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How Did MS-DOS Decide that Two Seconds Was the Amount of Time to Keep the Floppy Disk Cache Valid?

posted by Fnord666 on Wednesday September 25 2019, @10:02AM   Printer-friendly
from the MS-what? dept.

upstart writes:

Submitted via IRC for Bytram

How did MS-DOS decide that two seconds was the amount of time to keep the floppy disk cache valid?

MS-DOS 2.0 contained a disk read cache, but not a disk write cache. Disk read caches are important because they avoid having to re-read data from the disk. And you can invalidate the read cache when the volume is unmounted.

But wait, you don't unmount floppy drives. You just take them out.

IBM PC floppy disk drives of this era did not have lockable doors. You could open the drive door and yank the floppy disk at any time. The specification had provisions for reporting whether the floppy drive door was open, but IBM didn't implement that part of the specification because it saved them a NAND gate. Hardware vendors will do anything to save a penny.

[...] Mark Zbikowski led the MS-DOS 2.0 project, and he sat down with a stopwatch while Aaron Reynolds and Chris Peters tried to swap floppy disks on an IBM PC as fast as they could.

They couldn't do it under two seconds.

So the MS-DOS cache validity was set to two seconds. If two disk accesses occurred within two seconds of each other, the second one would assume that the cached values were still good.

I don't know if the modern two-second cache flush policy is a direct descendant of this original office competition, but I like to think there's some connection.

Original Submission

 
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  • (Score: 1, Insightful) by Anonymous Coward on Wednesday September 25 2019, @03:04PM (4 children)

    by Anonymous Coward on Wednesday September 25 2019, @03:04PM (#898527)

    MAC is a hardware address. A Mac is a computer.

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  • (Score: 2) by RS3 on Wednesday September 25 2019, @03:33PM (3 children)

    by RS3 (6367) on Wednesday September 25 2019, @03:33PM (#898556)

    My first ATM card was called MAC: Money Access Center. It's good we have pedants like you to keep the world running smoothly. /s

    • (Score: 2) by EEMac on Wednesday September 25 2019, @04:24PM (2 children)

      by EEMac (6423) on Wednesday September 25 2019, @04:24PM (#898598)

      I've always wondered why people call Apple computers "MAC"s. "Mac" is short for Macintosh, not an acronym. But enough people do it (and have always done it) that there's some mental quirk there . . .

      • (Score: 1) by MindEscapes on Wednesday September 25 2019, @06:47PM

        by MindEscapes (6751) on Wednesday September 25 2019, @06:47PM (#898685) Homepage

        Macintosh
        Apple
        Computer

        :)

        --
        Need a break? mindescapes.net may be for you!

      • (Score: 2) by RS3 on Friday September 27 2019, @01:43AM

        by RS3 (6367) on Friday September 27 2019, @01:43AM (#899376)

        It's possible that people who are Mac fans are also more high-strung, very detail-oriented, maybe almost persnickety. And that's okay, but don't expect everyone to be like that.

        I don't care so much. All my life I've had to learn that there are no absolute rules in English. In some cases it seems there are more exceptions than rule-abiding.

        Mac, MAC, who cares- it's an abbreviation anyway. I would think the pedants would be more annoyed with "Mac" rather than fully "Macintosh".