When a NIC is configured, you are setting the parameters which tell the computer network software where to find the adapter (base address) and who is "tapping the CPU on the shoulder"
(IRQ). The base address is the pointer to the rest of the world that says "Here I am at base address xxx!". The IRQ is the "tap on the shoulder" to the CPU that says "Hey, it's IRQx, I've got something important to say!". The Upper Memory Block is the NIC's BIOS or actual program in the NIC's ROM. It is set to a free area of memory in the PC's upper memory - to avoid conflicts with other devices (video cards, internal modems, SCSI drivers, etc..).
IRQ - Interrupt Requests
IRQ stands for Interrupt ReQuest. It is a "tap on the shoulder" to the CPU by a peripheral card plugged in an ISA slot to tell the CPU that the peripheral has something to say (also used by EISA and MCA slots). Common peripherals are modems, NICs (network interface cards), sound cards, SCSI adapters, hard-drive controllers, floppy drive controllers, COM ports and printer ports.
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. 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. When 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 not 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 preceding 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 than IRQ11 is available for use for another NIC card or another device. Most autodetecting software or operating systems expect to see the IRQs assigned as above.
Standard COM Port Assignment
Note that COM1 (DB9 on the back of the PC) and COM3 share IRQ4. This is allowed as long as only one device is active at a time. This means that if you are running a mouse on COM1 then you cannot use COM3 for an internal modem. You will run into a conflict.
Some communication packages will allow you to do this but most will choke or cause flaky operation. A common sympton is if you move the mouse, you see garbage on your terminal program.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.
Port IRQ Function
COM1 4 Mouse
COM2 3 Not used or plotter or external modem COM3 4 Not used (conflicts with mouse)
COM4 3 Not used or internal modem
DMA -Direct Memory Access
DMA stands for Direct Memory Access. This is a method that allows channels to be openned by the peripheral to read/write directly to memory without going through the CPU. This off-loads some of the work from the CPU to allow it to do more important tasks.
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 card 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 card Yes 16 bit 16 bit
DMA6 SCSI Yes 16 bit 16 bit
DMA7 Available Yes 16 bit 16 bit
* - DMA4 is cascaded to the first 8 bit DMA controller and is not available.
Note: DMA0 is on the 16 bit ISA bus but is only 8 bits wide.
*** 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. You may have one DMA channel assigned to two devices ONLY if one device is active at a time.
Base Addresses
Base addresses are also called I/O ports, I/O addresses, I/O port addresses and base ports. They are memory locations that provide an interface between the operating system and the I/O device (peripheral). The peripheral communicates with the operating system through the base address.
Each peripheral must have a UNIQUE base address. Standard Base Address assignments (h - hexadecimal):
Base Address Function
060h + 064h Keyboard controller
170h + 376h Secondary IDE Hard-drive controller 1F0h + 3F6h Primary IDE Hard-drive controller
220h Sound Card
*** 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.