Systems, devices, and methods, including program instructions, are provided for firmware filters and patches. One method embodiment includes executing extended firmware interface (EFI) program instructions during runtime to modify a firmware implementation underlying an interface and invoking the modified firmware implementation, while the original firmware code in non-volatile memory remains unmodified. A further embodiment includes adding a runtime service in an system abstraction layer (SAL) interface and adding a subordinate runtime service that describes a device not known to a firmware image on a system but useable by a particular operating system (OS).