---------------------------------------------------------------------
  CLASS: SpaceBitmapDesc     --DVM
  PUBLIC METHODS:
   SAME AS ALL TaggedDescriptors:
    Error error()      - returns error if in error state.
    void descLen()     - returns length of the descriptor
    void clear()       - sets state to same as when instantiated
    void toISO()       - Translates to ISO form in supplied buffer
    void fromISO()     - Translates from ISO form in supplied buffer
    Error verifyISO()  - Compares object with ISO form in buffer
  USAGE NOTES:
   - CRC is NOT calculated over the entire bitmap, but rather only
     over the header (i.e., over the structure defined here).
   - descLen() is ALWAYS 24 bytes !!! On read, too?
  
 ----------------------------------------------------------------------