PIO Addressing and DMA Addressing

PIO Addressing and DMA Addressing

The addressing needs of PIO access and DMA access are different.

PIO deals in small amounts of data, typically single bytes or words. PIO is directed to device registers that are identified with specific VME bus addresses. The association between a device register and its bus address is fixed, typically by setting jumpers or switches on the VME card.

DMA deals with extended segments of kilobytes or megabytes. The addresses used in DMA are not fixed in the device, but are programmed into it just before the data transfer begins. For example, a disk controller device can be programmed to read a certain sector and to write the sector data to a range of 512 consecutive bytes in the VME bus address space. The programming of the disk controller is done by storing numbers into its registers using PIO. While the registers respond only to fixed addresses that are configured into the board, the address to which the disk controller writes its sector data is just a number that is programmed into it each time a transfer is to start.

The key differences between addresses used by PIO and addresses used for DMA are these:

PIO addresses are relatively few in number and cover small spans of data, while DMA addresses can span large ranges of data.
PIO addresses are closely related to the hardware architecture of the device and are configured by hardware or firmware, while DMA addresses are simply parameters programmed into the device before each operation.

In Crimson systems, portions of the physical address space are permanently mapped to portions of the VME address space. PIO is performed to these designated, fixed physical addresses. Some DMA addresses are also assigned fixed mappings, but additional segments are assigned dynamically as needed.

In Challenge and Onyx systems, all VME mappings are dynamic, assigned as needed. Kernel functions are provided to create and release mappings between designated VME addresses and kernel addresses.

It is important to realize that the kernel functions provided for dynamic mapping in the Challenge or Onyx systems also work in the Crimson line. A device driver that uses these functions is portable to all systems supported by IRIX 6.2.