Chapter 7: C790 Bus/G-Bus Bridge
TX7901 User’s Manual (Rev. 6.30T – Nov, 2001)
7-2
7.2 Address Space Decode and Translation
The C790 accesses G-Bus devices through one CG internal register window, one ROM
window and four PCI windows. The upper 28-bit address pAddr[31:4] seen on the C790 bus
is copied directly onto the G-Bus while the lower 2–bit address is derived from the byte
enable bits. G-Bus Masters access the main memory through the memory controller on the
C790 Bus via one GC internal register window and five memory windows. The upper 28-bit
address seen on the G-Bus is copied directly into the C790 Bus while the lower 2 address
bits on the G-Bus are used to specify the location of the word/double-word in a quad-word.
(See Figure 7-2.)
Figure 7-2 G-Bridge Address Translation
7.3 Bus
Transactions
The C790 can issue either a single quad-word operation or 4 quad-word burst operations on
the C790 bus. When the C790 initiates a bus cycle to a G-Bus device, the bridge detects the
address and posts the written data into the CGFIFO if it is a write transaction, or initiates a
G-Bus read transaction. Such transactions are called CG (C790 to G-Bus) transactions.
The masters on the G-Bus can initiate a single double-word operation or a burst operation of
up to 8 quad-words (16 double words). When G-Bus masters initiate a bus cycle to main
memory, the bridge detects the address and posts the written data into the GCFIFO if it is a
write transaction, or initiates a C790 Bus read transaction. Such transactions are called GC
(G-Bus to C790) transactions.
The C790 Bus supports the Wrap-Around addressing mode while the G-Bus supports the
linearly-incrementing addressing mode. 4-quad-word burst operation on the C790 Bus is
translated into 4-quad-word aligned burst transaction on the G-Bus.
The C790 and G-Bus Masters can issue their transactions independently and the bridge can
handle and arbitrate their concurrent transactions correctly.
C790 Bus Addr
C790 Bus Addr
ByteEnable
Byte En.
31
4
15
0
31
4 3 2 1 0
7
0
Encode
Reduce
Содержание TMPR7901
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