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Theoratical DMA question

Posted on 1998-05-29
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Last Modified: 2010-04-27
Hi,
    I have a question regarding DMAs. I understand the concept that the DMA is used to organise the movement of data so that the processor can get on with processing it. What are the implications if 2 DMAs are used? Is there any sort of limit to the number of DMAs you could use? Would it cause bigger problems with keeping the cache consistent?
Thanks in advance,
Nik.
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Question by:PiersBull
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datn earned 400 total points
ID: 1135682
DMA or direct memory access are used by high speed devices that bypasses the CPU, therefore, "direct". Serial and parallel ports are not DMA, but sound cards and SCSI cards often are. In IBM-AT compatible machines, you have access to 7 DMA channels. If you have the same DMA channel being used at the same time, you will have a resource conflict. However, if the devices do not need the DMA simultaneously, it is possible to share the DMA. For example, if you have a tape backup and a network adapter, you can do backups, but not while being on the network.
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by:PiersBull
ID: 1135683
Hi datn,
    Does 7 DMA channels imply that there are 7 DMAs? i.e. 7 seperate pieces of hardware?

Cheers.
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by:datn
ID: 1135684
You actually have two DMA controllers per card. The first 4 DMA does transfers at 8-bit, while the 2nd 4 DMA has 16-bit transfers. Channel 0 does not appear on most 16-bit cards. If it does, it still only transfers data at 8-bits. Channel 1 and 5 are usually for the sound, with channel 5 being only 16-bit. Channel 2 is universally used by the floppy controller. Channel 3 is for ECP parallel ports. Channel 6 is available usually as a SCSI. Channel 7 is also a 16-bit available.
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