PI7C8150A
2-PORT PCI-TO-PCI BRIDGE
4.4
VGA SUPPORT
PI7C8150A provides two modes for VGA support:
VGA mode, supporting VGA-compatible addressing
VGA snoop mode, supporting VGA palette forwarding
4.4.1
VGA MODE
When a VGA-compatible device exists downstream from PI7C8150A, set the VGA mode
bit in the bridge control register in configuration space to enable VGA mode. When
PI7C8150A is operating in VGA mode, it forwards downstream those transactions
addressing the VGA frame buffer memory and VGA I/O registers, regardless of the values
of the base and limit address registers. PI7C8150A ignores transactions initiated on the
secondary interface addressing these locations.
The VGA frame buffer consists of the following memory address range:
000A 0000h–000B FFFFh
Read transactions to frame buffer memory are treated as non-prefetchable. PI7C8150A
requests only a single data transfer from the target, and read byte enable bits are forwarded
to the target bus.
The VGA I/O addresses are in the range of 3B0h–3BBh and 3C0h–3DFh I/O. These I/O
addresses are aliases 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 0’s. VGA BIOS addresses starting at C0000h are not decoded in VGA mode.
4.4.2
VGA SNOOP MODE
PI7C8150A provides VGA snoop mode, allowing for VGA palette write transactions to be
forwarded downstream. This mode is used when a graphics device downstream from
PI7C8150A 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
PI7C8150A claims VGA palette write transactions by asserting DEVSEL_L in VGA snoop
mode.
When VGA snoop bit is set, PI7C8150A forwards downstream transactions within the
3C6h, 3C8h and 3C9h I/O addresses space. 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 aliases every 1KB throughout the first 64KB of I/O space.
Note:
If both the VGA mode bit and the VGA snoop bit are set, PI7C8150A behaves in the
same way as if only the VGA mode bit were set.
Page 46 of 111
APRIL 2006 – Revision 1.1
06-0057
PERICOM [ PERICOM SEMICONDUCTOR CORPORATION ]
