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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?

  • What DOS? AppleDOS? AmigaDOS? – idrougge Jul 11 at 9:09
  • @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 at 9:43
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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 cylinder, 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.

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 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.)

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.

  • Common for stepper motor drives to need to be parked whereas voice coil drives did not. Also, on some OEM customized versions of DOS that I used the command was "spindown" rather than "park". – Brian Knoblauch Jul 2 at 13:11
  • If I recall correctly, the prompt didn't return after running PARK. But my memory may be wrong; it's 25+ years ago. – gerrit Jul 2 at 13:25
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    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 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 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 – Christian Lescuyer Jul 3 at 20:45
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This command is supposed to place HDD heads on "park" position.

  • 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 at 4:11

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