The Free On-line Dictionary of Computing (30 December 2018):

bit bashing
bit diddling

   (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.