> DMA works for devices, because the device does the memory access. RAM to RAM DMA would need something to do the accesses.
Isn't a blitter exactly that sort of device? Assuming that it can access the relevant RAM, why couldn't that be used for general-purpose memory copying operations?
Yes, but PCs have only rarely had general purpose blitters. They were integrated in some video cards, but that's more or less like DMA; Intel had one for a while recently [1]; FreeBSD loads a driver for it on my Xeon L5640 hosted server, but I don't see any evidence that anything actually uses it. and I'm not sure there was enough actual performance improvement enabled by offloading copies, so Intel stopped including these. Linux marked their driver as broken because it caused issues with copy-on-write [2]
[1] https://lwn.net/Articles/162966/ [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin...