Re: what was it ?

From: Boris Karloff (modelt20_at_canada.com)
Date: 09/18/05

  • Next message: Jeff D. Hamann: "laptop question..." 
    To: dgmm <freebsd01@dgmm.net>, freebsd-questions@freebsd.org
Date: Sun, 18 Sep 2005 13:27:16 -0500

    >On Sunday 18 September 2005 12:34, *** 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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