Source : Free On-Line Dictionary of Computing

bit bashing
     
        (Also "bit diddling" or {bit twiddling}).  Any of several
        kinds of low-level programming characterised by manipulation
        of {bit}, {flag}, {nibble}, and other
        smaller-than-character-sized pieces of data.  These include
        low-level device control, encryption algorithms, checksum and
        error-correcting codes, hash functions, some flavours of
        graphics programming (see {bitblt}), and assembler/compiler
        code generation.  May connote either tedium or a real
        technical challenge (more usually the former).  "The command
        decoding for the new tape driver looks pretty solid but the
        bit-bashing for the control registers still has bugs."  See
        also {bit bang}, {mode bit}.