only available to system mode (unprotected) programs) Segment 8: shared virtual memory for attachment of named address spaces (NASs) (read-only) Segments 9-55: private virtual memory (user segments, read-write to any program) Segments 56-59: private virtual memory (system segments, only available to system mode (unprotected) programs) Segments 60-63: shared virtual memory for attachment of named address spaces (NASs) (read-only) Some real memory is not addressable using virtual memory addresses and so is only available to UMMPS or real memory job programs. Read-only virtual memory may be changed by privileged programs that turn memory protection off (usually for very limited periods of time). Named address spaces (NASs) allow the attachment of named segments of virtual memory. They are shared virtual memory spaces that may be attached and detached from a given job's virtual address space and the same addresses may have different contents depending on which named address spaces are attached. NAS support is mostly used by MTS to attach VM segments preloaded with system components as a way to extend shared virtual memory without using VM address space below the magic 16 MB line and thus keeping more of this valuable address space available for use by 24-bit user programs. Signon and project IDs Everybody who uses MTS is assigned a signon ID (also called userids or Computing Center IDs, CCIDs). Signon IDs are always 4 characters long. If necessary shorter IDs are automatically padded on the right using the string ".$.". Thus, the IDs "MTS.", "DAB.", "ME$." or "C.$." could be written as "MTS", "DAB", "ME" and "C", respectively. Signon IDs are protected using passwords which must be given at the start of each session (as part of or more often immediately after the $SIGNON command). Exceptions are jobs submitted via *BATCH* that run under the same ID that submitted the new job, jobs scheduled to run repeatedly at a particular time or on a particular day when the jobs run under the same ID that scheduled them, or jobs initiated from the operator's console. Passwords are from 1 to 12 characters long, lower case letters are converted to uppercase, special characters other than comma and blank are allowed. Passwords can be changed using the $SET PW command. Changing a password from a terminal session requires entering the original password, and the new password must be entered twice for verification. Entering an incorrect password is counted and reported to the