In the mid-1970s, Digital Equipment Corporation (DEC) revolutionized computing with the introduction of Virtual Address Extension, or VAX. This groundbreaking 32-bit Instruction Set Architecture (ISA) and the computer systems built upon it marked a significant leap forward in computing power and versatility.
The Birth of VAX: Bridging High-Level Languages and Hardware
VAX was designed with a clear mission: to execute programs written in high-level programming languages more efficiently than ever before. This innovative architecture replaced the earlier PDP ISA and served as the foundation for the VAX/VMS operating system, both creations of DEC.
VAX: More Than Just an Architecture
VAX represents both a 32-bit ISA and a family of computers that leveraged its power. These systems, ranging from desktop computers to mainframes, typically ran on the VMS operating system. Large VAX multiprocessing clusters could support thousands of users, positioning VAX as a formidable midrange server with virtual memory capabilities.
Why VAX Stood Out
VAX quickly gained favor for several key reasons:
- Impressive processing power
- Exceptional reliability
- User-friendly design
- Versatility in supporting multiple programming languages (FORTRAN, BASIC, PASCAL)
VAX's Innovative Features
VAX introduced several groundbreaking features:
- Virtual memory mapping
- Updated processor status registers
- New addressing modes (pre-decrement, post-decrement, literal, byte, long displacement, word, register deferred, pre-decrement deferred, post-increment deferred)
- Four hardware-implemented privilege modes (User, Kernel, Supervisor, Executive)
A unique aspect of VAX was its use of register masks at the beginning of subprograms, specifying which registers to preserve when passing control.
VAX and Operating Systems
The native operating system for VAX was the Virtual Memory System (VMS), later renamed to OpenVMS. Other supported operating systems included:
- BSD UNIX variants
- ULTRIX-32
- VAXELN
- Xinu
- NetBSD
- OpenBSD
VAX Computers: A Legacy of Innovation
The VAX-11/780, released in 1977, was the first computer based on the VAX ISA. It paved the way for a wide range of systems, including:
- MicroVAX minicomputers
- VAXstation workstations
- VAX 4000, 6000, 8000, and 9000 series
- Fault-tolerant VAXft systems
- Alpha-compatible VAX 7000 and 10000 series
VAX's influence extended beyond DEC, inspiring both authorized and unauthorized clones worldwide, from the UK to Hungary, East Germany, and even China and the Soviet Union.
VAX as the Quintessential CISC Architecture
VAX is often considered the epitome of Complex Instruction Set Computer (CISC) design. Its hallmarks include:
- A vast array of programmer-friendly assembly language instructions
- Multiple addressing modes
- An orthogonal instruction set architecture
- Specialized instructions for complex operations
VAX's Enduring Legacy
Despite the march of technology, VAX systems continue to play a crucial role in many organizations. The architecture's reliability and compatibility with modern systems have ensured its longevity. The transition of OpenVMS to x86 platforms has further extended VAX's influence, maintaining its reputation for unparalleled reliability.
Conclusion
Virtual Address Extension stands as a testament to innovation in computing architecture. Its diverse features and enduring presence in various computing environments underscore VAX's significant impact on the evolution of computer systems. As we continue to push the boundaries of computing, the legacy of VAX reminds us of the importance of robust, versatile architectures in shaping the digital landscape.