Talk:Cylinder-head-sector

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[edit] Surplus Sectors?

The lines: "Most modern drives have a surplus space that doesn't make a cylinder boundary. Each partition should always start and end at a cylinder boundary. Only some of the most modern OSs may disregard this rule, but this can cause some compatibility problems, especially if the user wants to boots up to more than one OS on the same drive."

...seem a bit out of place, because there's a section on IDE drives immediately following.

It is talking about IDE drives, when it says "most modern drives," ...right?

I don't understand about the "surplus space" - doesn't Zone Bit Recording take care of that?

Or is it that, even under ZBR, there's a little bit of extra space on each cylinder?

Doesn't ZBR make the cylinder completely transparent, and you're basically just addressing sectors? How would a program know where the cylinder boundary is, since each cylinder has a different number of sectors? LionKimbro

  • No, this refers to the fact that under an operating system (such as DOS and most Windows(tm) versions) which requires its partitions to end on a pseudo cylinder boundary (usually of 255 heads and 63 sectors), there will almost always be some odd number of user accessible sectors on a disk that don't divide exactly into that geometry. Daniel B. Sedory 00:01, 11 June 2007 (UTC)

"Only some of the most modern operating systems"??? Hasn't Linux always (i.e., for 15 years, as of 2006) allowed any kind of partition boundaries?

  • The History section here is obviously a bit out of date, since CHS tuples haven't actually corresponded to disk hardware in a very long time! Other than that, yes, Linux has been using different boundaries than DOS ever since it could; probably to mitigate the difference between the pseudo-geometry used and the number of surplus (unused) sectors left at the end of the disk. Face it: Any OS that's based on pseudo-CHS values (even Linux), rather than individual sectors, will likely have SOME sectors that can't fit into its partition boundaries for whatever arbitrary values you choose! Daniel B. Sedory 00:01, 11 June 2007 (UTC)

There is no trusted source for this statement:

"For operating systems such as Microsoft DOS or Windows (before Vista), each partition must start and end at a cylinder boundary."

My personal experience: at least since Win98SE theese non cylinder boundary partitions and partition tables can be read, although their partitioning programs don`t create non cylinder boundary partitions. Only Symantec/Notrton Partition Magic 8.x won`t handle such partitions. Non of you ever had mixed linux and windows installations on a single harddrive? Nerver used GParted for creating windows partitions? 89.58.155.42 (talk) 18:58, 2 April 2008 (UTC)

[edit] Two heads/platter: Not always!

I'm aware of drives (the current 5200RPM Hitachi 2.5" drives) some of which have an odd number of heads (the 20GB drive has one platter with one head, the 60GB drive has two platters with three heads). I suspect that this is rather common.

[edit] CHS count

This article might want to mention something about how the cylinders, heads and sectors are counted. I was just reading about the classical PC (DOS) partition table and how the first sector is located at cylinder 0, head 0, sector 1 (CHS 001). This is where the master boot record (MBR) is stored. However, the address CHS 000 seems to be illegal. Are all CHS addresses ending in 0 invalid, or just this first one? --Jwinius 15:49, 19 April 2007 (UTC)

In case anyone else wants to know as well, the answer is that all CHS specified locations must conform to a few range rules to be valid (legal). The allowed values are:
1. Cylinder: 0 - 1023 (for a maximum total of 1024).
2.     Head: 0 - 254  (for a maximum total of 255).
3.   Sector: 1 - 63   (for a maximum total of 63).

As one proceeds through LBA counts of 0, 1, 2, etc., the following are a few corresponding CHS tuples that may be very helpful in understanding their relationship (for drives that have 63 sectors per head):

LBA 0 = CHS 0,0,1
LBA 62 = CHS 0,0,63
LBA 63 = CHS 0,1,1

Likewise, for drives that have a pseudo-geometry of 255 heads per cylinder (which may be determined by your operating system these days; e.g., many Linux installs are based on only 16 heads/cylinder, whereas the same physical disk might have 255 heads/cylinder under a Windows OS), the following would be true:

