DMA transfer RAM-to-RAM

15
votes

A friend of mine has told me that on x86 architecture DMA controller can't transfer between two different RAM locations. It can only transfer between RAM and peripheral (such as PCI bus).

Is this true?

Because AFAIK DMA controller should be able between arbitrary devices that sit on BUS and have an address. In particular I see no problem if both source and destionation addresses belong to the same physical device.

3
cwindowsx86dma
Why to copy RAM from one location to another? x86 has a memory paging sistem, so any memory page can be visible in virtual memory at any address. – GJ.

3 Answers

13
votes

ISA (remember? ;-) DMA chips certainly have a Fetch-and-Deposit transfer type.

However, from the MASM32 forums:

Hi,

Checking in "The Undocumented PC", he says memory to memory DMA is possible. He then goes on to say that there can be problems, limitations, and that the CPU can do the copy faster than the DMA hardware anyway (MOVSD on 386+).

So it seems to be a yes you can, but who cares, kind of thing.

Regards,

Steve N.

8
votes

Yes, memory to memory transfer is possible well up to 80386 family I have tried with "modern" x86's :)

Specify RAM for source and destination. You might have to watch out for coherence of the L1 cache depending on the device you are programming and if you have enabled the cache.

You might find some code in the Linux kernel for refreshing video RAM pages in shadow memory. This rings a bell.

2
votes

There are definitely DMA engines that cannot transfer between 2 ram addresses, so the second part of the question is already based on an incorrect premise.