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Q79777: AutoCAD Version 2.62 May Not Function with HIMEM.SYS

Article: Q79777
Product(s): Microsoft Disk Operating System
Version(s): MS-DOS:4.x,5.x,6.0,6.2,6.21,6.22
Operating System(s): 
Keyword(s): 
Last Modified: 17-DEC-2000

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The information in this article applies to:

- Microsoft MS-DOS operating system versions 4.0, 4.01, 5.0, 5.0a, 6.0, 6.2, 6.21, 6.22 
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SYMPTOMS
========

AutoCAD version 2.62 may not load, or if it loads, may not show hidden lines,
garble the text, or exhibit other unusual behavior with HIMEM.SYS installed.

CAUSE
=====

Autodesk confirms that AutoCAD 2.62 uses an A20 wraparound (when addressing
memory above 640K) that may conflict with HIMEM.SYS or any other extended memory
manager that manipulates the A20 address line.

WORKAROUND
==========

To prevent these problems, either disable the HIMEM.SYS DEVICE= line in the
CONFIG.SYS file, or upgrade to AutoCAD version 9 or later.


MORE INFORMATION
================

The conflict with AutoCAD version 2.62 and HIMEM.SYS is tied to AutoCAD's method
of addressing memory. All Intel chips in the 8086 family (8086, 80286, 80386,
and 80486) address memory in a method known as "overlap and add" addressing. The
8086 family uses 16-bit registers. Instead of putting them end-to-end for 32-bit
addressing, 8086 family processors overlap them and add the two registers
together for 20-bit addressing. Note that this allows for many possible
combinations of numbers in the two addressing registers, which can result in the
same 20 bit address.

Example
-------

Offset F13E

Segment   +203E
---------------

Address 2F51E

This is the 20 bit address on the 8086. The same result can be realized by adding
the following together:

Offset E13E

Segment   +213E
---------------

Address 2F51E

Because there are 4096 different combinations for the two registers that can add
up to the same result, programmers who use the 8086 family arrived at a
convention, or agreed-upon standard, to simplify the addressing scheme. In the
segment register, only the most significant hexadecimal digit is used (far
left). The other three are set to zero, as follows:

Offset XXXX

Segment   +X000
---------------

Address XXXXX

This scheme sets up natural 64K groups in memory. The segment register specifies
which block of 64K to address while the offset register designates how far into
a block of 64K to address.

To address more than 640K of memory, AutoCAD 2.62 ignores the above convention
and places hexadecimal numbers in the first three places of the segment register
to achieve a "wraparound" address. AutoCAD takes a picture of what is in
conventional memory at the time it loads, and places the information on the hard
disk. This includes MS-DOS, which is temporarily replaced by AutoCAD as the
operating system. When AutoCAD finishes, it puts memory back to the way it was
before loading, thus giving control of the operating system back to MS-DOS.
Wraparound addressing is in direct conflict with HIMEM.SYS or any third party
XMS memory manager that uses straight addressing methods.

There are two possible solutions to this situation:

- Have a separate CONFIG.SYS for running AutoCAD 2.62 that does not have the
  statement:

     device=c:\dos\himem.sys

  (where c: is the drive and \dos is the directory location of the HIMEM.SYS
  file.)

- Upgrade your AutoCAD software (version 9 follows version 2.62).


The product included here is manufactured by a vendor independent of Microsoft;
we make no warranty, implied or otherwise, regarding this product's performance
or reliability.

Additional query words: 6.22 3rdparty 3.0 3.0a 3.00 3.00a 5.00 5.00a 6.00 6.20

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Keywords          :  
Technology        : kbMSDOSSearch kbMSDOS400 kbMSDOS621 kbMSDOS622 kbMSDOS620 kbMSDOS600 kbMSDOS500 kbMSDOS401 kbMSDOS500a
Version           : MS-DOS:4.x,5.x,6.0,6.2,6.21,6.22

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