The microprocessor is microprogrammed and partially pipelined and consists of six major functional units, the I-Box, E-Box, M-box, bus interface unit (BIU), cache, and control store and microsequencer. The I-Box fetches VAX instructions from the cache and decodes them (parses) into macroinstructions. The I-Box has an IROM (Instruction decode ROM) that holds the information required to do so