what was it ?

Boris Karloff modelt20 at canada.com
Sun Sep 18 11:27:29 PDT 2005


>On Sunday 18 September 2005 12:34, dick hoogendijk wrote:
>> I'm building an old msdos machine for a little kid (very
nostalgic).
>> But I seem to rememeber that there was an issue about the
space of the
>> harddrive. Some kind of limit I don't remember. How large
can a ms-dos
>> partition be?

>How old?

>32MB is one number which comes to mind from MS-DOS 3.2
days.

>I've just repaired an old PC for a friend (Pentium 133MMX).
 While testing, I 
>used an old 10GB HDD I had lying around.  The BIOS would
only see it as 8GB  
>It didn't understand the existing 10GB bootable Win98
partition/OS on it.

>-- 
>Dave

When IDE drives were first introduced, they stored
information in CMOS as to their size. Sectors were always
512 bytes in size, and they were limited to 1024 cylinders,
255 heads and 63 sectors. Multiplying this out, you get
about 504Mb (some manufacturers claimed 528Mb, because they
were counting a megabyte as 1,000,000 bytes instead of the
programming standard of 1,048,576 bytes). 

To break this barrier, Logical Block Addressing (LBA) was
instituted about 1994. Some refer to this as int13
extensions, because it translated cylinder, head and sector
(CHS) values to 28 bit values. This translated value was
then used to map the location on the hard disk.

MS DOS and Windows 3.1 used FAT16 architecture, which was
unable to address a partition larger than 2Gb (2047Mb).
Windows 95 was originally released with this limitation
also. Since there are exactly 4 primary partitions on any
hard disk, this limited the size of the hard disk to 8Gb
(some called this the 8.4Gb barrier).

About 1998, Windows 95(B) and later changed to a FAT32
architecture. In theory, this should be able to address 2
Terabytes, but BIOS limitations with int13 limited this to
8.4Gb per partition. This created a limitation of about 32Gb
(some manufacturers refer to this as a 37Gb limit). For the
most part, however, Windows 95(B), Windows 98 and Windows Me
could not efficiently use even 32Gb, since the minimum space
allocated for a file was also increased, and the time to
calculate the free space at startup and retrieval slowed
with increasing size. Windows 2000 claimed to be built on
Windows NT, but it recognized FAT32 as well as NTFS (Windows
NT4.0 did not recognize FAT partitions). As a result, it was
easy to set up Windows 2000 with really sluggish behavior
(thus the phrase 'Windoze').

BIOS programmers then applied LBA technology to newer
drives, and drives larger than 32Gb were now available for
Windows. This adaptation allowed drives as large as 128Gb to
be used (137Gb barrier). However, a limitation of FAT32
prevented a single file from being larger than 4Gb. 

Starting with Windows NT, and the default in Windows XP is
the NTFS file system; which addresses some of these limits.
These use a 32 bit architecture, which places the limits in
the 2 Terabyte range. Other limitations prevent this from
being realized, however. 

To answer the question originally placed:

Early versions of MSDOS cannot utilize a drive larger than
2Gb (4 partitions of 504Mb). MSDOS 5 and 6 could handle
drives up to 8.4Gb (4 partitions of 2047Mb). Old machines
that only run MSDOS usually have a BIOS limitation hard
wired, but using LBA helps if available.

Many old computers with these limitations will not recognize
a drive larger than 8.4Gb at all. Generally, if the computer
relies on int13 extensions, a drive larger than 8.4Gb will
not be detected during powerup, and 'no bootable drive
present' is reported.

This, of course, assumes you are using an IDE drive, not one
of the other variants that MSDOS was notorious for (like
ESDI,MFM or RLL). Using a SCSI controller allowed larger
drives to be used, but still FAT16 couldn't address larger
partitions.

The question to ask you is: WHY would you want to use
MSDOS??????
FreeBSD 4.11 has minimum hardware requirements that rival
most DOS systems, and its more stable, more robust, and more
powerful. WHY perpetuate the arcane in the new
generation????? Encourage your youth to learn a real OS
(like BSD) from the start.

Harold.


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