8/2/2019 chap8-10 -IRQ & DMA

1/8

What is an IRQ ?

An IRQ is really an interrupt Request line. In

the case of expansion board , and it is an

actual contact(finger) on the board, and it is acorresponding pin in the sockets on the bus

connected by a trace on the motherboard . A

device sends a interrupt to the motherboardsinterrupt circuitry by changing the voltage

level on the interrupt request line.

8/2/2019 chap8-10 -IRQ & DMA

2/8

(Cont..)

This voltage change acts via interrupt controller

circuitry to interrupt the processor to service the

device needing the CPU attention.Simply put, if ,

for example , a serial port is receiving data from a

MODEM, it can wait until its buffer overflows for

the CPU to come to its assistance.

8/2/2019 chap8-10 -IRQ & DMA

3/8

(cont..)

The CPU will then temporally store theprogram and data it is working on in an area

of memory know as the stack and load aroutine (program) to service the interruptingdevice, service the device(move the data outof its buffer ) and then restore the program

and then restore the program and it waspreviously working on and resume what itwas doing.

8/2/2019 chap8-10 -IRQ & DMA

4/8

(cont..)

A CPU can be interrupted many times a

second a marvelously complicated affair

when one considers that a CPU can beworking on one interrupt request and be

interrupt in this task by still another IRQ

request of a higher priority , and that itusually does all of this juggling without

losing a beat or data

8/2/2019 chap8-10 -IRQ & DMA

5/8

IRQ USUAL FUNCTION / DEVICE

0 System Timer

1 Keyboard

2 2nd IRQ controller

3 COM2:

4 COM1:

5 Free ( used by LPT2: or sound card)

6 Floppy disk

7 LPT1

8 Real Time Clock (RTC)

9 Free ( May be labeled/appear as IRQ2)

8/2/2019 chap8-10 -IRQ & DMA

6/8

IRQ USUAL FUNCTION/DEVICE

10 Free ( often used by sound cards)

11 Free

12 PS/2 mouse

13 Math coprocessor on CPU (used)

14 Primary IDE15 Secondary IDE

(cont..)

8/2/2019 chap8-10 -IRQ & DMA

7/8

DMA

DMA stands for Direct Memory Access. DMAallows computer peripherals to communicatedirectly with the computers RAM memory, by

passing the processor. With DMA, peripheralswork faster and use less processor power.In moderncomputers, DMA access is negotiated by thecomputers BIOS during startup, or by the

operating system.Computer have a limited numberof DMA channels available, but modern computersrarely run out.

8/2/2019 chap8-10 -IRQ & DMA

8/8

DMA Bus line Typical Default Use Other common uses

0 no Memory refresh None

1 8/16 bit Sound card(low DMA) SCSI host adapters, ECP parallelports, tape accelerator cards,network cards, voice modems.

2 8/16 bit Floppy disk controller Tape accelerator cards

3 8/16 bit none ECP parallel ports, SCSI hostadaptor ,tape accelerator cards,

sound card(low DMA), networkcards, voice modems, hard diskcontroller on old PC/XT

4 no None:Cascade for DMAs 0 - 3 None

5 16 bit only Sound Card ( high DMA) SCSI host adapters, network cards

6 16 bit only None Sound cards (high DMA) , network cards

7 16 bit only None Sound cards (high DMA) network

cards