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
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.