A personal computer system compatible with application programs and operating system software. The personal computer system includes a microprocessor electrically coupled to a data bus, non-volatile memory electrically coupled to the data bus, volatile memory electrically responsive to the data bus and a direct access storage device electrically responsive to the data bus, the direct access storage device storing a second portion of operating system microcode. The non-volatile memory stores a first portion of operating system microcode and the direct access storage device stores a second portion of operating system microcode. The second portion of operating system microcode includes a boot program. The first portion of operating system microcode verifies the integrity of the boot program prior to loading the boot program into the volatile memory.