Chapter 8: PCI/G-Bus Bridge
TX7901 User’s Manual (Rev. 6.30T – Nov, 2001)
8-9
8.2.3 PCI Master Writing to G-Bus Slave (Bridge Target Write)
The PGB performs posted writes for all memory and I/O write transactions to the
G-Bus. PCI configuration transactions to the PGB are not forwarded to the G-Bus.
Although PCI bursts may be of arbitrary length, burst transactions on the G-Bus can only
have “packet” sizes as shown in the table below. Because of this, a Bridge target write
strategy is implemented as described below.
Table 8-2 G-Bus Burst Sizes
GBTZ
# of Quad-word
Equivalent
# of Transfers for
64-bit Device
# of Transfers for
32-bit Devices
000
1
2
4
001
2
4
8
Transactions on the G-Bus, whether for memory or I/O, can also be non-burst 32-bit or 64-
bit single data transactions.
8.2.3.1 Posted
Write
The PGB breaks large PCI bursts of data into smaller bursts that fit into the PCI Bridge write
FIFO. The PGB write FIFO can hold a burst of size two quad-wards. The core provides the
word count of the current burst to the G-Bus. Any burst transaction that either comes in with
a size not conforming to the sizes of Table 8-2 or has some bytes disabled is broken down
into a minimum number of smaller size bursts that conform to the sizes in Table 8-2. Posted
writes from the PCI bus to the G-Bus can happen in parallel with posted writes from the G-
Bus to the PCI bus. Bursts transmitted onto the G-Bus have all Byte Enables active. When
the target is a 32-bit data target, the PGB drives the upper 32 data bits to zero. For such a
transaction, the PGB sets the upper four Byte Enable bits to ones. (One means “disabled”.)
Summary of Contents for TMPR7901
Page 1: ...TX System RISC TX79 Family TMPR7901 Symmetric 2 way superscalar 64 bit CPU ...
Page 14: ...Handling Precautions ...
Page 15: ......
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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