BCM1250/BCM1125/BCM1125H
User Manual
10/21/02
B r o a d c o m C o r p o r a t i o n
Page
196
Section 8: PCI Bus and HyperTransport Fabric
Document
1250_1125-UM100CB-R
There is a reset time configuration bit southOnLDT (on generic bus IO_AD[21]) that sets whether the route to
the southbridge is via the PCI bus or HyperTransport fabric. This is done as a hardware configuration option
to allow use of the southbridge before the PCI bus and HyperTransport fabric have been configured (this is in
line with section C.3 of the HyperTransport specification rev 0.17, and section E.3.1 of the HyperTransport
specification rev 1.0). The PCI and HyperTransport device enumeration will discover the southbridge as it
configures the system and can check that the southOnLDT bit is set correctly (if it is not correct there is a
serious hardware error, or an option card has been added with a new southbridge -- both these events should
be reported). If the southbridge is reached through the HyperTransport bridge then the configuration software
should check (and possibly configure, if they are not also setup in hardware) any HyperTransport-
HyperTransport and HyperTransport-PCI bridges on the route to accept and forward packets with the
COMPAT bit set (and from there on set any PCI-PCI bridges to do subtractive decode). If the southbridge is
on the direct PCI bus or is bridged from there by anything other than the internal HyperTransport bridge (which
logically appears as if on the PCI), then the southOnLDT bit must be clear (and any bridges off the PCI must
be set to do subtractive decode).
Since subtractive decode is only used for legacy devices it will only be used for I/O addresses in the bottom of
the range. In this interface, locations in the first 32 KB of the I/O address space can
only
be used for the legacy
subtractive decode devices. This area is shown as region H in
. Any access to this
range will be directed according to the southOnLDT bit, if it is set they will be sent out as HyperTransport
requests with the COMPAT bit set, if it is clear they will be sent to the PCI for subtractive decode. (On the
BCM1125 the access is always made to PCI.)
Similarly, the bottom 16 MB of the memory space region is used for subtractive decode only. If the southOnLDT
bits is set any accesses in this region will be sent on the HyperTransport bus as a 24 bit address with the
COMPAT bit set, otherwise they are sent on the PCI for subtractive decode.
The other legacy device that the PCI and HyperTransport specifications have as special is the VGA display
controller. This has historically used memory addresses
0A_0000
-
0B_FFFF
in the address map and registers
X3B0
-
X3BB
and
X3C0
-
X3DF
in I/O space (per PCI Spec AD [15:10] are ignored for this decode). These
addresses are in the compatibility regions. Rather than being routed based on the SouthOnLDT configuration
bit, the VGA range is routed using the VgaEn bit in the HyperTransport Bridge Control Register. If the VgaEn
bit is clear (indicating the bridge should not forward VGA accesses) VGA accesses are done to the PCI bus,
if the VgaEn bit is set the access is sent to the HyperTransport Fabric but the COMPAT bit is not set allowing
the request to be routed by address to the VGA controller. As with all the compatibility space accesses the
address is masked to a 24 bit address with the upper bits zero.
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