1. A GCR write control apparatus for use in a data processing system wherein raw data is group coded recorded prior to being written on magnetic tape, comprising: first memory means for storing a plurality of instructions; second memory means for storing a special character table and a code conversion table for converting raw data into GCR data; first addressing means for accessing said first memory means; second addressing means for accessing said second memory means, said second addressing means coupled to the output of said first addressing means and to a source of raw data; transmitting means coupled to the output of said second memory means and synchronized with said raw data for transmitting special characters and converted data to said magnetic tape; and control means for incrementing said first addressing means.