software that boots the computer. You can still attach two floppy drives, boot into DOS 6.1, and use it as you would have in the early 90's, with floppy drives A and B. So largely the reason for starting the hard drive at C is for backwards compatibility. While the OS has abstracted data storage to some degree, it still treats A and B differently, in such a way that allows them to be removed from the system without altering the OS, caching them differently, and due to early viruses treating their boot sector with more caution than the hard drive's boot sector. | |||||||||||||||
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A: and B: were used for floppy disks. Edit: Someone asked for pics so here's an 8", 5.25" and 3.5" floppy disk. (8" disks were not used as standard on IBM compatible personal computers.)
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Less an answer, more of an anecdote. In this Microsoft article, it says:
So when I built a new computer recently with two internal drives, one for the OS and one for data, I thought, hey!, I'll make my data drive "A". I felt all rebellious until I discovered that Windows will not index drives lettered A or B. :( Took me quite a while to figure out what the problem was, but I found some other people who suffered the same issue when they used A or B for a drive. As soon as I assigned that drive a different letter, windows indexed the drive. So much for being rebellious. lol. | |||||||||||||||||
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"Drive letters A and B are reserved for floppy disk drives. However, if your computer does not have a floppy disk drive, you can assign these letters to removable drives." Source: http://support.microsoft.com/default.aspx?scid=kb;en-us;307844 | |||||||||||||||
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Ah... the good ole days. A: was the first disk device, B: the second, and so on - in CP/M. As somebody else posted, this ran on the 8-bit 8080 and Z-80 machines that pre-dated MS-DOS. MS-DOS in turn was a 16-bit (8086) knock-off clone of CP/M, or more specifically, CP/M-86, so it used the same drive letter conventions. Back in those days the only conventions that were pretty much universal were set by CP/M (for example, the disk naming on the Apple-2 and Tandy TRS-80 was something different again... I once used a TRS-80 with 4 floppy drives... oh the power!). With something like CP/M the first hard drives, when they came out, just appeared as the next available drive letter. MS-DOS, for reasons best known to itself, allocated 2 floppy drives pretty much universally and placed the hard drive at C: This is PROBABLY because the early IBM-PC's running MS-DOS all had 2 floppy drives, so this was a natural configuration. This convention has been preserved since, though as pointed out here numerous times - in windows - you can change this. As far as I know in MS-DOS you can't. Slightly off topic: Back in the 1980's it was pretty common on these early machines for disks to be a mixed bag of different types with legend incompatibilities. The IBM-PC imposed a bunch of standards with the format of its 5.25 inch floppies - before that you had all sorts of different arrangements with hard and soft sectoring, number of sectors / track, number of tracks, etc etc. Data interchange between machines using floppy disks was a very hit and miss affair, which only got sorted with MS-DOS and the IBM-PC. The most reliable way to transfer files between machines back then was to write some programs (in assembler) to to file transfers using the serial port, and then wire them up. It was never very slow, because files were never very big. My favourites from then were the high density drive my employer bought - it was an 8 inch floppy attached by a huge long ribbon cable. I think it stored about 200K, which was huge when developing software on a machine with 2 floppy drives, each of them 89K. Compiler on one, source and object code on the other. A compilation took minutes while the drives clanked away. At the time the IBM PC was introduced you could get it running either CP/M-86 or MS-DOS, and there was no clear indication that one or the other would win. Eventually MS-DOS won the day - some bundling deals might have had a bit to do with that. The first IBM machine my employer bought did make things a lot easier... and the port of stuff to MS-DOS was pretty easy - mainly because of the same conventions for disk drive letters, and also because the system calls to read / write files and populate file control blocks (remember FCBs and the weird format they had in memory?) was the same. | |||||||||
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A and B were used for floppy disk drives. As far as I'm aware, there are two because the older computers (with no hard drives) would boot the OS from A:\ and run applications from B:\ | |||||||||||||||||||||
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They are floppy drives. If you're curious to the order of the drive assignments, wikipedia has more information http://en.wikipedia.org/wiki/Drive_letter_assignment | ||||
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A: is the 5.25" 1.2Mb removable magnetic media, and B: is a more modern 3.5" 1.44Mb drive in my coding machine. | |||||||||||||||||||||
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The two horizontal slots on the front of this PC are the A: and B: drives (5.25" floppies in this case). Note that you had to physically "mount" the drive by turning the lever on the front of the drive. Ahh, simpler times.
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If I remember correctly on my DOS systems they were used for floppy drives but It was a virtual floppy used when copying a floppy when there was only one drive. So you would copy from | |||||||||||||||
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A: was the boot device in the 8-bit CP/M operating system. It predates even the earliest MS-DOS. If you had a second drive, it was B:. A third would be C:, etc. Systems didn't have hard drives then. CP/M was originally targeted to the 8080 processor. CP/M was created in the 1970s. The very first small winchester disk drive was the ST-506 which boasted a generous 5 MB (five megabytes) of storage. They were expensive but they were fast (compared to what we were used to). Oh, and the primitive file system didn't have folders or subdirectories. There was no A:\. It was just A:. | |||||
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On my first PC in 1992, A: was the 3.5" floppy, and B: was the 5.25" old-style floppy. Other machines had two 3.5" drives, and having two floppies was fairly common, hence hard drives starting at C: to leave room. | ||||
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I feel old and I'm only 26. A and B traditionally are floppy drives. C traditionally is the main disk and D traditionally the CD-ROM My computer at home has CDEF as hard drives and G as DVD RW | |||||||
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They are a legacy from the DOS days - A: and B: were the floppy disk drives that were standard on the original IBM PC. C:, the hard drive, was the third disk. Today, of course, nobody has floppies. | |||||||||||||||||
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A:\ and B:\ still defaults to floppy drives, of course only visible if you have one installed (only have that for fun in some of my virtual machines) | ||||
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The first pc I used professionally was an Apricot Xen. The internal hard drive was A: and the 3.5" floppy was B:. This was back in the day of MS-DOS 2.11, before Ibm pcs came along. | |||||||||||||
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