Chapter 8: PCI/G-Bus Bridge
TX7901 User’s Manual (Rev. 6.30T – Nov, 2001)
8-1
8. PCI/G-Bus Bridge
8.1 PCI/G-Bus
Bridge
(“PGB”)
This section describes the TX7901’s PCI to G-Bus Bridge implementation.
8.1.1 Overview
The PCI bus is an industry-standard computer bus and the G-Bus is an on-chip bus
proprietary to Toshiba. The PGB maps transactions between PCI and the G-Bus. The PGB
is fully compliant with the PCI 2.1 specifications with minor exceptions, which are described
later. It implements delayed transactions and posted writes to improve performance. The
PGB also integrates a standard PCI arbiter for five PCI devices.
8.1.1.1 General
Specifications
The G-Bus is a Toshiba-proprietary on-chip bus that has the following important
characteristics:
•
G-Bus operates at up to 66 MHz.
•
G-Bus has non-multiplexed 32-bit addressing and 32-/64-bit data transfers.
•
G-Bus can support multiple masters.
•
G-Bus has a “Retry” mechanism to support delayed transactions.
The PGB is designed with the following features:
•
Fully compliant with Revision 2.1 of the PCI bus specification.
‒
No support for VGA palette snooping.
•
Supports 32-bit PCI bus.
•
PCI bus can run up to 66 MHz.
•
Forwards memory and I/O cycles from PCI to the G-Bus.
•
Forwards memory, I/O cycles, and configuration cycles from the G-Bus to the PCI bus.
•
Provides concurrent operation on both the PCI and G-Bus to isolate traffic between the
two buses.
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