VAX  VAX is a line of computers developed by Digital Equipment Corporation (DEC) in the mid-1970s. The VAX-11/780, introduced on October 25, 1977, was the first of a range of popular and influential computers implementing the VAX instruction set architecture (ISA). A 32-bit system with a complex instruction set computer (CISC) architecture based on DEC's earlier PDP-11, VAX (""virtual address extension"") was designed to extend or replace DEC's various Programmed Data Processor (PDP) ISAs. The VAX architecture's primary features were virtual addressing (for example demand paged virtual memory) and its orthogonal instruction set. VAX has been perceived as the quintessential CISC ISA, with its very large number of assembly-language-programmer-friendly addressing modes and machine instructions, highly orthogonal architecture, and instructions for complex operations such as queue insertion or deletion, number formatting, and polynomial evaluation. It is historically one of the most studied and commented-on ISA's in computer history. VAX was succeeded by the DEC Alpha instruction set architecture. The name "VAX" originated as an acronym for "Virtual Address eXtension", both because the VAX was seen as a 32-bit extension of the older 16-bit PDP-11 and because it was (after Prime Computer) an early adopter of virtual memory to manage this larger address space. Early versions of the VAX processor implement a "compatibility mode" that emulates many of the PDP-11's instructions, giving it the 11 in VAX-11 to highlight this compatibility. Later versions offloaded the compatibility mode and some of the less used CISC instructions to emulation in the operating system software. 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 higher-level language, 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.