An Interrupt Is An Opportunity, Not A Problem
The PC uses 16 interrupts (often abbreviated as IRQ, short for Interrupt
from 0 to 15. All of the devices hooked up to the system get the CPUs attention through an
interrupt. It's best for
each piece of the personal computer that needs an IRQ, to have its OWN unique IRQ.
Sometimes you can gamble and get
away with two cards, ports, whatever sharing an interrupt. This often does not
work. In any case, if two
devices try to use an interrupt at the same time, one may not work, both may not work, or the
whole system may freeze.
As "goodies" get added to a PC, the pool of IRQs shrinks.
An Interrupt Is An Opportunity, Not A Problem
The PC uses 16 interrupts (often abbreviated as IRQ, short for Interrupt ReQuest), numbered from 0 to 15. All of the devices hooked up to the system get the CPUs attention through an interrupt. It's best for each piece of the personal computer that needs an IRQ, to have its OWN unique IRQ. Sometimes you can gamble and get away with two cards, ports, whatever sharing an interrupt. This often does not work. In any case, if two devices try to use an interrupt at the same time, one may not work, both may not work, or the whole system may freeze. As "goodies" get added to a PC, the pool of IRQs shrinks.
|IRQ 0||System timer interrupt from TIMER-0. No user-definable options.|
|IRQ 1||Keyboard controller|
|IRQ 2||Cascade for IRQs 8-15. IRQ 9 cascade to IRQ 2 through a software redirect.|
|IRQ 3||Available. The standard IRQ for COM 2/COM 4.|
|IRQ 4||Available. IRQ 4 is the standard for COM 1/COM 3 -- the serial mouse connection. Don't try to have the mouse share an IRQ.|
|IRQ 5||Available. This is the default IRQ for most sound cards. Network card and LPT 2 are often set to IRQ 5, IF a sound card is not used.|
|IRQ 6||Floppy Drive controller. No user-definable options.|
|IRQ 7||Primary parallel (printer) port - LPT 1. If a parallel port is not used, then IRQ 7 can be assigned to another device.|
|IRQ 8||Real Time Clock. No user-definable options.|
|IRQ 9||Cascades to IRQ 2.|
|IRQ 10||Available. If a sound card is using IRQ 5, then this IRQ can be used for a network card.|
|IRQ 11||Available. Often used for a SCSI controller.|
|IRQ 12||Available. Often used for the PS2 style mouse.|
|IRQ 13||Math Coprocessor. No user-definable optinos.|
|IRQ 14||Primary IDE (hard drive, CD ROM) controller. The 2 IDE devices on the same cable use the one IRQ.|
|IRQ 15||Secondary IDE channel. Available if the secondary channel is disabled.|
|Mouse COM1||IRQ 4|
|Modem COM 2||IRQ 3|
|Extra Serial COM 4||IRQ 3 (Possible conflict with COM 2)|
|Sound Card||IRQ 5|
|SCSI Card||IRQ 9|
|Network Card||IRQ 10|
|SCSI Card||IRQ 11|
|PS2 Mouse||IRQ 12|
|Primary IDE||IRQ 14|
You can squeeze extra interrupts out by disabling everything that you do not use! Disabling COM 2 makes IRQ 3 available. Disabling the secondary IDE channel will let you use IRQ 15 for something else. Disabling a PS2 mouse port releases IRQ 12. To get "good to the last drop," use a SCSI hard drive. You can then disable both IDE channels. This way both IRQs 14 and 15 get their freedom.
COM Ports (short for Communication Ports) are serial interfaces to the system. Each one must have an interrupt. Mice, modems, serial printers, all use COM ports. The system can have up to 4 COM Ports: COM 1, COM 2, COM 3, and COM 4. Systems generally have 2 in use: mouse (usually on COM 1) and modem (Usually on COM 2 or COM 4). Serial ports end in either 9-pin or 25-pin connectors. Adapters are available to convert the 25 pin to the 9.
|COM 1||IRQ 4||3f8h|
|COM 2||IRQ 3||2f8h|
|COM 3||IRQ 4||3e8h|
|COM 4||IRQ 3||3f8h|
Even though you can install 4 COM ports, traditionally there are only 2 IRQs for them. This way, even with 4 serial devices attached, only 2 can be in use at a time. since the mouse is constantly used, interrupt sharing with the mouse is not possible. That basically leaves COM 1 hogging IRQ 4. COM 2 and COM 4 can be set to IRQ 3 (as long as only one is used at a time. New modems and motherboards often allow a wide range of IRQ settings.
For BBS, or ISP applications, you can get a Multiport Board which can handle from 2 to 128 ports. These are not cheap. For a shoe-horn, you can buy a box that will allow several external devices to use one port - one at a time.
For the standard installation of an internal modem, the easiest thing to do is to disable the COM 2 on the motherboard or the IDE card (for older systems). The Internal modem is then set to COM 2, IRQ 3.
|LPT1||7||378h , 3BCh, or 278h|
|LPT2||5||278h , 378h|
Formerly PCs only needed 1 Parallel port, since it was only used for the printer. Now with ZIP drives, and digital cameras, a second parallel port is becoming a necessity. Switch boxes, as for the serial, are used to allow several external devices to share one port. But, again, only one at a time.
A large number of shareware programs are available for checking IRQs, COM ports, and other facets of hardware.
I Installed A New Card And Now Nothing Works!
If the situation is not so simple, take out one of the other cards WITH THE POWER OFF, OF COURSE. Don't pull the video card, for then the machine can't do anything. Turn on the system. If the computer comes back to life, you've found the opposition to the new hardware. If the system still won't boot, then remove a different card and try again.
In general, I prefer to try to adjust the settings on the new card.
If after removing just about everything no progress has been made, start to investigate if the new card is just plain defective
Double Click on the SYSTEM ICON
to Investigate And Change Hardware Settings