The Seagate U Series hard disk Product manual states that the drive has 16 "Default read/write heads" while it has 4/3/2/1 "Physical read/write heads" (depending on actual model). Now, one or two disks is completely normal for a cheap drive 2001.

But what on earth is a "default" head and how can it have 16?

I have never heard of this term. If it refers to -- for example -- the maximum number of addressable heads by the controller logic, then it's a pretty marginal information and why include it among basic and vital information like number of sectors or cylingders? (http://www.codemicro.com/support/disc/manuals/ata/u6pmrevb.pdf, see page 9: Specification summary table)

3 Answers 3


The physical geometry how many heads the drive actually has is not the same as the logical geometry of how many heads is presented to the PC by the drive.

By translating the geometry, the drive can be fully addressable to up to the maximum of approximately 8 gigabytes in CHS mode, as the IDE interface is limited in CHS mode to 16 heads, 63 sectors and 16383 cylinders.

If there was no translation, and the drive reported only the 4 physical heads, only the first about 2 gigabytes would be accessible via CHS addressing mode.

The CHS geometry translation parameters are configurable, so the manual reports the geometry translation parameters that are the defaults that are selected after a powerup or drive reset.

  • So they use "virtual" heads to address cylinders beyond 16383? The manual never states the actual number of cylinders.
    – Dercsár
    Commented Oct 12, 2020 at 17:57
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    Now I have just realized how every disk except from the jurassic era seems to have 16383 cylinders and 63 sectors. They probably have way more than that.
    – Dercsár
    Commented Oct 12, 2020 at 18:05
  • And I always wondered how on earth do they squeeze eight disks into that slim housing. So that's how they do.
    – Dercsár
    Commented Oct 12, 2020 at 18:06
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    That is correct, the physical storage is virtualized. Talking about how many physical cylinders there actually is won't be relevant, and most likely cylinders at inner edge and outer edge have different amount of sectors per track too so it cannot be represented as single number.
    – Justme
    Commented Oct 12, 2020 at 18:20
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    That's why modern disks completely dropped the CHS terminology and went to LBA (logical block addressing) - you simply don't care.
    – tofro
    Commented Oct 13, 2020 at 8:10

Modern hard drives are not addressed by Cylinder, Head and Sector (CHS) coordinates, but by Linear Block Address (LBA). This is mostly due to the number of sectors per track changing across the width of the disk, as more sectors can be squeezed into the larger circles described by the outer tracks than the inner ones. The drive electronics automatically translate the linear logical address into a physical one.

This sectoring scheme began to be used before LBA addressing became the norm, so there was a long interim period in which logical CHS addresses were sent to the drive with no obvious relationship to its physical geometry, and the drive electronics had to translate them anyway.

The drive would report a "default mapping" which the manufacturer considered to be a good fit for the drive's capacity, but a different mapping could be assigned by software (ie. the PC BIOS). Very old BIOSes didn't know how to read the default mapping and had to be manually configured, which is why the default mapping is listed on the drive label.

  • 4
    Correction: LBA = Logical Block Addressing (as opposed to CHS being a form of physical block addressing).
    – ssokolow
    Commented Oct 13, 2020 at 3:27

Some older computers used to identify 512-byte sectors by specifying a combination of a head number, sector number, and cylinder number. For historical reasons, the head number was often limited to the range 0-15, sector number 1-63, and cylinder number 0-1022. This limited disks to 8,455,200,768 bytes--a little bit under 8 GiB. Newer drives simply access all sectors sequentially by number.

I think what that manual is saying is that if it receives a command to read or write a sector at a specified cylinder/head/sector combination, it will access sector (cylinder * 1008) + (head * 63) + (sector - 1). This would likely not yield meaningful results if one wanted to use the drive to its full capacity, but it would allow the drive to be connected to an older system and--if formatted on that system--used as though it were an antique just-under-8 GiB drive that used cylinder/head/sector addressing.

  • 1
    The numbers don't add up. 16 heads, 63 tracks and 1024 cylinders is approximately 512 megabytes, i.e. 504MB limit, as the cylinders is limited to 0..1023 by the standard PC BIOS and the heads are limited to 0..15 by the standard IDE CHS mode. 8 gigabytes is the IDE CHS limit due to 0..16383 cylinders.
    – Justme
    Commented Oct 12, 2020 at 17:58
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    I think there was two sets of limitations, probably in succession of each. PC specs are such an evolutionary mess...
    – Dercsár
    Commented Oct 12, 2020 at 18:01
  • Yes, well there are the limits everywhere. Limits of IDE interface, BIOS interface, limits how disk translates the geometry, limits how BIOS translates the CHS or LBA the geometry to OS, and how OS can access the drive and if it uses the standard BIOS interface or the extended BIOS interface.
    – Justme
    Commented Oct 12, 2020 at 18:27
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    @MichaelKarcher I worded it badly - I meant that if the BIOS does CHS to CHS translation, it can support logical BIOS CHS up to 1024/256/63 which is about 8 gigabytes, and due to that 8GB limit, that translates into physical IDE CHS of up to 16384/16/63. If it is an old BIOS which cannot do CHS translation, then you are limited to 1024/16/63 or 504MB. So via standard BIOS interface, the drive won't be accessed but up to 16383 cylinders for 8GB. Yes, BIOS geometry translation usually limits to 255 heads for DOS compatibility, so maximum sectors for DOS is 1024*255*63 or about 8032MB.
    – Justme
    Commented Oct 13, 2020 at 7:51
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    @Justme That's correct and I messed up my comment by claiming the BIOS limit to be 16384/256/63, whereas you have the correct limit limit 1024/256/63 in your comment. There is nothing that inherently limits to 16384 cylinders, neither in the BIOS nor in the IDE interface - but if you remap the available 8GB from the BIOS/DOS interface limit to the maximum cylinder size of 255(or 256)/63, 16384 cyclinders is what you can address. Commented Oct 13, 2020 at 8:00

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