In a multiprocessor system having a plurality of main memories and a shared extended memory, each main memory is associated with an extended memory partial write control. When an extended memory partial write instruction is issued, tag information identifying updated portions of main memory data is transferred to the associated extended memory partial write control along with the main memory data. Each time a subblock of the main memory data arrives, the extended memory partial write control performs a partial write operation to substitute those portions of the main memory data which are identified by the tag information for the corresponding portions of a data subblock in a specified extended memory area. During this partial write operation, that specified extended memory area is kept locked.