Classic PCI Configuration Space
51
March 5 2007 June 2011
SCPS154C
Table 417. Bridge Control Register Description (Continued)
BIT
FIELD NAME
ACCESS
DESCRIPTION
9
SEC_DT
RW
Selects the number of PCI clocks that the bridge waits for the 1394a OHCI master on the
secondary interface to repeat a delayed transaction request. The counter starts once the delayed
completion (the completion of the delayed transaction on the primary interface) has reached the
head of the downstream queue of the bridge (i.e., all ordering requirements have been satisfied
and the bridge is ready to complete the delayed transaction with the initiating master on the
secondary bus). If the master does not repeat the transaction before the counter expires, then the
bridge deletes the delayed transaction from its queue and sets the discard timer status bit.
0 = The secondary discard timer counts 215 PCI clock cycles (default)
1 = The secondary discard timer counts 210 PCI clock cycles
8
PRI_DEC
R
Primary discard timer. This bit has no meaning in PCI Express and is hardwired to 0b.
7
FBB_EN
RW
Fast back-to-back enable. This bit allows software to enable fast back-to-back transactions on the
secondary PCI interface.
0 = Fast back-to-back transactions are disabled (default)
1 = Secondary interface fast back-to-back transactions are enabled
6
SRST
RW
Secondary bus reset. This bit is set when software wishes to reset all devices downstream of the
bridge. Setting this bit causes the PRST signal on the secondary interface to be asserted.
0 = Secondary interface is not in reset state (default)
1 = Secondary interface is in the reset state
5
MAM
RW
Master abort mode. This bit controls the behavior of the bridge when it receives a master abort or
an unsupported request.
0 = Do not report master aborts. Returns FFFF FFFFh on reads and discard data on writes
(default)
1 = Respond with an unsupported request on PCI Express when a master abort is received on
PCI. Respond with target abort on PCI when an unsupported request completion on PCI
Express is received. This bit also enables error signaling on master abort conditions on
posted writes.
4
VGA16
RW
VGA 16-bit decode. This bit enables the bridge to provide full 16-bit decoding for VGA I/O
addresses. This bit only has meaning if the VGA enable bit is set.
0 = Ignore address bits [15:10] when decoding VGA I/O addresses (default)
1 = Decode address bits [15:10] when decoding VGA I/O addresses
3
VGA
RW
VGA enable. This bit modifies the response by the bridge to VGA compatible addresses. If this bit
is set, then the bridge decodes and forwards the following accesses on the primary interface to the
secondary interface (and, conversely, block the forwarding of these addresses from the secondary
to primary interface):
Memory accesses in the range 000A 0000h to 000B FFFFh
I/O addresses in the first 64 KB of the I/O address space (address bits [31:16] are 0000h) and
where address bits [9:0] are in the range of 3B0h to 3BBh or 3C0h to 3DFh (inclusive of ISA
address aliases – address bits [15:10] may possess any value and are not used in the decoding)
If this bit is set, then forwarding of VGA addresses is independent of the value of bit 2 (ISA), the I/O
address and memory address ranges defined by the I/O base and limit registers, the memory base
and limit registers, and the prefetchable memory base and limit registers of the bridge. The
forwarding of VGA addresses is qualified by bits 0 (IO_ENB) and 1 (MEMORY_ENB) in the
command register (offset 04h, see Section
4.3).
0 = Do not forward VGA compatible memory and I/O addresses from the primary to secondary
interface (addresses defined above) unless they are enabled for forwarding by the defined
I/O and memory address ranges (default)
1 = Forward VGA compatible memory and I/O addresses (addresses defined above) from the
primary interface to the secondary interface (if the I/O enable and memory enable bits are
set) independent of the I/O and memory address ranges and independent of the ISA enable
bit
Not Recommended for New Designs