HP Compaq dc7100 DT Technical Reference Manual page 61

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. 2.3) 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 Data Register
I/O Port 0CFCh, R/W, (8-, 16-, 32-bit access)
Bit Function
31 Configuration Enable
30..24 Reserved—read/write 0s
23..16 Bus Number. Selects PCI bus
15..11 PCI Device Number. Selects PCI
10..8 Function Number. Selects function of
7..2 Register Index. Specifies config. reg.
1,0 Configuration Cycle Type ID.
Two types of configuration cycles are used. A Type 0 (zero) cycle is targeted to a device on the
PCI bus on which the cycle is running. A Type 1 cycle is targeted to a device on a downstream
PCI bus as identified by bus number bits <23..16>. With three or more PCI buses, a PCI bridge
may convert a Type 1 to a Type 0 if it's destined for a device being serviced by that bridge or it
may forward the Type 1 cycle unmodified if it is destined for a device being serviced by a
downstream bridge. Figure 4-2 shows the configuration cycle format and how the loading of
0CF8h results in a Type 0 configuration cycle on the PCI bus. The Device Number (bits <15..11>
determines which one of the AD31..11 lines is to be asserted high for the IDSEL signal, which
acts as a "chip select" function for the PCI device to be configured. The function number (CF8h,
bits <10..8>) is used to select a particular function within a PCI component.
Register 0CF8h
Results in:
AD31..0
(w/Type 00
Config. Cycle)
NOTES:
[1] Bits <1,0> : 00 = Type 0 Cycle, 01 = Type 1 cycle
Type 01 cycle only. Reserved on Type 00 cycle.
Figure 4-2. Configuration Cycle
Technical Reference Guide
0 = Disabled
1 = Enable
device for access
selected PCI device.
00 = Type 0
01 = Type 1
3 2 2
Reserved
IDSEL (only one signal line asserted)
PCI Configuration Address Register
I/O Port 0CF8h, R/W, (32-bit access only)
Bit Function
31..0 Configuration Data.
1 1 1 1
Bus Device Function
Number Number
Function
361834-002
System Support
8 7 2 1 0 [1]
Register
Number Index
Register
Number Index
4-3