DMA Confusion

elathrop

I took a practice core exam from another site. 1 Question asked how many DMA channels an AT had. Their answer is 8. Mike Meyers book tells me DMA channels 0 and 4 are the same. To me this means there are 7 DMA channels altogether, which is the way I answered the question. To add a little more confusion, Christopher Crayton’s book, “A+ Exam Guides”, says AT computers had one DMA controller. I guess Mr. Crayton’s answer to the question would be 4! Mike Meyers writes that the two 8237 DMA controller chip system started with the 286 which, I believe is an AT.

I’m wondering how the sage Tech-Notes populace would answer this question and that’s what I’ll go with if I’m asked on the test.

ss002d6252

AFAIK There are 8 DMA channels, its just that 2 are allocated for specific use (linking the chips) . I guess its like IRQ's where there are channels 0-15, but 2 & 9 are unavailable as they link the 2 chips together and to the mobo.

See here http://en.wikipedia.org/wiki/Direct_memory_access

elathrop

Thanks for the input. The more I think about it, the more I, too, believe it is 8.

What got me thinking about this, was a practice question on IRQ's. It seems there are 15 IRQ's because 2 & 9 are tied together and count as one.

With DMA channels, I guess, it's different.

kevozz

In my Test Yourself A+ book it says there are 2 DMA controllers on virtually all Intel based microprocessors since 80286. Four channels on each controller for a total of 8. Earlier PC systems based on the 8088 processor had one DMA controller. Hope that helps.

elathrop

Yes it does help, thank you. The evidence is now overwhelming; there are 2 DMA controller chips on AT's and ATX's. However, if you get the chance, take a look at Mike Meyers' AIO 5th edition pages 223 & 224. He teaches that DMA channels (DRQ's) 0 & 4 are cascaded, so, together, they can only be used as 1 DMA channel which brings the grand total to 7. Mike Meyers is a hard source to contradict.

Maybe, I'm just overthinking this one, though, I mean, there are 8 legs comming off of these chips!?