A special purpose communications protocol processor (CPP) provides more efficient processing of layered communications protocols--e.g. SNA (Systems Network Architecture) and OSI (Open Systems Interconnection)--than contemporary general purpose processors, permitting hitherto unavailable operations relative to high speed communication links. The CPP contains special-purpose circuits dedicated to quick performance (e.g. single machine cycle execution) of functions needed to process header and frame information, such functions and information being of the sort repeatedly encountered in all protocol layers, and uses instructions architected to operate these circuits. The header processing functions given special treatment in this manner include priority branch determination functions, register bit reshaping (rearranging) functions, and instruction address processing functions. Frame processing functions so handled include CRC (cyclic redundancy check) computations, bit insertion/deletion operations and special character detection operations.