A computer system firmware is provided that includes functionality for allowing a calling application to invoke firmware functions through the use of firmware services for getting and setting firmware variables. Firmware functions may be defined and mapped to firmware variable names. When a request is received by the firmware to get or set a value for a particular firmware variable, the firmware determines whether a custom firmware function has been defined that corresponds to the requested firmware variable. If a custom function has been defined that corresponds to the requested firmware variable, then the corresponding custom function is executed rather than the requested get or set operation. A firmware setup application might utilize this mechanism to obtain information from a firmware for use in modifying the configuration of a computer system that would not otherwise be available to the setup application.