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PCI to Physical Memory Address
The window base address must be on a naturally aligned boundary address depend-
ing on the size of the window
the address space of any PCI device can be located anywhere in the PCI's 4GB mem-
ory space, and this scheme is compatible with the PCI specification:
A PCI device specifies the amount of memory space it requires via the Base reg-
isters in its configuration space. The Base Address registers are implemented so
that the address space consumed by the device is a power of two in size, and is
naturally aligned on the size of the space consumed.
A PCI device need not use all the address range it consumes (that is, the size of the
PCI address window defined by the base address) and it does not need to respond to
unused portions of the address space. The one exception to this is a PCI bridge that
requires two additional registers (the base and limit address registers). These regis-
ters accurately specify the address space that the bridge device will respond to
are programmed by the power-on self-test (POST) code. The 21174, as a PCI host-
bridge device, does not have base and limit registers
addresses defined by the window base register (that is, all addresses within a win-
dow).
Figure 1–17 shows how the DMA address ranges of a number of PCI devices are
accepted by the PCI-window ranges. PCI devices are allowed to have multiple DMA
address ranges, as shown for device 2. The example also shows that the window can
be larger than the corresponding device's DMA address range, as shown for device
0. Device 1 and device 2 have address ranges that are accepted by one window.
Each window determines whether direct mapping or scatter-gather mapping is used
to access physical memory.
1 For example, a 4MB window cannot begin at address 1MB. It must start at addresses
4MB, 8MB, 12MB, ... .
2 A PCI bridge device responds to all addresses in the range: base ≤ address < limit.
3 Host-bridge devices, because they are under system control, are free to violate the rules.
System Address Space
A–34
1
. This rule is not particularly difficult to obey, because
3
, but does respond to all the
2
and
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