To:	opendos AT delorie DOT com
Date:	Thu, 2 Nov 2000 05:20:05 -0800
Subject:	RE: FDISK
Message-ID:	<20001102.052005.-3943975.1.domanspc@juno.com>
X-Mailer:	Juno 4.0.11
MIME-Version:	1.0
X-Juno-Line-Breaks:	0-6,8-48,50-53,55-57,59-60,62-73,75-89,91-97,100-104,106-111,113-122,124-133,135-136,138-152,154-161,163-167,170-172,174-190,192-201,203-204,206-209,211-218,220,222,224,226-236,238-248
X-Juno-Att:	0
X-Juno-RefParts:	0
From:	Robert W Moss <domanspc AT juno DOT com>
Reply-To:	opendos AT delorie DOT com

I think I may have stumbled on something which resembles 
the answer to the first question in this thread. 

There is a 'signature block' on all hard disks partitioned with 
FDISK for DOS 'of any brand'.  This 'signature block' consists 
of the last two bytes in Cylinder 0, Head 0, Sector 1, and 
consists of 55AAh.  If these two bytes are damaged then the 
computer will not boot and will act as if there are no partitions at all.


My scource is partial to Pure MSDOS, (or as Patrick would say 
"pure c*** DOS", so the following information doesn't relate to 
Compaq DOS, PC DOS, IBM DOS, DR DOS per se, but may help 
to give some people a general understanding of FDISK, since 
we had a lot of wandering in the thread. 

In order for DOS to use a disk it must be partitioned and this is the 
task of the FDISK Program. Partitioning a hard disk is the act of 
defining areas of the disk for an operating system to use as a volume. 
To DOS, a volume is an area of the disk denoted as a drive letter; 
for example the first volume will always be C, the second will be D, 
and so on. Some people think that a drive must be partitioned only 
if you want to divide it into more than one volume. This is a 
misunderstanding; a disk must be partitioned even if it will be the 
single volume c.  

When a disk is partitioned, a master partition boot sector is written 
at Cylinder 0, Head 0, Sector 1 - rhe first sector on the hard disk. 
This sector contains data that describes the partitions by their 
starting and ending cylinder, head, and sector locations.   The 
partition table also indicates to the ROM BIOS which of the partitions 
is bootableand, therefore, where to look for an operating system to 
load.  A single hard disk can have 1 to 24 partitions.  This number 
includes all of the hard disks installed  the system, which means that  
you can have as many as 24 hard disks with one partition rach, a 
single hard disk with 24 partitions, or a combination of disks and 
partitions such that the total number of partitions does not exceed 24. 
If you have more than 24 drives or partitions, DOS does not reconize 
them , although some other systems may.  What limits DOS is that a 
letter is used to name a volume, and the Roman alphabet ends with 
Z - the 24th volume, when you begin with C.    This will include any 
CDROM drives and removable drives (Syquest, tape drives, etc. 
 L120 drives will show up as floppy drives A or B).  

The FDISK program is the accepted standard for partitioning hard disks. 
Partitioning prepares the boot sector of the disk in such a way that the 
DOS FORMAT peogram can operate correctly;  it also enables different 
operating systems to coexist on a single hard disk.  

If a disk is set up with two or more partitions, FDISK shows only two
total 
DOS partitions:  the primary partition and the extended partition.  The 
extended partition is then divided into logical DOS volumes, which are 
partitions themselves .  FDISK gives a false impression of how the 
partitioning is done.  FDISK reports that a disk divided as C, D, E, and
F 
is set up as two partitions ,  with a primary partition having a volume 
designator C and a single extended partition containing logical DOS 
volumes D, E, and F.  But in the real structure of the disk , each
logical 
DOS volume is a separate partition with an extended partition boot  
sector describing it.  Each drive volume constitutes a separate partition

on the disk,  and the partitions point to one another in a daisy-chain 
arrangement.  

Different versions of DOS have had different partitioning capabilities, 
as follows:   

* DOS 1.x had no support whatsoever for hard disk drives. 

