504MB DOS IDE limit - an explanation
John Naismith
consider to be a definitive explanation (if such a thing exists) of the 504MB
"limit".
The capabilities of DOS are constrained by a combination of the IDE spec & the
BIOS. The IDE spec allows for hard disk sizes of up to 130GB, whilst a
"standard" (non-LBA) PC BIOS can handle disks of up to 8GB in size. The 504MB
"limit" occurs because of a mismatch between the drive geometries supported by
each hardware standard. The problem is that DOS uses the "lowest common
denominator". ie -
BIOS IDE DOS
Max sectors/track 63 255 63
Max Heads 255 16 16
Max Cylinders 1024 65536 1024
To calculate sizes from Sectors, etc, use the following formula :
sectors x heads x cylinders x 512 = capacity in *bytes*. To convert to MB divide
your answer by 1,048,576.
This gives : BIOS limit = 8,032.5MB; IDE limit = 130,560MB; DOS limit = 504MB
MS-DOS 6.x does *not* support drives up to 2GB natively, unless your BIOS has an
LBA mode available. It supports *partitions* (important distinction) of up to
2GB. Why you'd want a 2GB FAT partition is beyond me, as you'd lose about 300MB
(ballpark figure) due to "slack" space.
If you don't have LBA, and you use DOS, then you have four viable options :
1) Get an EIDE controller card - either with on-board BIOS or with s/w drivers.
I would suggest that this is the preferred solution, as TSRs such as Ontrack,
etc have compatibility problems with s/w. eg - if the TSR is 16 bit (and it
will be if it's for DOS) then this may force crap like Win95 to run in MS-DOS
compatibility mode.
2) Use Ontrack (or something similar) to convince DOS that your BIOS is capable
of more than the above figures.
3) Upgrade your motherboard BIOS to one supporting LBA. This is generally *not*
easy, as before the "modular" BIOSs, a BIOS chip was programmed for use on a
*specific* model/make of m/b. You'll generally find that if your m/b is a couple
of years old then there is *no* upgrade available.
4) Get rid of MS-DOS/Win3.x/Win95 and get a *real* operating system.
I *know* that this is all difficult to remember - if you just remember that a
combination of MS-DOS and your BIOS are the culprits, then you'll do OK. There
is an *awful* lot of confusion about this subject - even amongst some of the
experts in this group. Also note that *none* of the above applies to SCSI
disks/controllers.
I do hope that this clears the matter up once and for all.
--
-----------------------------------------------------------
John Naismith jo...@naismith.demon.co.uk
-----------------------------------------------------------
David W Hoskin
<Jo...@naismith.demon.co.uk> writes
>I do hope that this clears the matter up once and for all.
Er, possibly not for those of us utterly and deeply confused by this
problem.
Thanks, John, for the info which IS clear and concise, but I'm still not
sure how it applies to my situation:
I have a 486 VESA m/b with AMI WinBios dated 12/93 which DOES have a
Hard Disk LBA enable/disable option. My i/o board is a VESA IDE one
(NOT an EIDE). I am running DOS 6.2 and WFWG 3.11.
i) Is it likely that I can install, say, a 850Mb drive without changing
the I/O board?
ii)If this is not posible as one partition, can it be installed with 2
equal partitions?
iii) If I do have to get a new EIDE i/o board what do the modes 3,4 & 5
refer to?
I you are able to provide a straighforward answer to this I would be
extremely grateful, and I guess others would find it a worthwhile
addition to the explanation you have already given.
-- **************************************************
Regards, The business of finding fault is very easy: that
David Hoskin of doing better, very difficult.(Francis de Sales)
East Yorkshire, UK **************************************************
John Chapple
>
Now how about one describing what the various modes mean?, (how do you
know whether your card, hard disk supports it?, are the modes somehow
related to whether your bios supports LBA or LARGE(etc))?
John Chapple J...@ch-w.demon.co.uk
'There are too many BRIGHT lights going out' Phone (+44) 01233 712868
Dave Powers Fax (+44) 01233 713698
Brian Lewis
<da...@hoskin.demon.co.uk> writes:
<snipped, edited>
> Regards, The business of finding fault is very easy: that
> David Hoskin of doing better, very difficult.(Francis de Sales)
> East Yorkshire, UK *************************************************
Hi Dave,
Did you realise that adding the "*****" to the end of your sig separator
"-- " will defeat those programs which are rigorous in their
interpretation of the definition?
The sig separator IIRC should be hyphen hyphen space _and then nothing
else_
Cheers!
