HP DC71XX, DX61XX 4.2.1 PCI 2.3 Bus Operation, I/O and Memory Cycles

System Support

82915G/GV [1]

Integrated

GMCH

RGB Monitor

Memory

Graphics

Cntlr

Controller

Function

PCI Bus 0

Host-PCI Exp.

PCI Express x16 graphics slot [2]

Bridge

Host-DMI Bridge

DMI Link

DMI

82801 ICH6

PCI Bus 1

IDE

SATA

USB I/F

LPC

AC97

PCI 2.3

PCI Exp.

PCI Exp.

Cntlr

Cntlr

Cntlr

Bridge

Cntlr

Bridge

Port 1

Port 2

Function

Function

Function

Function

Function

Function

Function

Function

NIC

Cntlr

PCI Express x1 slot [1]

PCI 2.3 slot(s)

Notes:

[1]USDT form factor; 82915GV; SFF, ST, MT, and CMT form factors, 82915G

[2]SFF. ST, MT, and CMT form factors only.

Figure 4-1. PCI Bus Devices and Functions

4.2.1 PCI 2.3 Bus Operation

The PCI 2.3 bus consists of a 32-bit path (AD31-00 lines) that uses a multiplexed scheme for handling both address and data transfers. A bus transaction consists of an address cycle and one or more data cycles, with each cycle requiring a clock (PCICLK) cycle. High performance is realized during burst modes in which a transaction with contiguous memory locations requires that only one address cycle be conducted and subsequent data cycles are completed using auto-incremented addressing. Four types of address cycles can take place on the PCI bus; I/O, memory, configuration, and special. Address decoding is distributed (left up to each device on the PCI bus).

I/O and Memory Cycles

For I/O and memory cycles, a standard 32-bit address decode (AD31..0) for byte-level addressing is handled by the appropriate PCI device. For memory addressing, PCI devices decode the AD31..2 lines for dword-level addressing and check the AD1,0 lines for burst (linear-incrementing) mode. In burst mode, subsequent data phases are conducted a dword at a time with addressing assumed to increment accordingly (four bytes at a time).

4-2 361834-002 Technical Reference Guide