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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.
(Score: 2) by All Your Lawn Are Belong To Us on Thursday September 26 2019, @03:54PM
Back in the Halcyon days of college, when I attended one of the top-ten largest schools by enrollment in the Country and got my first job not working for my father in campus Computing Services, we had five primary computer labs on campus (in addition to coin-operated typewriters in the main library for people who still typed their papers manually). One was dedicated to VAX/VMS with more terminals than I care to think about, and IIRC had the terminals for access to the Cray-1 we had, two were IBM PC labs of around 100 machines each IIRC, and there were two Mac labs and one had around 60 and one had around 30 (Plus? SE?) and there was one Mac II in the 30 unit lab which was used for the speech therapy program to do something-or-other (it had a microphone hooked up to it).
There was also one, and only one, machine on the campus on which one could insert a Mac 3.5 floppy and I don't remember but it was probably a 5 1/4" IBM floppy, and be able to transfer data files from one disk to the other. I do remember it was an IBM PC machine, and it was actually in the VAX/VMS lab. I think people primarily used it to move .rtf files between the two. It wasn't in constant use, but it did get used.
This sig for rent.