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Scatter-Gather Addressing
Table A–13 Direct-Mapped PCI Target Address Translation
W_MASK<31:20>
0111 1111 1111
1111 1111 1111
Otherwise
1.14 Scatter-Gather Addressing
If the Wx_BASE_SG bit of the PCI base register is set, then the translated address is
generated by a lookup table. This table is called a scatter-gather map. Figure 1–20
shows the scatter-gather addressing scheme — full details of this scheme are pro-
vided later in Section 1.15, but for now a quick description is provided. The incom-
ing PCI address is compared to the PCI window addresses looking for a hit. The
translated base register, associated with the PCI window that is hit, is used to specify
the starting address of the scatter-gather map table in memory. Bits of the incoming
PCI address are used as an offset from this starting address, to access the scatter-
gather PTE. This PTE, in conjunction with the remaining, least-significant PCI
address bits, forms the required memory address.
Each scatter-gather map entry maps an 8KB page of PCI address space into an 8KB
page of the 21164 address space. This offers a number of advantages to software:
•
Performance: ISA devices map to the lower 16MB of memory. The Windows NT
operating system currently copies data from here to user space. The scatter-gather
map eliminates the need for this copy operation.
•
User I/O buffers might not be physically contiguous or contained within a page.
With scatter-gather mapping, software does not have to manage the scattered
nature of the user buffer by copying data.
In the personal computer (PC) world, scatter-gather mapping is not an address trans-
lation scheme but is used to signify a DMA transfer list. An element in this transfer
list contains the DMA address and the number of data items to transfer. The DMA
device fetches each item of the list until the list is empty. Many of the PCI devices
(such as an EISA bridge) support this form of scatter-gather mapping.
System Address Space
A–38
Size of Window
Translated Address <32:2>
2GB
Translated Base<33:31> : ad<30:2>
4GB
Translated Base<33:32> : ad<31:2>
Not supported
—
(Sheet 2 of 2)
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