Starting a the Overlay Loader, the command LOGI <username> signifies a desire to log in. The system will respond to the LOGI request with PASS? to prompt the user to enter their password. The system does not echo the password characters typed for security purposes. In the event of a successful login, the system responds with an OVL000. In the event of an unsuccesful login, an OVL message is returned indicating whether the interface has been locked for security reasons or not, and then OVL111 to signify that the user is not logged in.
The Meridian-1 (X11) and Succession 1000 (X21) interface is a command line interface (CLI) based on the SL1 overlay loader design. SL stands for Stored Logic. The SL1 overlay loader runs on top of a vxWorks operating system. The best explanation for why it is referred to as an “overlay loader” comes from the original hardware design of the SL1 PBX phone systems. In the 1970s, harddrives were not readily available in computing systems and RAM availability was scarce. Without harddrives and plentiful RAM, application data and configuration data could only be stored on floppy disks. In order to process calls and perform administrative functions, the admin functions were broken in to several “overlays,” each overlay was loaded in to memory using a load overlay command (e.g. “LD 20” is Load Overlay 20, which is a print overlay).
While an overlay was loaded, all sections of the database relevant to that overlay were accessible. This means that if you entered Load 11, you could perform administration of digital telephone sets, Load 10 for analog telephone sets, Load 12 for attendant consoles, and so forth.
Likewise, the limited disk space and memory also contributed to the usage of error codes instead of comprehensive error messages. Thus OVL111 indicates that the interface is idle and no one is logged in to the system. OVL000 indicates that the system is idle and there is no overlay loaded, but that a user is logged in. At last check there are more than 10,000 system messages.