Add-in board with enable-disable expansion ROM for PCI bus computers

摘要

An adapter or add-in card for use in a peripheral component interconnect (PCI) computer includes a universal module which couples the card to the PCI bus. The module includes a set of selectively programmable configuration registers which are loaded by a microprocessor on the adapter. A circuit arrangement on the module issues a command which inhibits the PCI processor from accessing the configuration registers until fully loaded. Another circuit arrangement presents the Expansion ROM base address register as a 'read/write' register or a read only register with all bits set to logical "0 " to the PCI computer. If the Expansion ROM base address register is presented as a read only register with all bits set to "0 ", the PCI computer concludes that no Expansion ROM exists on the add-in card, and its contents are not shadowed into the memory of the PCI computer. This disabling of the Expansion ROM causes memory space to be conserved in the computer. If the Expansion ROM is presented as a read/write register with non-zero values, the PCI computer concludes that an Expansion ROM exists and "shadows" its contents into the memory of the PCI computer.