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1.5 Cacheable Memory Space
Cacheable memory space is located in the range 00.0000.0000 to 01.FFFF.FFFF. The
21174 recognizes the first 8GB to be in cacheable memory space. The block size is
fixed at 64 bytes. Read and flush commands to the 21164 caches occur for DMA
traffic.
1.6 PCI Dense Memory Space
PCI dense memory address space is located in the range 86.0000.0000 to
86.FFFF.FFFF. This address space is typically used for memory-like data buffers
such as a video frame buffer or a nonvolatile RAM (NVRAM). Dense space does not
allow byte or word access, but has the following advantages over sparse space:
•
Contiguous locations — Some software, such as the default graphics routines of
the Windows NT operating system, requires memory-like transactions. These
routines cannot use sparse-space addresses, because they require transactions on
the PCI bus to be at adjacent 21164 addresses, instead of being widely separated
as in sparse space. As a result, if the user-mode driver manipulates its frame
buffer in sparse space, it cannot hand over the buffer to the common
Windows NT operating system graphics code.
•
Higher bus bandwidth — PCI bus burst transfers are not usable in sparse space
except for a 2-longword burst for quadword write transactions. Dense space is
defined to allow both burst read and write transactions.
•
Efficient read/write buffering — In sparse space, separate transactions use sepa-
rate read or write buffer entries. Dense space allows separate transactions to be
collapsed in read and write buffers (as the 21164 does).
•
Few memory barriers (MBs) — In general, sparse-space transactions are sepa-
rated by MB instructions to avoid read/write buffer collapsing. Dense-space
transactions only require barriers when explicit ordering is required by the soft-
ware.
Dense space is provided for the 21164 to access PCI memory space, not for access to
PCI I/O space. Dense space has the following characteristics:
•
It holds a one-to-one mapping between 21164 addresses and PCI addresses.
A longword address from the 21164 will map to a longword on the PCI with no
shifting of the address field. Hence, the term dense space. Sparse space, on the
other hand, maps a large piece of 21164 memory space (32 bytes) to a small
piece (such as a byte) on the PCI.
Cacheable Memory Space
System Address Space
A–15
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