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Scott Mueller's Upgrading and Repairing PCs Forum • View topic - Why use 240 Head Geometry?
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 Post subject: Why use 240 Head Geometry?
 Post Posted: Wed Oct 21, 2015 2:07 am 
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Hi Scott,

My question is: Why did/do many laptop/notebook manufacturers create their product so the BIOS would use a disk geometry having 240 heads? I've been around long enough to remember the first PCs could have disk drives with all sorts of different heads/sectors, and clearly remember a Desktop drive of about 3 or maybe 6 GB having 128 heads. But why have so many of these stuck with some value other than what became the standard 255 heads on Desktop PCs? (Hmm... looking this over again, I see it's really 2 questions: How did it start out this way, and why did it/has it continued in many cases? SEE BELOW)

Please feel free to reiterate and help teach others that geometry is a result of the BIOS that those makers use and not of the drive itself; which I think (just like myself long ago!) many people believe is a property of certain 2.5-inch drives; it's not, you can connect such a drive to a Desktop PC, and it will use 255 heads if you install a Windows OS on it.
Note: This can cause confusion for people trying to grab files off a drive made on some laptop/notebook using a desktop PC; and vice-versa, especially if they happen to use an EXTERNAL drive that can be easily connected to both kinds of PCs.

But again, I would really like to know WHY (or at least get your opinion of why you think this was done and still is being done)? My most simplistic guess is, they already had a BIOS written that worked, so why pay more money to change it! But then I'm still left with the "Why did they even start using values the Desktop PC makers never did?"


Thank you, Daniel (aka, TheStarman).


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 Post subject: Re: Why use 240 Head Geometry?
 Post Posted: Wed Oct 21, 2015 11:31 am 
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 Post subject: Re: Why use 240 Head Geometry?
 Post Posted: Fri Oct 23, 2015 6:43 pm 
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Thank you Scott,

I understand about why translation was necessary before PC BIOS were able to become more or less standardized for drives with capacities in the double digits of Gigabytes, but my main question is: "Why did a number of Laptop/Notebook BIOS continue to use a disk geometry of only 240 (or possibly some other values), instead of the 255 heads that all Desktop PCs have been using since, let's say, triple-digit Gigabyte capacity drives have been in use?" Note: This is *not* to say that you can not find any Notebook PCs that do not use a 255 head geometry with Windows XP or Windows 7 OSs installed on them. But there were enough of them for this to still be a problem for some of my web page readers trying to view files on their Notebook/Laptop when connecting the drive to a Desktop PC.

FOR ANYONE READING this page who has a Notebook/Laptop, perhaps you could take a look at the Partition Table of your drive and let us know what you see there? If you are using Windows, you can get a free demo of WinHex and learn how to view the Partition Table, or you could download HxD and look at the raw hex data of the first sector of the drive; there are also other free utility programs that will display this data for you.

(My web page here explains how to use/get some of these types of tools:
http://thestarman.pcministry.com/asm/mb ... lsRefs.htm

It may be that all new Laptops/Notebooks for the past few years all use 255 heads now; if they have not already started using the GPT/EFI/UEFI format. But I believe there are still some users out there; say those still running Windows XP, who will find something other than 255 heads in their Partition Tables.


TheStarman.


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 Post subject: Re: Why use 240 Head Geometry?
 Post Posted: Sat Oct 24, 2015 7:51 am 
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15/16 = 0.9375 X 256 = 240 ... still this doesn't answer the why does it?

Thanks for asking the question. I hadn't even noticed. Nor that there was a possibility of an issue between use of 2.5" drives in laptops versus desktops and the implications of external drives going back and forth in between.


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 Post subject: Re: Why use 240 Head Geometry?
 Post Posted: Sat Oct 24, 2015 10:39 am 
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 Post subject: Re: Why use 240 Head Geometry?
 Post Posted: Mon Oct 26, 2015 9:47 am 
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 Post subject: Re: Why use 240 Head Geometry?
 Post Posted: Tue Oct 27, 2015 2:46 pm 
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Last edited by TheStarman on Thu Oct 29, 2015 1:18 am, edited 1 time in total.

