Chapter 2
AMD-761™ System Controller Programmer’s Interface
89
24081D—February 2002
AMD-761™ System Controller Software/BIOS Design Guide
Preliminary Information
Bit Definitions
AGP 4X Dynamic Compensation (Dev0:F0:0xB4)
Bit
Name
Function
31–28
PVal
P Transistor Strength Value
This field reflects the P transistor strength value that was written to the non-strobed AGP
I/O pads according to Table 9 on page 90.
27–24
NVal
N Transistor Strength Value
This field reflects the N transistor strength value that was written to the non-strobed AGP
I/O pads according to Table 9 on page 90.
23
Reserved
Reserved
22
DisStrb
Disable Strobe
This bit allows the complimentary strobes ADSTB[1:0]# to be disabled when the AGP
interface is operating in 2X mode. Setting this bit causes these pins to be driven High.
21–16
Quantum_Cnt
Quantum Count
This field is used to determine the number of 100-ms intervals that elapse before a
dynamic compensation event is performed when the AlwaysCompensate bit is set. The
value allows for dynamic compensation time quantums to range from 100 ms to 6.4 s.
15–8
Reserved
Reserved
7
FW_Enable
Fast Writes Override
0 = Fast writes disabled
1 = Fast writes enabled (see below)
AGP fast writes are enabled by a combination of this bit and the Fast_Writes enable bit in
the AGP Command register (Dev 0:F0:0xA8, bit 4). The Fast_Writes status bit in the AGP
Status register (Dev 0:F0:0xA4, bit 4) is 0 by default, indicating that the AMD-761™ system
controller does not support this feature. Setting this bit forces the status bit to a 1 to
indicate support of fast writes. The fast writes feature is enabled only when this bit and the
Fast_Writes bit in the AGP Command register (Dev 0:F0:0xA8, bit 4) are set.
6
4X_Override
AGP 4X Override
This bit can be set to override the value in the read-only AGP Status register (Dev
0:FD0:0xA4). By default the rates field of the AGP Status register report 4X capability, but
setting this bit forces the 4X-capable bit to be 0, indicating a maximum of 2X support.
5
Comp3.3
Compensate for 3.3-V Signalling
This bit overrides the TYPEDET# value to force an AGP auto-compensation in a 3.3-V
signalling environment. This bit may be set in conjunction with the Do_Compensate bit to
enable BIOS to determine which drive strength values the auto-compensation circuit
selected for this motherboard.
Note:
This bit must be set only while the AGP interface is
disabled
. Setting this bit
while the AGP interface is enabled results in unpredictable behavior.
4-3
Reserved
Reserved
2
PCI
PCI
As shown in Table 9 on page 90, this bit, along with BYP and AGP2X bits, controls the
drive strength of the output buffer and whether the input buffers are single-ended
or differential.