Why “C” is the Default Hard Drive Letter in Many Computers

Julie N. asks: Why is “C” the default drive letter in computers?

hard-driveFor nearly as long as hard disk drives have been placed in personal computers running certain popular operating systems (notably MS-DOS/Windows), the primary hard disk has been designated with the letter “C”.  But why?

The idea for designating different storage devices with simple letters is generally attributed to IBM’s virtual machine operating systems developed in the 1960s, starting with their CP-40 and CP/CMS systems, and later very notably, among others, copied by the CP/M operating system created by the company Digital Research, Inc. In the early systems (CP/CMS) the letters were used mostly to designated logical drives, although later (such as with CP/M), they were used to specify physical storage devices.

This all brings us to 1980 when IBM attempted to use the relatively popular CP/M operating system on the IBM Personal Computer.  Talks broke down between IBM and Digital Research, Inc, for reasons not totally clear today. The trouble is rumored to have started when Dorothy Kildall, the wife of CP/M creator Gary Kildall, refused to sign a non-disclosure agreement with IBM at the start of the negotiations. She supposedly told them she would not sign such a document without speaking with her husband first, who was out of town on business. This was a somewhat unusual move as Gary often left such business negotiations to her anyway.

This refusal to sign the non-disclosure agreement, which purportedly greatly annoyed the IBM representatives, was all supposedly at the advice of Gerry Davis, Digital Research’s attorney. But given this sort of thing is standard practice for many business negotiations, the whole thing seems decidedly odd looking back, with those involved not helping with their conflicting accounts.

What happened after isn’t any clearer. Gary Kildall later claimed, upon returning from his little business trip, he and his wife reached a handshake agreement with IBM’s representative, Jack Sams, while aboard a flight to a vacation the couple were taking. He claimed IBM didn’t honor that agreement.  Sams said that none of that ever happened.

Whatever the case, what we do know for sure is that IBM moved on from the then relatively popular CP/M to instead dealing with Microsoft, who in turn purchased a license to a CP/M clone called 86-DOS. They then adapted 86-DOS for IBM’s new PC, with a few significant changes thrown in, and branded it MS-DOS, though called PC DOS by IBM.

Being based on a CP/M clone, among other things, MS-DOS borrowed the disk drive lettering schema from CP/M, which had borrowed it from the aforementioned previous IBM systems. By copying many elements of the CP/M system, it allowed popular software packages that could run on CP/M to be relatively easily ported over to MS-DOS and used on the new IBM PC.

This all brings us back to the specific drive lettering schema. Early PCs didn’t usually come with internal mass-storage devices due to the expense (though HDDs had been around since the 1950s). Instead, they generally had some form of a “floppy” disk reader, such as those used to read 5 1/4″ floppy disks, initially labeled as “A” in MS-DOS and certain other operating systems. Some systems came with two such floppy disk drives necessitating the need for a “B”. When the 3.5″ floppy disk (which wasn’t actually floppy at all unless you took it apart to get at its innards) was commonly added, using both “A” and “B” for floppy drives was firmly entrenched.

When hard disk drives became standard in most PCs in the later 1980s, since the first two letters were already commonly used for these floppy drives, they logically labeled the third storage device “C”, even though it now tended to be the main storage medium for the computer, including usually containing the operating system.

Despite that exceptionally few systems today still contain floppy disk drives, this schema of drive designation has stuck around anyway, with “A” and “B” often still by default reserved for floppy drives. Of course, these letters aren’t set in stone on modern systems and you can easily change, remove or add drive letters (representing both physical and logical drives) if you have administrative rights.

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Bonus Facts:

  • Notably, UNIX based systems (and similar, such as Linux-based) do not use drive letters, but rather a single hierarchical setup. So, for example, the root of the hierarchy is simply “/” instead of “C:”.  “/home” might really be its own separate physical disk (or logical) drive mounted to that point in the hierarchy or might not.  Similarly, any physical or logical drive can be mounted just about anywhere on this hierarchy.
  • MS-DOS didn’t always use “C” as the default for the hard drive on every system.  For instance, on the Apricot PC released in 1983, “A” and “B” were reserved for hard drives and “C” and “D” for floppy drives.
  • Bill Gates tops Forbes’ list of billionaires with a net worth as of March 2015 of $78.1 billion. His former partner in crime, Paul Allen, languishes at #51 with a paltry $17.4 billion. Steve Ballmer, who joined the company as an employee in 1980 and became its largest single shareholder in 2014, sits between them at #35 with $20.6 billion. Despite still topping the Forbes’ list, as of 2013, Bill Gates had given away about $28 billion through the Gates Foundation, which today has a total grants payment of $32.9 billion. These grants have included paying for expanding childhood immunizations, college scholarships, work toward a malaria vaccine and the eradication of polio, among many other things. The Gates Foundation Trust Endowment is now at $43.5 billion, which includes $15 billion in gifts from Warren Buffett, himself #3 on Forbes list.
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26 comments

  • If memory serves me correctly, my first IBM PC had two floppy drives; one for saving your work, and the other for the software driving the program you were using. Does anyone have a similar memory of the purpose of the two drives?

