24888 Rev 3.03 - July 12, 2004 AMD-8151
TM
AGP Tunnel Data Sheet
18
AGP Miscellaneous Control Register DevA:0x40
Default: 0000 0000h Attribute: See below.
Bits Description
31:8 Reserved.
7 Must be low. This bit is required to be low at all times; setting it high results in undefined behavior.
6 Must be low. This bit is required to be low at all times; setting it high results in undefined behavior.
5 Must be low. This bit is required to be low at all times; setting it high results in undefined behavior.
4 Must be low. This bit is required to be low at all times; setting it high results in undefined behavior.
3 FWDIS: fast write disable. Read-write. 1=DevA:0xA4[FWSUP] is low. 0=DevA:0xA4[FWSUP] is
high.
2 8XDIS: AGP 3.0 signaling mode disable. Read-write. 0=The IC drives A_MB8XDET# low to
indicate support for AGP 3.0 signaling. 1=The IC does not drive A_MB8XDET low. This bit may be
used in conjunction with DevB:0x3C[SBRST] to revert back to AGP 2.0 signaling. To do this,
software should (1) set DevB:0x3C[SBRST] in order to reset the AGP card, (2) set 8XDIS to cause
A_MB8XDET# to float high, and (3) clear DevB:0x3C[SBRST].
1 TYPEDET: AGP voltage type detection. Read only. This bit reflects the state of the A_TYPEDET#
pin. 0=The AGP master supports 1.5 volt signaling. 1=The AGP master requires 3.3 volt signaling
and is therefore not compatible with the IC. If this bit is detected high by BIOS, an error should be
signaled.
0 DBIEN: dynamic bus inversion enable. Read-write. 1= A_DBI[H, L] enabled to dynamically invert
the state of the A_AD signals when the IC is driving these. This only applies to AGP 3.0 transfers in
the downstream direction (fast writes and read responses to AGP master requests). For PCI transfers
in the downstream direction, A_DBI[H, L] are held inactive and no inversion takes place. 0=When
the IC drives the A_AD lines, A_DBI[H, L] are driven low. Note: this bit is only valid when 8x
transfer rates are enabled; if (1) DevA:0xA4[AGP3MD]=0 or (2) DevA:0xA4[AGP3MD]=1 and
DevA:0xA8[DRATE] is not 010b, then this field is ignored and the DBI is not enabled.
