Abstract: When a storage block is returned to the store allocator the master capability table (MCT) entry defining the block must also be deallocated. However, an arbitrary number of capability pointers to that block may still exist in the live system and these must be cancelled before the MCT entry is re-used. The cancellation operation ideally requires a system freeze which cannot be contemplated in a real-time environment. Accordingly each MCT entry includes a "garbage" bit and "visited" bit and the garbage bit is set each time a "load capability register" instruction is performed and a garbage collection process is used to detect blocks which can be freed.