Timeline for How does a floppy drive identify the first and last sectors and tracks?
Current License: CC BY-SA 4.0
16 events
| when toggle format | what | by | license | comment | |
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| Jan 25, 2022 at 13:42 | vote | accept | Bob Ortiz | ||
| Jan 17, 2022 at 17:45 | comment | added | Glen Yates | @supercat All I know is that I have converted many disks to be able to use the other side by only cutting out a new write protect notch. If you don't cut another notch, then when you flip the disk over the drive will report that it is write protected. I've never had to do anything about a sense hole. 5 1/4" Apple II drives if that makes any difference. | |
| Jan 17, 2022 at 17:36 | comment | added | supercat | @GlenYates: After doing that, rotate the disk to see if the edges of the index hole on the floppy disk itself are clean, and clean them up if not. It won't matter if the hole is a tiny bit bigger than it needs to be, since no information will be stored particularly close to it. Of course, if the systems that needed the index hole used double-sided drives, there would seldom be any need to add one. The main use would be to have a disk which could boot one of two ways depending upon which side was up. | |
| Jan 17, 2022 at 17:35 | comment | added | supercat | @GlenYates: Most drives that required the index hole had heads for both sides of the disk, and thus had no need to cut a write-protect notch to write the reverse side. Cutting an extra space for the index sensor would be tricky and time consuming, but hardly infeasible. Rotate the disk until the index hole is visible, mark its rotational position on the inside of the disk, rotate the disk so the index hole will be under the flip-side other sense position, and then punch a hole through both sides of the jacked and disk. | |
| Jan 17, 2022 at 17:28 | comment | added | Glen Yates | @supercat Drives not sensing track 0, yes, that is the reason for that wonderful nostalgic sound of di, di, di, di, dit, you hear coming from the drive every time you turn the computer on. | |
| Jan 17, 2022 at 17:22 | comment | added | Glen Yates | @abligh No, you do not need to punch another hole in the disk for the index hole, how would you possibly do this anyway without damaging the disk? The only thing you have to do is cut out another write protect notch. | |
| Jan 16, 2022 at 21:18 | comment | added | supercat | @abligh: There might be performance advantages to having the first sector of every track located at the same position around the disk, but there can also be performance advantages to having them skewed. A system that isn't bound by the constraints of an index hole can use either approach, as convenient. | |
| Jan 16, 2022 at 21:10 | comment | added | supercat | @abligh: No need to "monitor the rotation of the motor". For all operations but disk formatting, one can simply wait until one either sees a sector number has appeared, or one has gone too long without seeing it. To format a disk, one can start by indexing the head outward by 80 half tracks, writing an FM-encoded pattern [long sync] 255 [long sync] 254 [long sync] 253 ... [long sync] 0, followed by 16 sectors with no gap, switching to read mode, looking for a long sync, and seeing what number follows it. This will indicate how much gap to leave between sectors. | |
| Jan 16, 2022 at 16:51 | comment | added | abligh | @supercat - Yeah I was thinking of 8271/1770 based controllers. I guess if you monitor the rotation of the motor, and (for formatted disks) look for sector zero to give you an offset, that is an alternative to an index hole. | |
| Jan 16, 2022 at 16:05 | comment | added | supercat | @abligh: I think there may be a mentality among the chip manufacturers that a disk controller should affirmatively indicate that a track doesn't contain a header for a certain sector, and shouldn't assume a hard-coded rotational speed, but people who design drives as an integrated hardware/software system recognize that software can take care of such things. | |
| Jan 16, 2022 at 16:03 | comment | added | supercat | @abligh: Purpose-designed drive controller chips often rely upon the index hole, but machines from Apple and Commodore that use drive controllers built from discrete chips often ignore them. | |
| Jan 16, 2022 at 10:37 | comment | added | grahamj42 | @abligh, I agree for 5 1/4" disks in PCs and earlier CP/M machines. Same for 8" disks. | |
| Jan 16, 2022 at 9:32 | comment | added | abligh | @supercat re the index hole, I've never heard of it not being used on 5.25" disks anyway. That's surely why we needed to punch another hole in the envelope in order to use disks in single sided drives the wrong way up (that and the write protect notch) to use the reverse side. Is my memory failing me? | |
| Jan 14, 2022 at 17:01 | history | edited | user3840170 | CC BY-SA 4.0 |
typos
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| Jan 14, 2022 at 15:46 | comment | added | supercat | Many floppy drives don't have sensors for track 0 nor the index hole. Track 0 may be located by simply running the stepper motor out 40 tracks, letting its motion be blocked by the end stop. Sectors may be placed starting at an arbitrary rotational position, and missing sectors may be dealt with via timeout. | |
| Jan 14, 2022 at 13:33 | history | answered | Justme | CC BY-SA 4.0 |