Computer system with plug-in override of system ROM

A portable computer system with a special connector, on the motherboard, into which a field-installable boot card can be inserted. The special motherboard connector is wired so that the operator, by setting connections on the field-installable boot card, can bypass the boot memory on the motherboard and force the computer to boot from the memory on the boot card. This permits a technician, in the field, to temporarily override the internal nonvolatile memory which holds the basic system software. This permits recovery of a system in which the basic system software has been corrupted. Preferably the motherboard boot memory is a flash EPROM, and can be rewritten, by setting appropriate jumpers on the boot card, after the computer has booted from the boot card. The motherboard connector is preferably located on the motherboard, and is accessible through a removable cover. This connector can also preferably be used for temporary attachment of a diagnostic display card.