There are several writable fields in the standard PCI Configuration Header for PCI devices. They are:
Some of these are written by the firmware and should never be changed by the device driver. The PCI Configuration Header Programming Table must be followed when programming the PCI Configuration Header registers.
PCI Configuration Header Programming Table
| Register/Bit Name | Firmware Action
(Boot or ibm, configure-connector call) |
Software (Device Driver) Action |
|---|---|---|
| Command/Fast Back-to-Back Enable | Write to a value of 0 on platforms capable of PCI Hot Plug. May be written to a value of 1 on non-Hot-Plug capable platforms if all I/O devices on the same PCI bus are capable of Fast Back-to-Back transfers. | Preserve value |
| Command/SERR# enable | Write a value of 1 | |
| Command/Wait cycle control | Write to a value of 0 (may be hardwired to a 1, so may be a 1 when read even after being written to a 0) | |
| Command/Parity Error Response | Write a value of 1 | |
| Command/VGA Palette snoop | Write a value of 0 | |
| Command/Memory Write and Invalidate Enable | Write to 0 (reset value) | |
| Command/Special Cycles | ||
| Command/Bus Master | Write to 0 (reset value) unless boot device. | Must write to a 1 before the first DMA operation. Must write to a 0 before unconfiguring device driver. |
| Command/Memory Space | Write a value of 0 (reset value) unless boot device, in which case does not write a value of 1 until BARs and Expansion ROM Base Address are set. Only written to a 1 if that specific address space is used for that I/O device. | Must write to a 1 before the first operation (if any) to the I/O devices memory space. Must write to a 0 before unconfiguring device driver. |
| Command/IO Space | Must write to a 1 before the first operation (if any) to the I/O devices I/O space. Must write to a 0 before unconfiguring device driver. | |
| Build-in Self Test (BIST) | Write a value of 0 | If BIST is implemented, can write to a 1 to initiate BIST |
| Latency Timer | Initialize to a system-specific value | Preserve value |
| Cache Line Size | ||
| Base Address Registers | Initialize based on size requested and address space available | Writes based on the ODM M.n and O.n customized attributes |
| Expansion ROM Base Address | Writes based on the ODM M.n and O.n customized attributes. Write LSB to a 0 before enabling the Command/Memory Space if Expansion ROM not used by software. | |
| Interrupt Line | Ignore | Ignore - get information from ODM |