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Thread: How are sectors dealt with on an MFM drive...

  1. #1

    Default How are sectors dealt with on an MFM drive...

    I've got a 30M ST-4038 and I think it says it has 17 sectors per track. I've read about MFM encoding and I'm sure there has to be some other encoding that identifies a sector beginning, the sector number, the data, and probably a crc or checksum of some kind. I get that hard sectored floppy disks use a physical method of locating at least the beginning of sectors. I don't have a clue about how soft sector works, or what the ST-4038 does. If it is soft sectored, how does it locate the right sector? The PC tells the controller which sector it wants, then does the controller just start reading the entire track reading one sector after another until it hits the one it wants? I see how that could work on the read, but how can it do the write without risking running into other sectors? Or does it read the sector number and there is some space area for it to decide, yes, this is the one we want to write, and then trigger/switch to writing?

  2. #2
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    No, in its simplest form, a ST506 interface drive is very similar to plain old floppy drive. The drive itself, like the floppy, is basically stupid. It can step the head carriage back and forth, can detect when the heads are over track 0, but what comes and goes to and from the disk is entirely up to the controller. So, yes, there sector ID headers, sector data headers, CRCs and in some drives, more elaborate error checking code (e.g. bigger polynomials). As a matter of fact, there are controller chips that will work with either floppy or hard disks--the big difference is the data rate.

    So, though we may call it an "MFM" drive, there's no particular restriction on the encoding used. You could have an ST506-interface drive that uses FM, MMFM, RLL of any flavor, or even conceivably NRZI.

    As an example, consider the Shugart SA-1000 encoding guide here

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    As Chuck points out everything is soft-sectored layed down by a low level format. The LLF encodes pre-ambles, header marks, inter-sector gaps, etc. The sector ordering around the track could and often was optimized for the specific host system. Based on when sectors were requested during typical IO and how easily the controller could acquire them from one to the next affected the ideal sector interleave. 1:1 was a linear ordering. Other interleave factors would place them non-contiguous around the track (e.g 1, 4, 7, 10, 13, 16, 2, 5, 8, 11, 14, 17, 3, 6, 9, 12, 15) requiring multiple rotations to read all the sectors.

    During write the controller would assert the write head immediately after locating the target sector header for the waiting payload.

    MFM was 5 MHz @ 3600 rpm = 83.3 Kbps with 17 sectors per track - always. RLL was 7.5 MHz @ 3600 rpm = 125 Kbps with 26 sectors per track. I believe ESDI was 20 MHz @ 3600 rpm.
    "Good engineers keep thick authoritative books on their shelf. Not for their own reference, but to throw at people who ask stupid questions; hoping a small fragment of knowledge will osmotically transfer with each cranial impact." - Me

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    Characterizing MFM vs. (2,7) RLL as 7.5 vs. 5.0 MHz is a bit misleading. It's mostly a matter of symbol encoding. The drive with (2,7)RLL encoded data has exactly the same fmax requirement than one with MFM encoded data.

    The maximum frequency is the same, as is the minimum frequency (i.e. bandpass), but the position of each pulse requires more accuracy for RLL. A drive that exhibits too much jitter would not do well for RLL, where it might do just fine for MFM. Plated media pretty much made RLL (of one flavor or another) the de-facto standard by the time IDE drives rolled into the picture, as it's easier to get accurate edge timing with plated media than with coated media.

    As an example, consider 8" floppy FM vs. MFM -the bandwidth is the same for both, even though FM has only half the data rate that MFM has.

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