Int 5C Access Lost in NetBIOS/Protocol Problem (104043)

SYMPTOMS

A user reported that after adding dynamic protocol loading under MS-DOS, Int 5C (NetBIOS interrupt) could no longer be accessed and a custom NetBIOS routine could no longer determine the permanent node name (a unique network card ID burned into the card) using the NetBIOS LanAdapterStatus call.

CAUSE

Since dynamic protocol loading had just been added, its presence was suspected to have caused the problem. However, Int 5C is always accessible if a fully-NetBIOS-compatible protocol is used, regardless of how that protocol was loaded.

Two probable causes for the problem as reported are:

The user may have switched to a protocol that doesn't (correctly) support the LanAdapterStatus NetBIOS call. Microsoft's TCP/IP is an example of such a protocol.

Even more likely (at least more common) is the possibility that the user loads more than one protocol, and that the first to load (LANA 0) doesn't have a NetBIOS interface. The problem doesn't lie in the configuration of the protocols, but in the fact that most programs using NetBIOS calls are 'dumb' in that they call only the first network stack loaded on a system and therefore fail if they call one that doesn't support NetBIOS.

RESOLUTION

A good test in this instance is to load only NetBEUI and see if that enables you to access the NIC address. If it does, change the protocol loading order so that LANA 0 has a protocol that is fully NetBIOS compatible.