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>Memory Mapped IO</H1
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>Getting Access to the Device</H1
><P
> The most widely supported form of IO is memory mapped IO.
That is, a part of the CPU's address space is interpreted
not as accesses to memory, but as accesses to a device. Some
architectures define devices to be at a fixed address, but most
have some method of discovering devices. The PCI bus walk is a
good example of such a scheme. This document does not cover how
to receive such an address, but assumes you are starting with one.
Physical addresses are of type unsigned long.
</P
><P
> This address should not be used directly. Instead, to get an
address suitable for passing to the accessor functions described
below, you should call <TT
CLASS="FUNCTION"
>ioremap</TT
>.
An address suitable for accessing the device will be returned to you.
</P
><P
> After you've finished using the device (say, in your module's
exit routine), call <TT
CLASS="FUNCTION"
>iounmap</TT
> in order to return
the address space to the kernel. Most architectures allocate new
address space each time you call <TT
CLASS="FUNCTION"
>ioremap</TT
>, and
they can run out unless you call <TT
CLASS="FUNCTION"
>iounmap</TT
>.
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