AXP (later renamed Alpha), and their own Alpha-based microprocessor, the DECchip 21064, a high performance 64-bit design capable of running OpenVMS. In August 2000, Compaq announced that the remaining VAX models would be discontinued by the end of the year, but old systems remain in widespread use. The Stromasys CHARON-VAX and SIMH software-based VAX emulators remain available. VMS is now developed by VMS Software Incorporated, albeit only for the Alpha, HPE Integrity, and x86-64 platforms. Processor architecture Virtual memory map The VAX virtual memory is divided into four sections. Each is one gigabyte (in the context of addressing, 230 bytes) in size: For VMS, P0 was used for user process space, P1 for process stack, S0 for the operating system, and S1 was reserved. Privilege modes The VAX has four hardware implemented privilege modes: Processor status longword The Process Status Longword contains 32 bits: VAX-based systems The first VAX-based system was the VAX-11/780, a member of the VAX-11 family. The high-end VAX 8600 replaced the VAX-11/780 in October 1984 and was joined by the entry-level MicroVAX minicomputers and the VAXstation workstations in the mid-1980s. The MicroVAX was superseded by the VAX 4000, the VAX 8000 was superseded by the VAX 6000 in the late 1980s and the mainframe-class VAX 9000 was introduced. In the early 1990s, the fault-tolerant VAXft was introduced, as were the Alpha compatible VAX 7000/10000. A variant of various VAX-based systems were sold as the VAXserver. SImultaneous Machine ACceSs (SIMACS) System Industries developed an ability to give more than one DEC CPU, but not at the same time, write access to a shared disk. They implemented an enhancement named SImultaneous Machine ACceSs (SIMACS), which allowed their special disk controller to set a semaphore flag for disk access, allowing multiple WRITES to the same files; the disk is shared by multiple DEC systems. SIMACS also existed on PDP-11 RSTS systems. Canceled systems Canceled systems include the BVAX, a high-end emitter-coupled logic (ECL) based VAX, and two other ECL-based VAX models: Argonaut and Raven. Raven was canceled in 1990. A VAX named Gemini was also canceled, which was a fall-back in case the LSI-based Scorpio failed. It never shipped. Clones A number of VAX clones, both authorized and unauthorized, were produced. Examples include: Systime Computers Ltd of the United Kingdom produced clones of early VAX models such as the Systime 8750 (equivalent to the VAX 11/750). Norden Systems produced