LAN Adapter, 16-Bit VGA Adapter Shared Memory Conflict (63862)

SYMPTOMS

Conflicts in memory addressing may occur when shared-memory architecture LAN adapters (for example, the Western Digital WD8003E) are used with 16- bit VGA adapters.

The LAN adapter software does not load properly: either the MAC driver refuses to load, NetBind fails, or LAN operation is severely impaired.

CAUSE

This conflict exists because some VGA adapters operating in 16-bit mode use two 128K memory segments (0xA0000 - 0xBFFFF video RAM, 0xC0000 - 0xDFFFF ROM). If the VGA adapter does this, it blocks out the memory regions that many LAN adapters need. Under MS-DOS, LAN adapters with "Autoswitch" may function properly; however, this Autoswitch feature does not work with OS/2 and the LAN adapters will probably fail.

RESOLUTION

There are four possible solutions to this problem:

Run the LAN adapter in a nonshared-memory mode. Some LAN adapters (for example, the 3Com 3C503 adapter) are capable of running in either a DMA, Programmed I/O, or shared-memory mode.
In some computers, it may be possible to base the LAN adapter's shared- memory region at 0xE0000 (this region is used in IBM equipment for BASIC ROMs, but should be free in Compaqs and most compatibles). A determination of whether or not this region is free can be made under MS-DOS using DEBUG or a memory-analysis tool.
Set the VGA adapter to run in 8-bit BIOS mode (on some VGA adapters this can be done independently of the memory access mode). This will impact the performance of MS-DOS, but not that of OS/2 or Windows. On VGA boards where the memory access mode cannot be set independently of the BIOS mode, this will severely impact performance in all operations.
Set the VGA adapter to run in "Autoswitch" mode. This will work only in MS-DOS. It may not work under Windows, and will not work under OS/2.

For more information on this topic, consult the LAN adapter manufacturer.