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Assuming a 3.5" or 5.25" diskette placed on a surface with its label side facing upwards, which side of the magnetic disc within does head #0 of the drive reside on?

3 Answers 3

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Head 0 is normally on the bottom, i.e. opposite side of the label. This is true for both 3.5" and 5.25" disks.

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    Thanks! Is there a standard denoting the basic specs like that one? (including the spinning direction and the location of track zero)
    – Mark
    Nov 6, 2023 at 12:14
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    @another-dave Now, that's nonsense. Of course, both sides of the disk are accessible to heads.
    – tofro
    Nov 6, 2023 at 13:36
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    @Mark ECMA-147, §3.3: “The direction of rotation shall be counter-clockwise when looking at Side 0”, §8.2.4: “The track number shall be a two-digit decimal number (from 00 to 79) for each side. It identifies the tracks consecutively, starting at the outermost track (00).” This spec only covers 90 mm floppies (so-called “3½ inch”), but I’d be surprised if the older 5¼″ floppies were much different. Nov 6, 2023 at 15:32
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    @tofro - Oh, you're absolutely right. 3.25" disks have the metal slide on the hole. Who the hell knows what I was thinking of. When writing that, I definitely had a mental image of an uncovered hole in the disc enclosure. Maybe an 8" single-sided disc for a DEC RX01? It's been a long time since I touched any type of removable disc.
    – dave
    Nov 6, 2023 at 15:33
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    @Mark the linked ECMA standard is from 1990 but sources like "how we invented the Macintosh" hagiography tend to refer to 3.5" disks as being a Sony invention that a Sony sales team went around and peddled. So it was likely standardised after the fact.
    – Tommy
    Nov 6, 2023 at 18:18
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According to ECMA-125, a standard specifying high-density 90 mm floppies (so-called “3½ inch”), §4.14:

Side 0 is the side engaged by the spindle. Side 1 is the opposite side.

The spindle engages the disk from the bottom, i.e. the opposite side of the label.

The ECMA standards for 5¼ inch disks, of which I looked at ECMA-78 2nd Ed. and ECMA-99, are much less clear in their description, and only say (in §3.3.5):

For convenience of description the two sides are defined as Side 0 and Side 1; they are shown in Figs. 1-4 and Fig. 8.

However, the figures clearly show that side 0 is the one which puts the index hole window on the left:

Figure 4 from the ECMA-99 specification

So it is again unambiguously identified. Whether it should be called the “top” or the “bottom” is, however, much more debatable: 5¼ inch disks never had as strong a convention about it (or the placement of the label) as 90 mm floppies.

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    And certainly in the UK they were (are) called 3½" disks, Nov 6, 2023 at 18:44
  • And in the RSA, 3½" disks were (are) called "stiffies" to differentiate them from 5¼" disks, which are floppy (bendable), whereas stiffies are, well, stiff. Nov 8, 2023 at 10:56
  • The convention for 5 1/4 inch disks was well-established in the earlier days when single-sided drives with only one read/write head were popular (for cost reasons). This head was always on the lower side of the disk (when the drive was mounted horizontally), as it wouldn't need to move with the clamping mechanism on the other side. If you'd put your single head on the top instead, you wouldn't have been able to read disks from other manufacturers' drives! Double-sided drives naturally followed the same convention of side 0 on the bottom for compatibility's sake.
    – Kaz
    Nov 8, 2023 at 19:48
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Side 0 is on the 'bottom' side of the drive, or more exactly it's on the non-moving side when closing/opening that drive. Which is, at least with all early drives also the side the drive motor is acting on. That side is traditionally mounted bottom to improve adjustment but it's also a more natural handling to close downward.

This is historically due to the very first 8" drive being single side. Having the head mounted on the non-moving side simplifies mechanics. For later two-sided drives it wouldn't have mattered, as one head must be on the closing side, but numbering sequence had to be kept for compatibility when operating single-sided disks on dual-sided drives.

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