How does the PIIX4E perform PC/PCI DMA?
PC/PCI DMA will support most I/O devices on the PCI bus via the REQ#/GNT# signals. This I/O device needs to arbitrate for communication with the PIIX4E’s DMA controller on the PCI bus, similar to a PCI Bus Master. To accomplish this, PC/PCI serial protocol must be followed. The requesting agent (I/O device) has to support the proper channel encoding, while the GNT# signal needs to decode the granted information. This is required for proper device synchronization between the I/O device and the PIIX4E, even if it is the only device using a PC/PCI DMA channel. DMA 0-3 are 8-bit count-by-byte channels while DMA 5-7 are 16-bit count-by-word channels. The channel can be configured for any mode, except cascading. Additional information is available in chapter 8.5 of the PIIX4E datasheet.
