Abstract: Duplicate copies of all or a portion of a display screen may be maintained at two or more computer systems with efficient use of memory, processor and bandwidth resources. The display screens are segmented into a two-dimensional matrix of blocks. A value, such as a CRC, is computed for each of the blocks and stored with a pointer to the corresponding block of the display screen. Changes in the display screen are detected by repeatedly calculating the values and comparing with previously stored values for the corresponding block. When the values are different, the pointers are temporarily stored until a predetermined period of time or all the blocks have been checked. When at least one of these criteria is met, adjacent blocks are transmitted as a group, preferably using compression.