Interrupts and LAN Manager

ID: Q96715

SUMMARY

Many hard-to-diagnose problems can often be traced to a simple interrupt conflict. Most frequent among the errors are a failure to bind to the network interface card on LAN Manager DOS workstations. Unfortunately, interrupt conflicts on servers cause more subtle problems, often inconsistencies in receiving broadcasts or an inability for workstations to see the server on the network, although server function seems normal.

You cannot proceed with an assumption that the default interrupt will always be correct. The most popular interrupt for ISA card defaults is Int 3. If you have a serial mouse as well as a modem attached to a given PC, you can assume that this interrupt will no longer be correct. Further complicating things, many PC manufacturers are now placing two serial ports as standard equipment on their new computers, thus making Int 3 impractical. However, even with the presence of two serial ports, you can sometimes use Int 3 if you first disable serial port 2 (Com2). If disabling a serial port is not an option, you will need to choose an alternative interrupt.

Alternative Interrupts

The interrupts available for your NIC are dependent on the card type. An 8-bit NIC will only have available the lower interrupts (2-7), while most 16-bit network interface cards will also have available Int 8-15. 32-bit cards are not an issue in this situation (see further below for a discussion).

The most commonly used second choice for interrupt setting is Int 5, but this is also conditional. On AT-type computers (80286 and above), this interrupt is often the location of the second printer port (if two are installed). Though a PC configuration that contains more than one printer port is more rare than a configuration that contains two serial ports, this can most often become an issue when configuring servers that may have the maximum three printer ports in order to support multiple attached printers. This is another reason (among many) why it is wiser to use at least 16-bit cards in servers.

If available, the higher interrupts are a preferred choice because it is less common for add-on boards to use these higher interrupts, thus making conflicts less frequent. The higher interrupts (in order of preference) are: 15, 10, and 9. Though it is unusual, these interrupts can also be unavailable due to the configuration of other boards.

The only way to be reasonably certain that you are using an unoccupied interrupt is to check your interrupts with one of the many diagnostic utilities available. Windows and LAN Manager contain the utility MSD.EXE, which can show available interrupts.

Interrupts are not an issue with 32-bit network cards. EISA and Micro Channel computers have their own way of handling interrupts, thus making conflicts highly unlikely. But with EISA controllers, the dual nature of this bus presents a problem. If you are using ISA cards in an EISA bus, all the aspects of interrupt conflicts and their prevention should be considered because ISA cards do not allow the EISA architecture to perform its normal interrupt resolution function.

You should keep track of all interrupts used by add-on boards. You might even want to keep a log or tape a label to the computer with the card and the interrupt being used.

Interrupts You Should Not or Cannot Use

0 Timer
1 Keyboard
6 Floppy disk controller
8 Clock
13 math coprocessor if present
14 fixed disk

These interrupts may vary slightly from PC to PC.

Additional query words: adapter 2.10

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