the codice_4 addressing supported in ibm-pc compatible bioses code used eight bits for - theoretically up to 256 heads counted as head 0 up to 255 (codice_5). however, a bug in all versions of microsoft dos/ibm pc dos up to and including 7.10 will cause these operating systems to crash on boot when encountering volumes with 256 heads. therefore, all compatible bioses will use mappings with up to 255 heads (codice_6) only, including in virtual codice_7 geometries.