IRQ, DMA and Addresses

(Interrupt Requests, Direct Memory Address)

DMA Direct Memory access

This is used to allow transfer of  data from one place to another in the computer without having the processor do it. It takes the load of the processor and results in faster overall data transfer. The standard computer (PC) has two DMA controllers. The first controls channels 0,1,2,3 and the second channels 4,5,6,7. Thus we have 8 channels altogether.  However, channel 4 is lost since it is used by the Direct memory access controller. The low channels 0-3 is so designated because they move one byte (8 bits) per transfer while the high channels moves 2 bytes per transfer.

Data being moved using DMA is not moved through the DMA controller. This is often referred to as "fly-by" and provides a limitation in the sense that DMA can only move data between devices (I/O ports)  and memory.  It cannot move data between ports or between memory.

DMA Channels                         Common Use

            What other items commonly use DMA channels ???  Parallel ECP ports needs a DMA channel and it has become more common to use buss mastering and DMA for Hard Drive interfaces.

Again sharing DMA channels is generally a NO NO!!!!!

Port Addresses

Each device needs a port address. It is a memory location used by the device for interfacing with the computer. This location is were data is sent by the device for retrieval by the computer and the location which the computer uses to transmit data to the device. Some devices might have a set of Addresses being used for  different functions while other have only one. The size of the address space may also vary from just one bit and up. Some are very standard and quite a few of the legacy devices had fixed port addresses set by jumpers on the card.

PORT ADDRESSES CANNOT BE SHARED

ALL the information about IRQ, DMA and I/O port addresses is found in Win95 by opening  Control Panel----> System Icon -----> Click Device Manager. Then hi light the applicable device and select properties. click the resource tab.

A list of everything is found by hi lighting Computer and then select the appropriate item you are interested in.

Installation Examples:

1. Tape controller - Conner

This is a legacy card. Thus the I/O information is set with jumpers on the card. Thus once these are set there is nothing that can be done within WIN95 to change them. If a conflict occurs your choice is to remove the card and reset the jumpers or to remove the conflicting device and reassign it. Thus, installing these device requires a little planning ahead. It is a very good idea to read the manufacturer's information and check in windows prior to installing.

Here we go !!!

We install the card which is a floppy accelerator card and hook up the tape drive with the cables provided. (The tape drive could have used the floppy drive cable instead but then the tape would have suffered severely in speed and I would not have had this example). We had set the jumpers on the card for default which is:

IRQ 6,  DMA 2 and I/O port 370-375

We boot Win95 and check. Nothing there ?? No Tape no accelerator??   hmm, maybe we should call the manufacturer or Microsoft and complain ?? Opps better read the instructions ???

Next we open Control Panel ---> Add New hardware. Well, it is no use of having window try to detect it. It didn't so it is up to us. We look through the list and on the very bottom it says Tape Controllers - select it. We get another list of manufacture's and there Conner is listed. (here you may have to use disk provided by manufacturer). A least Win95 provided drivers for this one. There is only one choice so we use it. We click OK. Hmmmm now what happened. Here this window appears  and what does it say:

IRQ 9,  DMA 3 and I/O 365-370.

It will not let me change anything either. Is Win95 just stupid ?? or is it trying to outsmart me?? Is it time to get on the phone ??? This is not going to work and it is asking me to reboot - what do I do ??  My time to outsmart Win95 !!!! Answer No !! 

Open control panel ---->system icon then click device manager. There it is ; the tape drive controller with one of those ! next to it. At least Win95 was smart enough to realize it messed up. We just click the plus in front of Tape drive controllers and there it says Conner Tape drive controller. Hi light it and select properties. click the resource tab.

First hi light Input/output then select change settings. A window appears in which we can type in the proper address or we can use the little button on the side and the proper one is one of the choices. We repeat this for the IRQ and DMA. then we select OK and we will be asked to reboot. Now it is time to say yes. In spite of Win95 feeble attempts to confuse us we have prevailed.

In boot up Win95 will now automatically detect the tape drive and install the appropriate drivers.

2. A PNP Modem using  A PNP motherboard.

Try this link

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Last revised: June 07, 1999