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SubjectRe: PCI drivers - memory mapped vs. I/O ports
On Mon, 18 Mar 2002, Ed Vance wrote:

> If a PCI device can be programmed equally well via I/O port space or memory
> space, what are the reasons to chose one space over the other when writing
> the driver?

o There is more 'memory' I/O space. Therefore you are more likely
to have enough space for your device when other boards are
installed. IOTW, you may find that there is no I/O space allocated
because there isn't any available.

o Memory address space I/O is faster on Intel Machines. You can
write or read whole buffers of I/O space in memory address space.
the best you can do in port space is one long-word at a time.

o PCI posted writes work when accessing memory-address I/O space.
Port I/O space cannot do this. The FIFO is written immediately
before any other access is allowed. This improves the access
speed in certain circumstances.

Basically, if you have a choice, it's hands-down to use memory-mapped
I/O space.

Dick Johnson

Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).

Windows-2000/Professional isn't.

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