21150-AB

Manufacturer Part Number21150-AB
DescriptionCommunications, Transparent PCI-to-PCI Bridge
ManufacturerIntel Corporation
21150-AB datasheet
 

Specifications of 21150-AB

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Page 71/164:

VGA Snoop Mode

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The VGA I/O addresses consist of the following I/O addresses:
3B0h–3BBh
3C0h–3DFh
These I/O addresses are aliased every 1KB throughout the first 64KB of I/O space. This means that
address bits <15:10> are not decoded and can be any value, while address bits <31:16> must be all
0s.
VGA BIOS addresses starting at C0000h are not decoded in VGA mode.
5.4.2

VGA Snoop Mode

The 21150 provides VGA snoop mode, allowing for VGA palette write transactions to be
forwarded downstream. This mode is used when a graphics device downstream from the 21150
needs to snoop or respond to VGA palette write transactions. To enable the mode, set the VGA
snoop bit in the command register in configuration space.
Note that the 21150 claims VGA palette write transactions by asserting DEVSEL# in VGA snoop
mode.
When the VGA snoop bit is set, the 21150 forwards downstream transactions with the following
I/O addresses:
3C6h
3C8h
3C9h
Note that these addresses are also forwarded as part of the VGA compatibility mode previously
described. Again, address bits <15:10> are not decoded, while address bits <31:16> must be equal
to 0, which means that these addresses are aliased every 1KB throughout the first 64KB of I/O
space.
Note: If both the VGA mode bit and the VGA snoop bit are set, the 21150 behaves in the same way as if
only the VGA mode bit were set.
Preliminary
Datasheet
21150
63