* DOS 2.x was the first version to include hard disk support, including 
the capability to partition a drive as a single volume with a maximum 
partition size of 16MB. DOS versions 2.x support only 16MB-maximum  
partitions due to the limitations of the 12-bit FAT system.  A 12-bit FAT

can  manage a maximum of only 4,096 total clusters on a disk.  

* The limit of 16MB did not come from the FAT, but from the high-level 
DOS FORMAT program, which aborts with an Invalid Media or Track 0 
bad - disk unusable error message if the partition is larger than 16MB.  
On a disk that has no bad sectors beyond the first 16MB,  you can 
ignore  the error message and continue the setup of the disk with the 
SYS command.  If the disk has defects after the first 16MB, those 
defects will not be properly marked in the FAT.  Most Vendors supplied 
modified high-level format programs that permitted partitions of up to 
32MB to be formatted properly.  Unfortunately, each cluster or minimum 
allocation unit on the disk then is 8,192 bytes (8K) because of the 
12-bit FAT.  

* DOS 3.x increased the maximum partition partition and, therefor, volume

size to 32MB but still could support only a single partition for DOS 
(assigned the C volume designator).  The size limit is 32MB, due to the 
limit of 65,536 total sectors in a partition. 

* DOS 3.3 introduce the concept of extended partitions, which enables 
DOS to see the drive as multiple volumes (drive letters).  The extended 
partition's  logical DOS volumes actually are partitions themselves.  In
the organization of the disk, the primary partition is assigned drive
letter C, 
and the extended partitions are assigned letters sequentially from D 
through Z.  Each drive letter Which is a volume or partition) can be 
assigned only as much as 32MB of disk space.  

* DOS 4.x increased the size of a single DOS partition or volume  to 2GB.
 
FDISK was modified to allocate disk space in megabytes rather than in 
individual cylinders, as with previous versions of DOS.  IBM DOS FDISK 
handled up to eight physical hard disk drives.  

* DOS 5.x had no changes in partitioning capabilities, but MS-DOS now 
could universally handle up to eight physical hard disk drives.  (IBM had

this capability in IBM DOS 4.x.)  

* DOS 6.x had no changes in partitioning capabilities, although both 
Microsoft and IBM added disk-compression software to DOS that created 
additional compressed volumes.  

The minimum size for a partition in any version of DOS is one cylinder;  
however , FDISK in DOS 4 and later versions allocate partitions in 
megabytes, meaning that the minimum-size partition is 1MB.  DOS 4.x 
and later versions permit individual partitions oe columes to be as large

as 2GB, whereas versions of DOS earlier than 4.0 have a maximum 
partition size of 32MB. 

FDISK is a very powerful program, and in DOS 5 and later versions, it 
gained some additional capabilities.  Unfortunately, these capabilities 
were never documented in the DOS manual and remain undocumented 
even in DOS 6.x.  The most important undocumented parameter in FDISK 
is the /MBR (Master Boot Record) parameter, which causes FDISK to 
rewrite the Master Boot Record code area,  leaving the partition tables 
intact.  Beware:  It will overwrite the partition tables if the two
signature 
bytes at the end of the sector (55AAh) are damaged.  This situation is 
highly unlikely , however. In fact, if these signature bytes are damaged,

you will know it:  the system would not boot and would act as if there 
were no partitions at all.  

The /MBR parameter seems tailor-made  for eliminating boot-sector virus 
programs that infect the Master Partition Boot Sector 
(Cylinder 0, Head 0, Sector 1) of a hard disk.  To use this feature, you 
simply enter  

   FDISK   /MBR  

FDISK then rewrites the boot sector code (with DR DOS it explains in the 
manual that this is a generic boot sector code),  leaving the partition 
tables intact.  This should not cause any problems on a normally 
functioning system, but just in case it is reccommended that you back up 
the partition tables information to a floppy disk before trying it.  You
can 
do this with the following command:    

         FDISK   /PARTN  

This procedure uses the MIRROR command to store partition-table 
information  in a file called PARTNSAV.FIL,  which should be stored on a 
floppy disk for safekeeping.  To restore the complete partition-table 
information,  including all the master and extended partition boot
sectors, 
you would use the UNFORMAT command as follows:  

        UNFORMAT   /PARTN  

This procedure causes the UNFORMAT command to ask for the floppy disk
containing the PARTNSAV.FIL file and then to restore that file to the
hard disk.  

