How did MS-DOS decide on two seconds to keep the floppy disk cache valid?

https://devblogs.microsoft.com/oldnewthing/20190924-00/?p=102915

by mpweiher • 7 years ago

109 9 7 years ago

devblogs.microsoft.com

bobx11• 7 years ago

Brings back great memories of Sierra’s King’s Quest and swapping out floppies as you reach different parts of the game. 2 seconds was definitely the minimum time I would imagine being able to put in a new floppy and not risk crushing it, but doesn’t that not take into account the ability of the hardware to find the sectors on the disk that need to be calibrated when you insert it?

rasz• 7 years ago

There is no calibration in floppies (and very old hard drives), track location is hardcoded in hardware and selected with stepper motor.

kevin_thibedeau• 7 years ago

Hard-sectored 8" and 5.25" floppies did exist.

coliveira• 7 years ago

Two seconds is the time it takes in Formula 1 for mechanics to lift a car, change four tires, and put the car back in movement. It is difficult to believe that you cannot swap flop drives in less than that time.

skykooler• 7 years ago

Doing it F1 style, you'd have one person take out the first floppy while a second person inserts the second floppy.

zeeZ• 7 years ago

You need a third person to push the button that ejects the floppy.

aidenn0• 7 years ago

I can do a 3.5" in less than 2s, but I assume they were testing with 5.25"

Those usually had a lever rather than a button so it was a 4 step process rather than a 3 step process:

3.5":

- Press button

- Remove Disk

- Insert Disk

5.25"

- Turn lever

- Remove Disk

- Insert Disk

- Turn lever

Add to this the fact that 5.25" floppies were less rigid, and expensive enough that you don't want to damage, and it takes a lot longer.

That being said, I think I did get down to just barely under 2s when I was restoring my hard disk from 97 5.25" floppies after getting a virus (the backup program being used bypassed DOS's system calls for accessing the disk, and polled multiple times per second so you didn't even need to press a key after inserting the disk).

How did MS-DOS decide on two seconds to keep the floppy disk cache valid?

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