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).

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