Note that if you are using WIN95 the MIRROR and UNFORMAT programs 
have been eliminated, and you will have to use a third party utility
program 
instead, such as Norton Utilities.   

FDISK also has three other undocumented parameters: 

/PRI: , /EXT: , /LOG: .  

These parameters can be used to have FDISK create master and 
extended partitions, as well as logical DOS volumes in the extended 
partition, directly from the command line rather than through the 
FDISK menus.  This feature was designed so you can FDISK in a 
batch file to partition drives automatically.  Some system vendors 
probably use these parameters (if they know about them, that is!) 
when setting up systems on the production line.  Other than that, 
these parameters have have little use for the 'normal' user, and in 
fact can be quite dangerous!  

There are many third party partitioning programs out there that will help

you over ride DOS's limitations but they should be used carefully.  They 
all use a non-standard formatting system and they almost all include a 
boot loader program which must be run before DOS loads.  If you use 
one of these and someone doesn't know about it and sits down at the 
computer and uses a DOS boot floppy you can just kiss your data 
goodby almost every time.  The oddball program sets up a special 
program in config.sys that allows DOS to see their hidden/compressed 
partition and a standard DOS boot disk cannot see the partition.  Theis 
other person may use NDD or PCTools to check out the drive and those 
programs will try to correct what they see as a problem.  They will write

to the disk and you will never be able to correct the problem unless you 
know a lot about data recovery software you will have lost all the stuff
on 
your computer. 

If you are new to DOS and want to learn more about DOS and/or just play 
around with it or if you have a second or  third computer you can load
just 
about anything you want on it and use it to practice with. 

I recommend anyone who wants to brush up on old stuff or new people 
who need to know a lot of stuff and have no reference books if you can 
afford to pay $50US, $71CAN, L46UK yo should go out and buy  a copy 
of Upgrading and Repairing PC's, by Scott Mueller, Published by QUE 
Books.  I got a copy of the Sixth Edition for $5US at a COMP USA store 
here in California.  the ISBN is 0-7897-0825-6.  You can problably get it
in 
a Barnes and Nobles or Amazon.com on the internet with a little
searching. 
There is a world of information in any of his manuals.  I believe he is
up to 
edition  11 or 12  now.  There are others out there but the two I like
best are 
Mueller's Upgrading and Repairing PC's and Mark Minassi's PC Upgrade & 
Maintenance , Published by SYBEX, which is up to about edition 11 or 12. 

Sometime back Paul Mattias wrote a post  about those little annoying 
differences between between the different versions of 
DRDOS/MSDOS/PCDOS/NDOS and the entry in the OEM Label of the 
Boot Sector. 

You can probably find it if you want to crash back through the archives 
about 6/8/10 months ago.  Or maybe if he reads this he will be so kind 
as to elucidate it for us again. If I can find it in my files I could
probly repost it.



BOB 'DOMAN' MOSS "Chocolate is a vitamin" 

Besides 'RTFM' there is:
 'SUE' (Stupid User Error) and 'DDT' (Don't Do That)  
The software engineers  use  these eupemisms for software "BUGS" 
called in by customers with problems.  Probably related to what the 
Old Tech was thinking to himself when he found that TV,VCR,Computer ,
etc.,  was not plugged into the wall receptacle.
________________________________________________________________
YOU'RE PAYING TOO MUCH FOR THE INTERNET!
Juno now offers FREE Internet Access!
Try it today - there's no risk!  For your FREE software, visit:
http://dl.www.juno.com/get/tagj.

- Raw text -