Blocks can be shared between processes via copy descriptors in the process stack. Thus, some processes may have write permission, whereas others do not. A code segment is read only, thus reentrant and shared between processes. Copy descriptors contain a 20-bit address field giving index of the master descriptor in the master descriptor array. This also implements a very efficient and secure IPC mechanism. Blocks can easily be relocated, since only the master descriptor needs update when a block's status changes.