    • Yes. This was in the days before hard drives were affordable… or even available.

    • Bathazar Xavier

      was it one that had the cpu (and whatever else) in one enclosure and the floppy (and whatever else) in another?

    • PC-DOS was usually booted from the A: drive.  Software was run from the A: drive in most cases, and a disk used in the B: drive for working storage.  Working with a PC with only one drive was painful and error-prone,

  • Gary Kildall probably got the idea to use drive letters while using (CP/)CMS when he was working at the Naval Postgraduate School (although drive letters and their assignment in CMS are a bit different). IBM probably liked that.

    And drive letter assignments on CP/M weren’t always the most stable or sane things, either.

    You might take a closer look at http://en.wikipedia.org/wiki/Drive_letter_assignment to get the “who copied from whom” straightened out.

  • My old commodore C 64 had an eight inch floppy drive to run whatever program I was using at the moment and I saved data to cassette tape. Some programs required the use of multiple floppies and tapes and constant switching of said multiple floppies and tapes. Ah the good old days. “Insert disk 9 of 17 into drive A.” Everything was DOS, no mouse, had to learn command prompts, bulletin board systems, phone cradle modems and paid outrageous phone bills!

    • What??? The C-64 didn’t have drive letters as you suggest, the drives were assigned numbers, and there was never an 8″ floppy drive… There was a 5 1/4″ floppy drive though. And yes, they did have their own version of DOS (Disk Operating System) but it was not very similar to MS-DOS. Also, the C-64 had one of the first GUIs, called GEOS, that did use a mouse, but this was not a standard. Also, not sure why you used a cassette to save data if you had a floppy drive… Cassettes were MUCH slower and held less data than the floppies. I don’t remember any games that were more than 3 floppies so 17?!? Did you really have a C64?

      • The first home/business based IBM word processor used 8 inch floppies. I had a box of them and eventually threw them out as I had no way to down load the information stored on there. Fortunately I printed out the contents before I returned the unit back to IBM. They were huge discs in paper sleeves. I was glad to see 3 1/4 floppies as they were a lot easier to store but back ups were mandatory as sometimes they would get corrupted and not readable.

      • I don’t think he ever had a C64. Also, not only were cassettes slower, they were vastly less reliable – it was not uncommon to wait through a half-hour program load and then see a load error (the system didn’t stop when it found an error, it waited until the end of the data…hello? Anybody actually design anything at that company?). Nothing came with multiple floppies for installing on a C64 because a C64 had only 64K bytes of memory and the floppies were larger than that. There might have been some game that used extended data, but it certainly wasn’t mainstream. Nobody would stand for a program that required constant disk-swapping.

        I own, and have used the heck out of, a Commodore PET (original), Commodore VIC-20, Commodore C64, Apple II, Apple Macintosh (original), Victor 9000 (8086 version of IBM-PC), IBM-PC, TRS-80 pocket computer, Kaypro 2000, Amiga 2000, Apple Mac II, and the latest version of a generic home-assembled PC about every year from 1993 to now. I found this article highly amusing, and somewhat sad that people don’t know the history of how the computers they use unfolded.

  • The famed ‘insert disc #11 of #17’ (sometimes this went up to #40) came in with PCs running WfW or Win3.11 and the like, long after the proprietary systems (C64 systems) had fallen from fashion. The whole point of the packs of installation discs was that the advent of cheap HDD storage allowed programs to become more complicated.

  • l was lucky enough to have a commodore c-65 which came with a 10″ floppy drive labeled A: and also a 11″ floppy drive labeled B: this way when it told you to insert a floppy into A: you knew it was the 10″ floppy, before they label them A an B many people ruined there floppys by sticking one too big in the wrong slot. Now you know the whole story.

  • I used to teach adult computer classes, starting in the mid 1980s, and our first classroom PCs had two floppy drives – one to load DOS and one for a “data disk”. Teaching dBase II on them was a trip. “At the dot prompt….”

  • CP/M named the primary floppy drive as A: and the secondary floppy drive as B:.  MS-DOS simply followed that practice.

    Incidentally, the software that Gates and Allen bought for use with the first IBM PC was QDOS from Seattle Computer products, supposedly paying $50 thousand for it.  With a few tweaks, QDOS became IBM’s PC-DOS 1.0, which was pretty lame software.  Despite over 35 years having elapsed since that time, Microsoft still produces a lame operating system.  Ask the folks in THE UK about it.  🙂

  • I always wonder why it starts with C and not with A. Thanks for this interesting article. 🙂

  • ha. you really span this story out unnecessarily. you could have skipped right over the whole NDA kildall ibm thing and just cut straight to the fact that computers had floppy disk drives in the A and B drive letter slots before hard drives became popular… i didn’t remotely need to know about kildall et wife

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