In 1994 or so, we had an old computer at my primary school. I remember finding out that it had a park command. From reading its documentation, it said that this command should be executed prior to shutdown. At home I would simply switch off when I saw the DOS prompt and there were no more I/O-indicating lights. I remember thinking at the time that the computer must have been very old if it had to be parked prior to switching it off.

What is this park command? Is it likely that, in 1994, we would have had a school computer that really had to be "parked"? What for?

  • 1
    What DOS? AppleDOS? AmigaDOS?
    – idrougge
    Jul 11 '19 at 9:09
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    @idrougge I have no idea, sorry. I probably didn't know it at the time and I certainly don't know it now.
    – gerrit
    Jul 11 '19 at 9:43
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    @idrougge maybe the DOS that has a PARK command. Just a guess. Feb 29 '20 at 10:38
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    Put the computer into reverse until it crashed? Jun 30 '20 at 16:17

Hard drives have read/write heads which fly above the spinning disks when the drive is powered. When power is removed, the heads no longer fly... For a long time now, the arms which hold the heads have been designed to “auto-park” the heads away from the disks’ surface, or over a safe “landing zone”, when they lose power¹, but early (up to the mid 80s) hard drives didn’t have this feature, so their heads would land on the disk surface, which could sometimes damage the surface.

So early PCs had a PARK command which would park the heads away from the disk surface. Typically, this would attempt to move the heads past the last “official” cylinder (over an “engineering cylinder” on MFM and RLL drives), or, starting with ATs, use the landing zone specified in the BIOS drive parameter table (accessed using the vectors stored at interrupts 0x41 and 0x46). You can see one such implementation in Roedy Green’s PARK which comes with source code, or in Jim Leonard’s disassembly of SpinRite’s PARK.

On PCs with auto-parking heads, it was safe to wait for the DOS command prompt, and the lights to switch off: COMMAND.COM ensures that I/O is finished before it displays the command prompt (and in-memory disk caches are supposed to honour that too).

(In fact, this feature is what allows Roedy Green’s PARK to work too: you’d wait for the command prompt, so there’s no outstanding I/O, then run PARK, which would be loaded from disk, then run with no I/O apart from parking the heads, then either loop forever or return to the command prompt which would normally not result in any I/O either, so the heads would remain safely parked. SpinRite’s PARK waits for the user to press a key, so the user can power the computer off without pressing a key and thus ensure there’s no untoward I/O.)

New PCs in 1994 wouldn’t need this, but it was common for schools to have very old computers, and an early PC requiring PARK wouldn’t be unheard of. Old habits die hard too, so it’s possible that the advice to run PARK was kept alive long after it stopped being relevant, but that would have involved copying the PARK command since it was system-specific and not part of DOS.

If I remember correctly, IDE drives never needed PARK, so you’d only find it on PCs equipped with pre-IDE drives (commonly referred to as MFM or RLL drives).

¹ Or nowadays when they detect a sudden movement.

  • 4
    I recall park. The lore was "run this before moving the computer". Apparently the disk heads landing wherever was no problem. The disk heads being shaken while moving the computer was. Shrug. Who knows what the lore really should have been.
    – Joshua
    Jul 2 '19 at 23:32
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    @Joshua The Zenith 8088 I had growing up had a command called "ship", which parked the heads. That confirms the "protect the computer while being moved" idea behind the command.
    – Tristan
    Jul 3 '19 at 14:20
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    This reminds me of the sync; sync; sync we used to do before shutting down Unix workstations. utcc.utoronto.ca/~cks/space/blog/unix/TheLegendOfSync Jul 3 '19 at 20:45
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    I had to try it - but Windows 10 says it doesn't know how to park. Well, at least I won't have to worry about where it's going after a date with that Linux guy... :-) Jul 4 '19 at 17:33
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    "it was common for schools to have very old computers" - in 1994 my primary school's computers didn't even have hard drives
    – OrangeDog
    Jul 5 '19 at 9:43

This command is supposed to place HDD heads on "park" position.

  • 4
    This answer is correct, but could be better by explaining where the park position is, why its a good idea, and perhaps why we don't need to do this nowdays.
    – Criggie
    Jul 6 '19 at 4:11

in 1989 when I purchased my Tandon pcx-20 it had and still does (yes i have it still) a park command and had a tandon tm503 full height 3600rpm 20mb mfm hd service manual for drive states any shock past 20g will damage the drive! In a tandon 501-503 series this COMPLETELY moves the heads past the media to a designated parking spot for the assembly.

Reason for park is the damage to head and media hitting each other during shipping would be much more catastrophic then vs now. servos like we have are not as delicate either today as they were in 1988.

Nowadays servo's autopark the head in newer drives,they don't even always have air inside now sometimes even helium is used!!!

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