are there more than on of DMA in computer or there is one for all devices?
if so, why there is DMA SELECT in DMA organization?
then , what is burst transfer?
and why it is used with hard disk?

lastly, cpu communicate with DMA to read and write from registers of DMA.
I know cpu communicate with DMA to write no. of word in word count reg. and read it then ; as I know cpu write the address in address reg. of DMA and write control info. in control reg. of DMA , but I don't know why and when it read AR and control reg. of DMA?
in block digram of DMA the lines for all registers of DMA are bidirectional.

please , I need help in this topic. I appreciate any help.

thanks in advance