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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: 3, Interesting) by RS3 on Thursday September 26 2019, @02:38AM (2 children)
Good for you! And you're bringing back very dusty memories. I probably used some single-sided floppies in the '80s, but I never owned one. First one was DSDD, then I jumped into 1.44M, bypassing 1.2M 5.25". But I ended up having to buy a 5.25" HD drive because I had Xenix on 5.25" floppy. I'm pretty sure I copied them to 3.5" right away, but Linux came along, and CDROM, and that was that.
(Score: 3, Interesting) by All Your Lawn Are Belong To Us on Thursday September 26 2019, @02:29PM (1 child)
Actually I started out on cassette tape, then upgraded to drives within a year or so and IIRC they were around 100K per disk. Then Seagate drives (darn if I remember the spec, though... could they have been DS?) IIRC our family's first IBM compatible (Commodore Colt) was also 5 1/4 drives-only. I had a Model 100 TRS-80 and later a 1400FD laptop that were on 3.5" (the 100 was a plug in accessory drive that used 4 AA's). Then I bought my first IBM laptop which both had color and a hard drive, on Windows 3.1 - think that was around 1994?
This sig for rent.
(Score: 2) by RS3 on Friday September 27 2019, @01:35AM
Computer history museum embodied you are!
Technically I used cassette tape first too- on a Kim-1, but I rarely messed with that.
Early 80s I had a job where they had 2 CPM machines- an Osborne 1 with dual floppies, which I loved, and some other machine that was much cooler- big screen, dual Z80s, I think it had dual floppies, or maybe a 5.25 and an 8". IIRC I mostly used it for RS232 stuff and wrote simple BASIC (to do RS232 stuff.) Wish I could remember the name. Molded fiberglass housing, very 2001 Space Odyssey looking thing. I think I had figured it out once and bookmarked it... somewhere...
I never had a TRS-80. Somewhere I have a Commodore 64 and floppy drive (iirc...) but never used it- had PCs by the time I got that.
I never heard of a Commodore Colt. Sad what happened to Commodore. They coulda been a contender. That's it- the Commodore Contender! That would have sold millions.