command was akin to SMS text messaging today, except a maximum of 80 characters could be sent per message. Very early versions of PRIMOS (revision 6) were originally called DOS (PRIMOS 2) and later DOSVM (PRIMOS 3), but starting with PRIMOS 4, on the P400 system, PRIMOS was the name that stuck. There were many major releases of PRIMOS. The last official revision (24.0.0.R52) was released July 3, 1997. By this time, a company called Peritus (which employed a number of ex-Prime engineers) was maintaining PRIMOS. From Revision 19, major portions of PRIMOS were written in the languages SPL and Modula-2, the usage of the Prime Macro Assembler _(PMA), FORTRAN IV and PL/P declined considerably around this time. Programs were guaranteed to run on all current Prime processors (subject to sufficient resources being available), as well as all subsequent Prime processors. In the versions of PRIMOS ca. 1977 and later, the filesystem included a distinctive construct known as the Segment Directory. Unlike more traditional directories, the files anchored in a segment directory were located using an integer index, effectively reducing searches of the directory to a simple hash function. Segment Directories were used in their Keyed-Index/Direct Access (KI/DA) file access system and in later versions of the system loader. Data access Indexed data could be stored in a MIDAS file: Multi-Indexed Data Access System and be accessed via COBOL or FORTRAN. Among the third party tools was a package named Queo, which was more powerful than COBOL despite being less verbose. The PRIMOS character set was basically ASCII but with the 8th bit inverted. The original 7-bit standard for ASCII left the 8th bit unspecified, but on the commonly available Teletype Model 33 ASR, the bit was customarily set to 1, and this became Prime's standard. This is vital to realize when transferring data from PRIMOS to almost any other system. User tools By the time of Prime Computer's demise, a list of languages supported by Primos included: Also available, but relatively uncommon, were: DBASIC Interpreted BASIC with double-precision arithmetic RPG Compiles an RPG II program (non-virtual) SPL Compiles an SPL program VRPG Compiles an RPG II program (virtual) Scripting Late versions of PRIMOS included a scripting language, CPL (Command Processing Language) that ESRI used as a basis for its platform-independent scripting languages AML (for ArcInfo) and SML (PC-ARC/INFO). This was a step beyond what already was available via: COMI