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Technical Reference Guide
Compaq Deskpro EN Series of Personal Computers
Desktop and Minitower Form Factors
Third Edition - September 1998
4-5
4.2.3 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).
4.2.3.1 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).
4.2.3.2 Configuration 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.
Bit Function Bit Function
31 Configuration Enable
0 = Disabled
1 = Enable
31..0 Configuration Data.
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 Address Register
I/O Port 0CF8h, R/W, (32-bit access only)
PCI Configuration Data Register
I/O Port 0CFCh, R/W, (8-, 16-, 32-bit access)