Compaq W4000 manual PCI BUS Transactions, 1.1 I/O and Memory Cycles, Configuration Cycles

Chapter 4 System Support

4.2.1 PCI BUS TRANSACTIONS

The PCI 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).

4.2.1.1I/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).

4.2.1.2Configuration Cycles

Devices on the PCI bus must comply with PCI protocol that allows configuration of that device by software. In this system, configuration mechanism #1 (as described in the PCI Local Bus specification Rev. 2.1) is employed. This method uses two 32-bit registers for initiating a configuration cycle for accessing the configuration space of a PCI device. The configuration address register (CONFIG_ADDRESS) at 0CF8h holds a value that specifies the PCI bus, PCI device, and specific register to be accessed. The configuration data register (CONFIG_DATA) at 0CFCh contains the configuration data.

PCI Configuration Address Register

I/O Port 0CF8h, R/W, (32-bit access only)

Bit	Function
31	Configuration Enable
	0 = Disabled
	1 = Enable
30..24	Reserved - read/write 0s
23..16	Bus Number. Selects PCI bus
15..11	PCI Device Number. Selects PCI
	device for access
10..8	Function Number. Selects function of
	selected PCI device.
7..2	Register Index. Specifies config. reg.
1,0	Configuration Cycle Type ID.
	00 = Type 0
	01 = Type 1

PCI Configuration Data Register

I/O Port 0CFCh, R/W, (8-, 16-, 32-bit access)

Bit Function

31..0 Configuration Data.

4-4Compaq Evo and Workstation Personal Computers Featuring the Intel Pentium 4 Processor

Second Edition – January 2003