This article will discuss the function and assignment of IRQs, DMAs and Base Addresses (IOPorts) used by IBM compatible computers. I've found that the documentation, with IBM compatible PCs and peripheral devices, list the resources that the device can use but do not give any guidelines as to standard resource assignments. Hopefully this article will shed some light on this troublesome area and provide some guidance as to the selection of IRQs, DMAs and Base Addresses.
An IRQ is a hardware interrupt, this means that there is a physical line run to each of the ISA slots on the motherboard. There are 2 types of ISA slots: 8 bit and 16 bit. The 16 bit consists of the 8 bit slot plus a 16 bit extension slot.
There are 8 IRQ (IRQ0-7) lines that run to the 8 bit ISA slot. There are 8 more (IRQ8-15) that run to the 16 bit ISA extension slot. For a total of 16 IRQs in a typical ISA bus PC. IRQ0 has the highest priority and IRQ7 the lowest priority. IRQ8-15 have "special" priority as will be explained.
When IBM introduced the AT computer, they added IRQ8-15. Now, in order to make AT (286) PCs backward compatible with 8 bit XT (8088) PCs and to "up" the priority of the new IRQ lines, they cascaded two interrupt controllers. This results in IRQ8-15 having the same priority as IRQ2. Priority means if two IRQs are active at the same time, the one with the higher priority is serviced first.
IMPORTANT: An IRQ can be assigned to only one active device at a time. If 2 devices share the same IRQ, this is called a CONFLICT. This means that when the IRQ line becomes active, the CPU does not know which device needs to "talk". For example, if a modem used IRQ5 and a NIC used IRQ5. If the modem had some information that needed to be passed on to the CPU, it would set IRQ5 active. The CPU would not know whether to talk to the NIC or modem. The computer may hang, or nothing would happen.
*** IRQ conflicts are the NUMBER 1 source of PC problems! ***
Here is a table that is used as a rule of thumb (guideline) in selecting IRQs for PCs. The IRQs are listed in order of priority. (Note that I lied earlier when I said that all IRQ lines go to the card slots)
IRQ Function Physical Line ISA Bus IRQ0 System Timer No - IRQ1 Keyboard Controller No - IRQ2 Cascaded to IRQ8-15 No - IRQ8 Real-time clock No - IRQ9 *-Available(IRQ2) Yes 8/16 bit IRQ10 NIC Yes 16 bit IRQ11 SCSI adapter Yes 16 bit IRQ12 Motherboard mouse/available Yes 16 bit IRQ13 Math coprocessor No - IRQ14 Primary IDE controller Yes 16 bit IRQ15 Secondary IDE controller Yes 16 bit IRQ3 Com2/Com4 Yes 8 bit IRQ4 Com1/Com3 Yes 8 bit IRQ5 Sound card/LPT2 Yes 8 bit IRQ6 Floppy drive controller Yes 8 bit IRQ7 Parallel port LPT1 Yes 8 bit*- IRQ9 appears as if it is IRQ2. Normally not used because it can cause interesting problems to appear. Is it really IRQ9 or is it the IRQ2 cascaded to IRQ9? Which do you set it to? What if you are using an 8 bit ISA modem in a 16 bit ISA slot? See what I mean...
The above table is a rule of thumb or guideline to selecting IRQs for your peripherals. For example if the PC does not use a SCSI adapter then IRQ11 is available for use for another NIC card or another device.
Most autodetecting software or operating systems like Linux expect to see the IRQs assigned as above.
COM2 (DB25 on the back of the PC) and COM4 have a similar problem except that most people don't use COM2. It is usually safe to configure an internal modem to COM4. If COM2 is used, it is typically used for an external modem or a plotter. Usually, both are not active at the same time.
Standard COM Port Assignment Port IRQ Function COM1 4 Mouse COM2 3 Plotter / external modem / not used COM3 4 Not used (conflicts with mouse) COM4 3 Internal modem / not used
There are 8 DMA channels available in the PC: DMA0-7. They are divided into 8 bit channels and 16 bit channels based on the 8 bit ISA slot and 16 bit ISA slot.
Here is a table that is used as a rule of thumb for selecting DMA channels:
DMA Function Physical Line ISA Bus Channel Width DMA0 Available Yes 16 bit 8 bits DMA1 Sound Yes 8 bit 8 bits DMA2 Floppy Disk controller Yes 8 bit 8 bits DMA3 ECP Parallel Port Yes 8 bit 8 bits DMA4 * - Not used No - 16 bit DMA5 Sound Yes 16 bit 16 bit DMA6 SCSI Yes 16 bit 16 bit DMA7 Available Yes 16 bit 16 bitNOTES:
*** DMA conflicts are the NUMBER 2 source of PC problems! ***
Like IRQs, you are only allowed to assign one DMA channel to an active device at a time. Otherwise you will have a conflict appear and things will not work properly. Notice the word active - this means turned on and a software program accessing the device. You may have one DMA channel assigned to two devices ONLY if one device is active at a time - risky but allowed.
Standard Base Address assignments (h - hexadecimal):
060h + 064h Keyboard controller 170h + 376h Secondary IDE Hard-drive controller 1F0h + 3F6h Primary IDE Hard-drive controller 220h Sound Card 300h NIC card 330h SCSI adapter 3F2h Floppy Drive Controller 3F8h COM1 2F8h COM2 3E8h COM3 2E8h COM4 378h LPT1 278h LPT2*** Base Address conflicts are the NUMBER 3 source of PC problems! ***
Unfortunately, the above table is only a small part of the Base Addresses used. The base addresses used will depend on what has been installed on the PC.
cat /proc/interrupts lists the current interrupts in use cat /proc/ioports lists the ioports (base addresses) in use cat /proc/dma lists the current dmas in use