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 been established (see the previous section), the configuration software must copy a code image into main memory and then disable the ROM address decoder (by clearing bit zero of the Expansion ROM Base Address Register to zero). In a non-PC environment, the area of memory the code image is copied to could be anywhere in memory space. The specification for that environment may define a particular area.

In a PC environment, the ROM code image must be copied into main memory into the range of addresses historically associated with device ROMs: 000C0000h through 000DFFFFh. If the Class Code indicates that this is the VGA's device ROM, its code image must be copied into memory starting at location 000C0000h.

The next section defines the format of the information in the ROM and how the configuration software determines which code image (yes, there can be more than one device driver) to load into main memory.