October 22, 2004
Hard Drives, MS-Dos, and Other Filesystems
MS-Dos's hard disk parameters originated with the WD-1003 controller
in 1984 which limited drives to 1024 cylinders x 16 heads x 63
sectors (10 bits, 4 bits, 6 bits, & 504MB). These values were the
actual bit width of the hardware registers built into the controller chip
and the O/S's, built in, hardware drivers.
The Int13 extensions include a fudge to allow the other
4 bits of the heads to be used for 1024 X 255 X 63 (8GB); a
later revision broke the 1024 cylinder barrier (I'll bet by using
the other 2 bits in the sectors value) for 32GB, and the latest Int13x
extensions themselves have recently changed to break the 137GB
barrier.
Why this is important to understand, is that to ensure
compatability with MS-Dos, we need to adhere to the rules
established during its evolution.
Fat 16 means there are two bytes per cluster in the
allocation table, used by the O/S to link file segments together. MS-Dos
3.3 contains the original hardware limitations and its partitions
will fail if the drive is over 504MB; but, MS-Dos 4, 5, & 6 can
handle 2GB because they support the Int13 extensions.
Fat32 in Windows 95 OSR2 doubles the number of bytes for each
cluster in those same allocation tables, allows for a variable sized root
directory, and adds two new partiton types which are Int13x
compatable.
When creating MS-Dos compatable filesystems both the Int13 and
filesystem rules apply. The first thing to change is the number of
heads in the partition table and master boot records. Using LBA mode
in the system's rom BIOS before creating partitions is the simplest way to
achieve this.
Unix, Linux, and several other operating systems have never
suffered from these limitations because they simply passed the drive
parameters to a single driver, which then handled all of the hardware
itself; though several older versions did restrict the heads and sectors
values within their drivers to the 16 & 63 defined in the orginal
WD-1003. Later versions removed those restrictions and now, too,
support LBA translation which is also best set in the rom before
partitioning.
If you feel that I need to add explanation, have further
questions, or have any feedback, please contact me below.
Sincerely,
Davis M McCarn
Computer-Help.Net
184
Eaglecrest Drive
Matthews, NC 28104
(704) 882-7551 or
Email: davism@computer-help.net
© Davis M McCarn 2004 All Rights Reserved