LBA 16,002 = CHS 0,254,1
LBA 16,064 = CHS 0,254,63
LBA 16,065 = CHS 1,0,1

And the maximum LBA sector that CHS can provide a valid corresponding tuple for is at: CHS 1023,254,63 which is equivalent to an LBA of 16,450,559 for 16,450,560 sectors, total; which was passed when hard disks began to exceed a size of 8,422,686,720 bytes, (about 8.42 GB; or 7.84 GiB). Daniel B. Sedory 22:31, 10 June 2007 (UTC)

[edit] Merging Disk sector into this article

Just a quick note here to mention that I'll be slowly merging the small amount of material at Disk sector into various sections here. Any comments would be appreciated. Daniel B. Sedory 19:27, 7 October 2007 (UTC)

I don't think a merge makes sense. The Disk sector article is about actual disk sectors while this article is about a hard-disk addressing method which uses physical cylinder, head, and sector to which the data is to be written as the address for the data. To merge these articles would be similar to merging House with House numbering. Sagsaw (talk) 00:50, 29 December 2007 (UTC)
First, the 'merge request' has been on these articles for a long time; I didn't place it here, but since I've spent much time editing both of these articles, thought I'd consider how to go about it and make a smooth transition. (It hasn't been a high priority for me though.) I'm not at all opposed to leaving them as they are; though I do see redundancy and that Disk sector is presently not much larger than 'dictionary-sized' entries rather than being encyclopedic. What I would really like is for one of the Wiki 'higher authorities' to make a judgment call on this. Daniel B. Sedory (talk) 04:06, 31 December 2007 (UTC)
It seems a bit weird that The Disk sector article is much smaller than The Cylinder-head-sector article. If a merge were to take place, I'd suggest CHS be a sub-part of the Disk sector article, since that's logically how things are. This would obviously take some careful planning and perhaps some paring down of the material on the CHS page (it seems to spend paras describing some terms where a wikilink should suffice). I'm not sure it is a good idea to merge them at present. I came to the Disk sector article to find out how large a disk sector was, and despite it being short, it's a concise article that told me exactly what I wanted to know. I wouldn't have wanted to page over loads of stuff about the largely historical CHS to get to that information. -- Jon Dowland (talk) 16:38, 7 March 2008 (UTC)

[edit] Block vs. Sector (as smallest storage unit on an HDD)

When I started editing this article, its use of the term 'Sector' in CHS (Cylinder,Head,Sector) as the geometrical sector (or slice) common to mathematics, etc. was (and still is) rather compelling. So, I kept that usage as best I could even when adding content to the article Disk sector; based upon what I'd read here and in Circular sector. As a reflection on this topic, I'm asking (musing) whether or not block should continue being used here as the smallest 'chunk of data' one can 'read from/write to' an HDD through its interface (controller card), and how best to point out that 'Sectors' in CHS notation doesn't necessarily mean the same thing as Sectors in HDD capacity calculations. I'm still thinking this through but wanted to present some data and ideas concerning this.

One possible reason the use of "block" was replaced by the now more common term "sector," may be that 'block' was viewed as being somewhat artificially inserted into formulas for computing a disk's capacity: We know many of these formulas use the pseudo drive values of: "63 sectors" and "255 heads," but what units are associated with them? If we use a formula often found in simplified web presentations: S sectors/head x H heads/cylinder x C cylinders, we can't help but notice both the 'head' and 'cylinder' units cancel each other out, giving us S x H x C sectors; apparently keeping all the units straight. But do these units actually correspond with reality?

Did the change from the real number of physical 'sectors' and drive 'heads' (perhaps when Zone bit recording became standard on all new HDDs?) signal the rise of 'sectors' vs. 'blocks'?

The formulas presented in this article arrive at drive capacity from some physically real units that still apply to floppy diskettes: S SectorSlices/Side x H Sides x C Cylinders = S x H x C (SectorSlices x Cylinders) (where Side is equivalent to 'head'). The only way to resolve this result into a single unit is by defining another term as equivalent to "SectorSlices x Cylinders"; which this article does by stating:

blocksPerPlatterSide = (cylindersPerPlatter)*(SectorsPerPlatter)

which is directly related to its definition of block as: The intersection of a cylinder and a sector (SectorSlices x Cylinders). So, what's wrong with the first formula I presented here from popular web sites? I'll have to leave that for another time! Daniel B. Sedory 10:34, 17 June 2007 (UTC)