138 21555 Non-Transparent PCI-to-PCI Bridge User Manual
List of Registers
Table 43. Downstream I/O or Memory 1 and Upstream I/O or Memory 0 Setup Registers
These registers may be preloaded by serial ROM or programmed by the local processor before
host configuration.
Bit Name R/W Description
0
Type
Selector
R/(WS)
• When 0, the BAR is requesting memory space, or is disabled.
• When 1, the BAR is requesting I/O space.
• Reset value is 0
2:1 Type R/(WS)
Type of space requested. Allowable values:
• 00b to indicate that the space requested by the BAR may be located
anywhere in memory space, must be used for I/O space
• 01b to indicate that memory space must be mapped below a 1MB
boundary
Other values may have unpredictable results.
Reset value is 00h.
3 Prefetchable R/(WS)
Indicates whether the space requested by the BAR is prefetchable.
• When 0, not prefetchable (required, but not hardware enforced, for I/O
space).
• When 1, prefetchable.
• Reset value is 0
5:4 Reserved R Read only as 0.
30:6 Size R/(WS)
These bits specify the size of the address range requested by the BAR.
• When a bit is 1, the corresponding bit in the BAR functions as a
readable and writable bit.
• When a bit is 0, the corresponding bit in the BAR functions as a
read
-only bit that always returns zero when read.
The PCI Local Bus Specification, Revision 2.2 states that the maximum
value requested for I/O space should not be greater than 256 bytes,
although this is not enforced in hardware. When configured as a memory
range, bits [11:6] should be set to a 0 as the minimum supported memory
range is 4KB.
• Reset value is 0 (disabled).
31 BAR_Enable R/(WS)
BAR enable.
• When 0, the corresponding BAR is disabled and reads as 0.
• When 1, the corresponding BAR is enabled, with size and type
specified by this setup register.
• Reset value is 0
Offsets Downstream I/O or Memory 1 Setup
Upstream I/O or Memory 0
Setup
Primary byte B3:B0h C7:C4h
Secondary byte B3:B0h C7:C4h
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