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