Contents of the TRAP2526.DOC file
The following text outlines the COMPAQ DOS 3.31
problem(s) TRAP2526.COM is designed to address.
Prior to COMPAQ DOS 3.31 a physical limit of 32 megabytes
was imposed on the intrinsic (non-external device driver support)
fixed disks supported by MS-DOS.
This limitation was a function of several variables, the
most pertinent being the number of logical sectors that could be
defined by 16-bits. This works out to be 65535 (16 bits) times
512 bytes per sector (the standard physical sector size of fixed
disk) = 32 megabytes.
Device drivers supplied by third party vendors to break
this "barrier" normally increase the logical sector size as
viewed by DOS. This method provides for addressing within the
logical bounds of DOS while increasing the physical capacity of
COMPAQ DOS 3.31 provides for intrinsic support of drives
greater than 32 megabytes but not by the method mentioned above.
Rather, they have increased the sector addressing capability from
16 bits to 32 bits - the resulting potential size now limited to
2 TERABYTES (although COMPAQ states there is a 512 megabyte limit
on the current version).
They have done so by using a standard extended disk
control block, as defined by Microsoft in the MS DOS 3.2
What they did at the same time was preclude low-level
reading/writing via DOS services to these new logical drives by
effectively changing the standard DOS-level input-output.
Utilities programs, such as MACE, NORTON, PC-TOOLS, that used the
old style of DOS-level I/O will not run on the new COMPAQ DOS
drives (until those companies change their code).
COMPAQ includes a caveat to users in their manual
regarding this. It should not be taken lightly.
As a consequence, the following patch is provided to
those readers who wish to modify COMPAQ DOS in order to run
utilities prior to official updates from the vendors.
1) at the dos prompt, invoke debug
2) at the debug prompt ( the '-') type in the following:
jmp short 19b4
3) You are now back at the DOS prompt
This patch will permit you to run your standard utilities
and still run all of the DOS utilities (CHKDSK, etc).
WARNING - this patch does not provide for deficiencies in
third party code that has trouble supporting extended control
blocks, nor does it allow for access by those utilities to areas
beyond the 32 megabyte (16 bit) boundary - contact your vendor
to find out about support.
For those users who do not wish to patch DOS directly, a
TSR which traps and modifies old-standard calls is available on
the Kolod Research bulletin board. It is called TRAP2526.COM.
The phone number of this board is (312) 291-1447. Parameters are
1200,8,N,1. Users should login under the GUEST account, and at
the '$' prompt type:
XT TRAP2526.COM. This will start the XMODEM transmit of the
file. To exit the board, type ^D (control D). There is no
charge for this program.
COMPAQ Computer can do the following to alleviate some of
1) Publish specifications for the new access methods (as
opposed to reverse engineering them as I did.)
2) Provide third party device driver vendors with
specifications so that they can implement drivers compatible with
the new COMPAQ DOS 3.31 standard.
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