Chapter 8: PCI/G-Bus Bridge
TX7901 User’s Manual (Rev. 6.30T – Nov, 2001)
8-30
8.4 Register
Dual-Porting
The configuration registers of the PGB consist of two groups; namely, the PCI configuration
register group and the G-Bus configuration register group. The G-Bus configuration register
group is used to configure the G-Bus interface of the PGB and is accessible at all times from
the G-Bus. The PCI configuration register group is used to configure the PCI interface of the
PGB as defined in the PCI 2.1 standard, but can only be written from the G-Bus side when
pgbCSR[2] (PCI Master enable) is set. Two registers are dual-ported between the G-Bus
configuration register group and the PCI configuration register group as described in this
section.
8.4.1 pgbCSR[1] Dual Porting
When pgbCSR[2] is set (i.e. the PGB is in the “HostMode”), the PCI configuration registers
can be written to from the G-Bus, which allows the G-Bus master to take complete control of
the PGB PCI configuration interface.
When pgbCSR[2] is cleared (i.e. the PGB is in the “SatelliteMode”), the PCI configuration
registers cannot be written to from the G-Bus. If pgbCSR[2] and pciCommand[2] are both
cleared, the G-Bus master will not be able to initiate PCI master transactions.
This is why pciCommand[2] is dual ported onto G-Bus register pgbCSR[1]. By dual porting
pciCommand[2] onto G-Bus register pgbCSR[1], the PGB can be in the SatelliteMode and a
G-Bus master can still become the PCI master.
8.4.2 p2gBase[3] Dual Porting
The p2gBase[3] register is dual ported into PCI configuration register [E8h], making it
directly accessible from both sides of the PGB. This allows a PCI master to control the G-
Bus target address of a PCI access to the p2gWindow through the PGB IO base address
range. By initializing the p2gBase[3] register to point to the base of the G-Bus registers, all
other PGB registers can be initialized from the PCI side of the PGB.
8.4.3 Protection
Strategy
The PGB can be set to various protected modes of operation as follows by using different
combinations of pgbCSR[2:1] and bits 1 (Memory Access Enable) and 0 (I/O Access
Enable) of the PCI Command configuration register:
Table 8-17 Protection Levels
Mode
PgbCSR[2:1]
PCI_Command[1:0]
Protection level
Run
0x1
0x1 , 0x2, 0x3
C790 and PCI can access the G-Bus.
Protected
0x2
0x0
PCI cannot access the G-Bus,.
C790 cannot access PCI.
Содержание TMPR7901
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