Chapter 8: PCI/G-Bus Bridge
TX7901 User’s Manual (Rev. 6.30T – Nov, 2001)
8-5
Signal Name
Signal Description
gbsgLastB
G-Bus Last signal In
gbsgBStartB
G-Bus Start In
gbsgBurstB
G-Bus Burst In
gbsgRetryB
G-Bus Retry In
gbsgBSizeB
G-Bus Quad-Word Count In
8.2 Theory of Operation
The PGB handles read and write transactions between PCI and the G-Bus. This section
describes four transaction cases.
Any read crossing the PGB causes writes posted in the same direction to be completed
before any read is done. Posted writes through both sides of the PGB can happen
concurrently. Posted writes and delayed reads can happen concurrently. The PGB will retry
the G-Bus Master if a G-Bus Master and a PCI Master initiate reads simultaneously. Writes
can be posted on the PCI side and reads can be delayed on the G-Bus side concurrently.
The reverse can also occur.
8.2.1 G-Bus Write to PCI (Bridge Master Write)
To improve performance on the G-Bus, the following Bridge master write strategy is used:
Receive write data in the PCI write FIFO if there is enough space to hold the
entire data burst. Retry the transaction if this is not the case.
Up to eight separate write transactions can be posted at any one time. Maximum burst size
supported is 16 words of 64 bits each. When an error occurs, an interrupt is posted to the G-
Bus and the address of the corrupted data is latched for use by the error handler.
Subsequent errors are ignored during the interrupt pending period. Posted writes from the
G-Bus to the PCI bus can happen in parallel with posted writes from the PCI bus to the G-
Bus.
Summary of Contents for TMPR7901
Page 1: ...TX System RISC TX79 Family TMPR7901 Symmetric 2 way superscalar 64 bit CPU ...
Page 14: ...Handling Precautions ...
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Page 17: ...1 Using Toshiba Semiconductors Safely 1 2 ...
Page 41: ...4 Precautions and Usage Considerations 4 2 ...
Page 42: ...TX7901 User s Manual Rev 6 30T November 2001 DOCUMENT NUMBER M 99 00004 07 ...
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