Product specifications
ProSavageDDR P4M266 – VT8751 P4 DDR SMA North Bridge
Revision 1.1, July 19, 2002 -54- Functional Description
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PCI Subsystem ID
The Subsystem ID and Subsystem Vendor ID are located in a
32-bit read only register at PCI Configuration Space Index 2C.
These registers reflect the content of 4 read/write CR registers
as follows:
Register
CR
Space
PCI Configuration
Space
Subsystem Vendor ID Low Byte CR81 Index 2CH
Subsystem Vendor ID High
Byte
CR82 Index 2DH
Subsystem ID Low Byte CR83 Index 2EH
Subsystem ID High Byte CR84 Index 2FH
Table 12. PCI Subsystem ID and Subsystem Vendor
ID Registers
These registers allow identification of particular vendors using
the same graphics chip. The following design allows the
subsystem identification to be handled by software (no
hardwiring).
All P4M266 motherboard designs will incorporate the video
BIOS into the system BIOS ROM. The system BIOS must
load the subsystem ID information in the P4M266 before any
ID scanning takes place. To do this, it must turn on the
P4M266, enable I/O accesses in the PCI configuration space,
unlock the CR registers, program the subsystem ID
information in the registers described above, then turn off the
P4M266.
Integrated AGP
P4M266 graphics conform with the requirements of Revision
2.0 of the AGP Interface Specification. Internal AGP P4M266
graphics are enabled by default, but the internal graphics
controller may be disabled in order to enable the external AGP
bus instead.
For the most part, AGP configuration is identical to PCI (V-
Link) configuration. PCI04[4] is hardwired to 1 to indicate
that P4M266 graphics implements a list of capabilities.
PCI34[7-0] point to the location of this list, which is at offset
80H. PCI80, PCI84 and PCI88 implement the register bits
required by the AGP specification. PCI88[2-0] select the data
rate. PCI88[8] = 1 enables AGP bus master operations.
PCI88[9] =1 enables sideband addressing. This is indicated
by PCI84[9] (1 = sideband addressing supported). The state of
PCI84[9] is determined by the state of CR70[7].