Motorola MCP750 Specifications Page 149

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Developing a Device Drive
r
10-3
Programmed I/O Support 10
A programmed I/O device does not directly access physical memory. Instead the device
supplies data to the CPU only when the CPU reads the data directly from device registers.
Data read from a programmed I/O device can be placed in the user’s I/O buffer via the
buffer’s virtual address, which is supplied on a call to the driver’s read routine.
Consult the technical reference manual for your board to determine whether it is a pro-
grammed I/O device.
Data Chaining Support 10
Often, a device must perform DMA transfers that are physically discontiguous. While the
virtual address space assigned to a buffer is contiguous, it might be made up of discontig-
uous physical memory pages.
To accommodate efficient transfers between the device and discontiguous physical mem-
ory, a device sometimes supports data chaining, or scatter/gather I/O. On a gather opera-
tion, the device reads from a number of discontiguous physical memory locations and
transfers the data to the device via DMA. On a scatter operation, the device writes device
data to a number of discontiguous physical memory locations via DMA. Examples of
devices supporting scatter/gather I/O are the HSA and the Interphase V/Ethernet 4207
Eagle device.
Consult the device technical reference manual for your board to determine whether the
device supports this feature. This features greatly simplifies the programming of DMA
operations in the device driver and allows for more efficient data transfer. Refer to
Chapter 12 (“Supporting Direct Memory Access (DMA)”) for an explanation of the pro-
gramming issues related to DMA transfers.
Installing and Testing the Device 10
The purpose of this section is to show how to install your VME device into the Series 6000
platform. Once you have installed your VME device, this section shows how to test the
functionality of the device using the console processor.
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