--
Brian Lewis ------ EMail at: br...@btext.demon.co.uk
Software: Win3.1 Twndsock2.0b WinDis3.1 NewsWin0.9 NetScape1.2b6
Mike Clarke
Naismith <Jo...@naismith.demon.co.uk> writes:
>The capabilities of DOS are constrained by a combination of the IDE spec & the
>BIOS. The IDE spec allows for hard disk sizes of up to 130GB, whilst a
>"standard" (non-LBA) PC BIOS can handle disks of up to 8GB in size. The 504MB
>"limit" occurs because of a mismatch between the drive geometries supported by
>each hardware standard. The problem is that DOS uses the "lowest common
>denominator". ie -
>
> BIOS IDE DOS
>Max sectors/track 63 255 63
>Max Heads 255 16 16
>Max Cylinders 1024 65536 1024
>
>To calculate sizes from Sectors, etc, use the following formula :
>
>sectors x heads x cylinders x 512 = capacity in *bytes*. To convert to MB divide
>your answer by 1,048,576.
>
>This gives : BIOS limit = 8,032.5MB; IDE limit = 130,560MB; DOS limit = 504MB
Thanks John for a very informative article which I duly archived for
future reference. The future has now arrived since I am considering
adding a 1.2Gb disk to my system, after dusting down and re-reading the
article I find that my system does not quite match the above figures and
am still a little uncertain as to whether or not I will will suffer from
the 504Mb limit.
My PC has an old (9/6/92) AMI BIOS so I initially assumed that I would
need either an EIDE card or something like Ontrack, but when I fiddle
about with the BIOS setup I find that it will let me specify up to 255
sectors/track instead of the expected 63. If I specify 255
sectors/track, 255 heads and 1024 cylinders the BIOS returns a size of
32,513MB. Combining these with the IDE limit suggests that my DOS limit
would be 2040Mb.
Considering the date of my BIOS I'm inclined to think the figures are
wishful thinking but I'd like to be certain of my requirements before
ordering the goods.
Has anybody had experience, favourable or otherwise, of using large
disks with the June 1992 AMI BIOS?
--
Mike Clarke
John Naismith
--
---------------------------
jo...@naismith.demon.co.uk
---------------------------
sh...@buzzcomp.demon.co.uk
>--
>Mike Clarke
You will need software such as On track to get passed the 504Mb limit.
So check with your supplier that you will get a copy of the correct
software for the drive (different drives, different software) before
ordering. If not use a supplier that will supply it.
*************************************
Colin Muirhead
E-mail:- co...@buzzcomp.demon.co.uk
Clive Kay
>My PC has an old (9/6/92) AMI BIOS so I initially assumed that I would
>need either an EIDE card or something like Ontrack, but when I fiddle
>about with the BIOS setup I find that it will let me specify up to 255
>sectors/track instead of the expected 63. If I specify 255
>sectors/track, 255 heads and 1024 cylinders the BIOS returns a size of
>32,513MB. Combining these with the IDE limit suggests that my DOS limit
>would be 2040Mb.
>
>Considering the date of my BIOS I'm inclined to think the figures are
>wishful thinking but I'd like to be certain of my requirements before
>ordering the goods.
>
>Has anybody had experience, favourable or otherwise, of using large
>disks with the June 1992 AMI BIOS?
>
the new disk guess what "504mb" boo....
Dont play around use Diskmanager 6.03b or later and you will get the
whole bit with no loss in speed.Dont forget to turn off the warning of
viruses in the boot sector in the Bios or it wont install..
--
Clive Kay
Carl Warnell
> Has anybody had experience, favourable or otherwise, of using large
> disks with the June 1992 AMI BIOS?
Yep. I (still) have this BIOS, and I bought a 1.27Gb Samsung (hey, it was
cheap...) HD about six months ago. It was fine setting up the drive
parameters in the BIOS, but DOS refused to format more than 528Mb. The
supplied software (Wide Disk or something) sorted that out, but it seemed a
bit kludgy so I bought a Promise EIDE controller. This is fine, when it
boots, which seems to be about two times in three, or less if it's cold...
--
_____ . . <|
Carl Warnell ' o/ FORE! . . |
b...@bogart.demon.co.uk | . . . . |
/| . . ...o
Mike Clarke
<Jo...@naismith.demon.co.uk> writes:
>How many sectors/heads/cylinders does your new drive have Mike ?
None yet but I'm considering the Seagate ST51270A 1.2Gb. The Seagate web
page quotes the logical organisation as 63 sectors/track, 16 heads and
2,485 cylinders. I suppose it's the 2,485 that's going to be the
problem.
I now see from the "Help - Installing new 1GB hard drive" thread that
Seagate drives have appropriate software supplied with them so hopefully
I should be OK.
--
Mike Clarke