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 Post subject: Re: Why use 240 Head Geometry?
 Post Posted: Tue Oct 27, 2015 3:02 pm 
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Scott,

I didn't miss the "Why" part of your post regarding HOW one arrives at 240 head geometry, but rather my questions were WHY was there a difference in how the BIOS of many Laptop/Notebook PCs handled disk geometry compared to Desktop PCs? I had only asked you this hoping you might have enough contacts in the industry or come across enough interview type articles from the industry to know that. Otherwise, we can only (as I first stated) guess that the BIOS company those Laptop/Notebook manufacturers used did not want to do any further development to add the LBA routines to their BIOS code either for monetary reasons, fear of possibly breaking the code or maybe due to some contractual or copyright reasons. That's the WHY I was asking about.


TheStarman.


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 Post subject: Re: Why use 240 Head Geometry?
 Post Posted: Wed Oct 28, 2015 12:27 am 
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 Post subject: Re: Why use 240 Head Geometry?
 Post Posted: Wed Oct 28, 2015 2:30 pm 
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Hi Scott,

Yes, that's what I had been trying to get at... that is a good answer for the first of what was really two questions: Why there could be a difference between a Laptop/Notebook and a Desktop PC in regards to disk geometry.

My 2nd question had ended up being: Why for some Laptop/Notebook manufacturers that difference continued even beyond the point of having drives with triple digit GB capacities; not just up to 8.42 GB?

I hesitated even asking that question, since I do not have any data from tests of my own to back-up that statement; only client's experiences. This is why I hope others in this forum can contribute some data about their own Laptop/Notebook PCs from various manufacturers, and discussed the utility you mentioned above.
Logically, you would think for drives over 8.42 GB, there shouldn't be any differences in how the drive is partitioned/used, since for any drive using the standard MBR partitioning scheme, only the LBA addressing should be in use. But, what happens at the beginning of such a drive in the area under 8.42 GB? Will you see for a common Windows 7 OS install, a disk geometry of 255 heads, or something else? (Not to mention, there could be some small FAT partition a name brand company might install in front of the Windows 7 partitions; making it possible to see 5 sets of different CHS values in that area: Both the Starting and Ending for that FAT partition, both for the "System Reserved" partition and the Starting set for the main Win 7 OS partition; its Ending CHS values could possibly still use only 239 for the head value instead of 255.)

And the most important question which must be backed-up by some real-world tested examples: Is it possible to read data from all Laptop/Notebook drives with a capacity, let's say that exceeds 137 GB, when connecting it to a Desktop PC as a slave drive, or will you have problems with some drives depending upon the manufacturer's BIOS in use on the Laptop/Notebook?


TheStarman.


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 Post subject: Re: Why use 240 Head Geometry?
 Post Posted: Thu Oct 29, 2015 9:52 am 
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 Post subject: Re: Why use 240 Head Geometry?
 Post Posted: Sat Oct 31, 2015 8:43 pm 
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Hi Scott and all,

Well of course I wouldn't expect there to ever be a problem reading from a Laptop or Notebook drive connected to my Windows PC when using WinHex, HxD or any other low level disk editing tool... provided the drive is physically/electrically healthy.
I can read sector data from a Mac drive, UNIX, Linux, etc. that way too; under Windows.

The possibility of a problem would only be when using a Windows OS to open the file system and attempt to read files from the drive. I'm also at this time, not sure if it may only be that Windows XP/2k would have an issue, but not Windows 7; or not after SP1.
As I stated earlier, this really needs to be fully tested using Laptop/Notebook PCs from all different kinds of manufacturers that were shipped with let's say at least 40 GB capacity drives or more, when there shouldn't have been any problems with the BIOS doing LBA to reach data that far beyond the 8.4 GB limitation. If I had access to some Toshiba, Acer, DELL, HP, etc. laptops, I would gather the data, or pull out the drives and run some tests by connecting them to my PC, but I do not. Which is why I'm hoping that some (with such machines) reading this would offer to do some testing. I can be contacted via email by writing it down from here (if you want to do that instead of posting the information here directly):



TheStarman.


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 Post subject: Re: Why use 240 Head Geometry?
 Post Posted: Wed Nov 04, 2015 7:38 am 
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I have connected many >8.4GB laptop drives to desktop systems (running XP and later) over the years without any sort of translation issues. I believe that is because for drives over 8.4GB the CHS parameters are simply not used. In that situation the drive is addressed purely via LBA, both from the BIOS to drive as well as the OS to BIOS. In my book I wrote a "Caution" with a warning about translation problems when moving drives between systems, but only for drives less than 8.4GB in capacity. Scott.


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 Post subject: Re: Why use 240 Head Geometry?
 Post Posted: Wed Nov 04, 2015 2:37 pm 
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