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Technical Reference Guide 361834-002 4-3
System Support
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.
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.
Figure 4-2. Configuration Cycle
PCI Configuration Data Register
I/O Port 0CFCh, R/W, (8-, 16-, 32-bit access)
PCI Configuration Address Register
I/O Port 0CF8h, R/W, (32-bit access only)
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
AD31..0
(w/Type 00
Config. Cycle)
Reserved
Function
Number
Register
Index
32211118
7 2 1 0 [1]
IDSEL (only one signal line asserted)
Function
Number
Register
Index
Register 0CF8h
Results in:
NOTES:
[1] Bits <1,0> : 00 = Type 0 Cycle, 01 = Type 1 cycle
Type 01 cycle only. Reserved on Type 00 cycle.
Device
Number
Bus
Number