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Register Description
R
92 Intel
®
82845 MCH for SDR Datasheet
3.6.21 BCTRL1—PCI-PCI Bridge Control Register (Device 1)
Address Offset: 3Eh
Default: 00h
Access: RO, R/W
Size 8 bits
This register provides extensions to the PCICMD1 register that are specific to PCI-PCI bridges.
BCTRL1 provides additional control for the secondary interface (i.e., AGP) as well as some bits
that affect the overall behavior of the “virtual” PCI-PCI bridge embedded in the MCH (e.g., VGA
compatible address ranges mapping).
Bit Descriptions
7
Fast Back to Back Enable (FB2BEN)—RO. Hardwired to 0. Since there is only one target
allowed on AGP, this bit is meaningless. The MCH will not generate FB2B cycles in 1x mode, but
will generate FB2B cycles in 2x and 4x Fast Write modes.
6
Secondary Bus Reset (SRESET)—RO. Hardwired to 0. MCH does not support generation of
reset via this bit on the AGP.
Note: The only way to perform a hard reset of the AGP is via the system reset either initiated by
software or hardware via the ICH2.
5
Master Abort Mode (MAMODE)—RO. Hardwired to 0. This means that when acting as a master
on AGP and a Master Abort occurs, the MCH will discard data on writes and return all 1s during
reads.
4
Reserved.
3
VGA Enable (VGA_EN1)—R/W. This bit controls the routing of host-initiated transactions
targeting VGA compatible I/O and memory address ranges.
0 = VGA compatible memory and I/O range accesses are not forwarded to AGP (Default). Rather,
they are mapped to primary PCI unless they are mapped to AGP via I/O and memory range
registers defined above (IOBASE1, IOLIMIT1, MBASE1, MLIMIT1, PMBASE1, PMLIMIT1)
1 = MCH forwards the following host accesses to the AGP:
• Memory accesses in the range 0A0000h to 0BFFFFh
• I/O addresses where A[9:0] are in the ranges 3B0h to 3BBh and 3C0h to 3DFh
(inclusive of ISA address aliases - A[15:10] are not decoded)
When this bit is set, forwarding of these accesses issued by the host is independent of the I/O
address and memory address ranges defined by the previously defined base and limit
registers. Forwarding of these accesses is also independent of the settings of bit 2 (ISA
Enable) of this register if this bit is 1.
Refer to Chapter 4 for further information.
2
ISA Enable (ISA_EN)—R/W. Modifies the response by the MCH to an I/O access issued by the
host that targets ISA I/O addresses. This applies only to I/O addresses that are enabled by the
IOBASE and IOLIMIT registers.
0 = Disable. All addresses defined by the IOBASE and IOLIMIT Registers for host I/O
transactions are mapped to AGP (Default).
1 = Enable. MCH does not forward to AGP any I/O transactions addressing the last 768 bytes in
each 1 KB block, even if the addresses are within the range defined by the IOBASE and
IOLIMIT registers. Instead of going to AGP, these cycles are forwarded to PCI0 where they
can be subtractively or positively claimed by the ISA bridge.
1
Reserved.