PCI Express System Architecture

ROM Shadowing Required

The PCI spec requires that device ROM code is never executed in place (i.e., from the ROM). It must be copied to main memory. This is referred to as “shadowing” the ROM code. This requirement exists for two reasons:

• ROM access time is typically quite slow, resulting in poor performance whenever the ROM code is fetched for execution.

• Once the initialization portion of the device driver in the ROM has been executed, it can be discarded and the code image in main memory can be shortened to include only the code necessary for run-time operation. The portion of main memory allocated to hold the initialization portion of the code can be freed up, allowing more efficient use of main memory.

Once the presence of the device ROM has ...