This is an old revision of the document!
The DEC VAX CPU is a legendary CISC CPU prized and praised by many low level assembly language coders. The CPU was used in many systems from the 1970's up until the 1990's. The acronym VAX came from the phrase “virtual address extension” this is because it's primary goal was to increase the address space of the PDP-11 which was a 16-bit machine. DEC was a very early implementer of the technique of using virtual memory. RAM was very expensive in those days. There were even compatible modes for the PDP-11 on early VAX machines, but later that capability was taken over by software based emulation. For more technical history on the VAX, we definitely recommend the Wikipedia article on the VAX.