AMD-761 Datasheet, PDF (101/264 Pages) Advanced Micro Devices – AMD-761-TM System Controller Software/BIOS Design Guide

24081D—February 2002
Preliminary Information
AMD-761™ System Controller Software/BIOS Design Guide
Bit Definitions
Bit Name
31–28 PVal
27–24 NVal
23 Reserved
22 DisStrb
21–16 Quantum_Cnt
15–8 Reserved
7 FW_Enable
6 4X_Override
5
Comp3.3
4-3 Reserved
2 PCI
AGP 4X Dynamic Compensation (Dev0:F0:0xB4)
Function
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.
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.
Reserved
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.
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.
Reserved
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.
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.
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.
Reserved
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.
Chapter 2
AMD-761™ System Controller Programmer’s Interface
89