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Chapter 4 System Support
Compaq Deskpro EN Series of Personal Computers
Desktop and Minitower Form Factors
Third Edition - September 1998
4-2
4.2 PCI BUS OVERVIEW
NOTE:
This section describes the PCI bus in general and highlights bus
implementation in this particular system. For detailed information regarding PCI bus
operation, refer to the PCI Local Bus Specification Revision 2.1.
This system implements a 5-V, 32-bit Peripheral Component Interconnect (PCI) bus operating at
33 MHz. The PCI bus uses a shared address/data bus design. On the first clock cycle of a PCI bus
transaction the bus carries address information. On subsequent cycles, the bus carries data. PCI
transactions occur synchronously with the Host bus at 33 MHz. All I/O transactions involve the
PCI bus. All ISA transactions involving the microprocessor, cache, and memory also involve the
PCI bus. Memory cycles will involve the PCI if the access is initiated by a device or subsystem
other than the microprocessor.
The PCI bus handles address/data transfers through the identification of devices and functions on
the bus (Figure 4-1). A device is defined as a component or slot that resides on the PCI bus. A
function is defined as the end source or target of the bus transaction. A device (component or
slot) may contain one or more functions (i.e., in this system the PCI/ISA Bridge function, EIDE
controller function, USB function, and ACPI function are contained within the South Bridge
component).
Figure 4–1.
PCI Bus Devices and Functions
32-Bit PCI Bus
ISA Bus
PCI Connector Slot 1
Host Bus
PCI Connector Slot 2
PCI Connector Slot 3
PCI Connector Slot 4
82371 South Brid
g
e
EIDE
Controller
Function
PCI/ISA
Bridge
Function
USB
I/F
Function
Power
Manage
Function
82443 North Bridge
Host/PCI
Bridge
Function
PCI/AGP
Bridge
Function
Minitower onl
y
PCI Connector Slot 5