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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: 2) by FatPhil on Wednesday September 25 2019, @03:38PM

    by FatPhil (863) <reversethis-{if.fdsa} {ta} {tnelyos-cp}> on Wednesday September 25 2019, @03:38PM (#898565) Homepage
    The mechanical answer makes sense. However, the difference in error messages would imply that there is a mechanism to detect between the lever being open and closed. It's just not asynchronous, so the OS can't know about it when it happens.

    To save costs, it could arguable that it might not be latched in the drive itself. So the article might be telling the truth about the lack of information flow about disk change to the OS, but for the wrong reason. Or it might still making a bit of a whiff - what do you make of INT13h AH=16h http://www.p-dd.com/chapter8-page36.html - that can't be done without both detecting and latching? Function 16h is one of DOS's low level disk I/O BIOS interrupt, that must have been there all the way back in the days of DOS 2.0.

    So the article is saying they didn't implement part of their own BIOS?

    It's still whiffing.
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