this compatibility. Later versions offloaded the compatibility mode and some of the less used CISC instructions to emulation in the operating system software. Instruction set The VAX instruction set was designed to be powerful and orthogonal. When it was introduced, many programs were written in assembly language, so having a "programmer-friendly" instruction set was important. In time, as more programs were written in high-level programming languages, the instruction set became less visible, and the only ones much concerned about it were compiler writers. One unusual aspect of the VAX instruction set is the presence of register masks at the start of each subprogram. These are arbitrary bit patterns that specify, when control is passed to the subprogram, which registers are to be preserved. Since register masks are a form of data embedded within the executable code, they can make linear parsing of the machine code difficult. This can complicate optimization techniques that are applied on machine code. Operating systems The native VAX operating system is Digital's VAX/VMS (renamed to OpenVMS in 1991 or early 1992 when it was ported to Alpha, modified to comply with POSIX standards, and branded as compliant with XPG4 by the X/Open consortium). The VAX architecture and VMS operating system were "engineered concurrently" to take maximum advantage of each other, as was the initial implementation of the VAXcluster facility. During the 1980s, a hypervisor for the VAX architecture named VMM (Virtual Machine Monitor), also known as the VAX Security Kernel, was developed at Digital with the aim of allowing multiple isolated instances of VMS and ULTRIX to be run on the same hardware. VMM was intended to achieve TCSEC A1 compliance. By the late 1980s, it was operational on VAX 8000 series hardware, but was abandoned before release to customers. Other VAX operating systems have included various releases of Berkeley Software Distribution (BSD) UNIX up to 4.3BSD, Ultrix-32, VAXELN, and Xinu. More recently, NetBSD and OpenBSD have supported various VAX models and some work has been done on porting Linux to the VAX architecture. OpenBSD discontinued support for the architecture in September 2016. History The first VAX model sold was the VAX-11/780, which was introduced on October 25, 1977, at the Digital Equipment Corporation's Annual Meeting of Shareholders. Bill Strecker, C. Gordon Bell's doctoral student at Carnegie Mellon University, was responsible for the architecture. Many different models with different prices, performance levels, and capacities were subsequently