82815 GMCH
R
76
Datasheet
3.4.31.
AGPCTRL—AGP Control Register (Device 0: AGP Mode Only)
Address Offset:
B0–B3h
Default Value:
00000000h
Access:
Read/Write
Size:
32 bits
This register provides for additional control of the AGP interface.
31
8
7
6
1
0
Reserved
GTLB_EN
Reserved
4X
Override
Bit
Description
31:8
Reserved
7
GTLB Enable ( and GTLB Flush Control)—R/W.
1 = Enables normal operations of the Graphics Translation Lookaside Buffer.
0 = Disable (default). The GTLB is flushed by clearing the valid bits associated with each entry. In this
mode of operation all accesses that require translation bypass the GTLB. All requests that are
positively decoded to the graphics aperture force the GMCH to access the translation table in main
memory before completing the request. Translation table entry fetches are not cached in the GTLB.
NOTE:
When an invalid translation table entry is read, this entry is still cached in the GTLB (ejecting the least
recently used entry).
The GMCH flushes the GWB when software sets or clears this bit to ensure coherency between the
GTLB and main memory.
This bit can be changed dynamically (i.e., while an access to GTLB occurs).
6:1
Reserved
0
4X Override.
When this bit is set to 1 the Rate[2] bit in the AGPSTAT register will be read as a 0. This
“back-door” register bit allows BIOS to disable AGP 4X mode.
The introduction of universal AGP cards and universal motherboards has raised some potential
problems that this bit alleviates. AGP 2X can operation at 1.5V or 3.3V. AGP 4X can operate only at
1.5V.
In a system that is supporting 3.3V operation, and therefore cannot support a 4X transfer
rate, it is the responsibility of the BIOS to make sure that 4X mode is not selected.