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VAX
VAX (an acronym for virtual address extension) is a series of computers featuring a 32-bit instruction set architecture (ISA) and virtual memory that was developed and sold by Digital Equipment Corporation (DEC) in the late 20th century. The VAX-11/780, introduced October 25, 1977, was the first of a range of popular and influential computers implementing the VAX ISA. The VAX family was a huge success for DEC, with the last members arriving in the early 1990s. The VAX was succeeded by the DEC Alpha, which included several features from VAX machines to make porting from the VAX easier.
VAX was designed by Digital Equipment Corporation (DEC) as a successor to the 16-bit PDP-11, one of the most successful minicomputers in history with approximately 600,000 units sold. The system was designed to offer backward compatibility with the PDP-11 while extending the memory to a full 32-bit implementation and adding demand paged virtual memory. The name VAX refers to its virtual address extension concept that allowed programs to make use of this newly available memory while still being compatible with unmodified user mode PDP-11 code. The name "VAX-11", used on early models, was chosen to highlight this capability. The VAX ISA is considered a complex instruction set computer (CISC) design.
DEC quickly dropped the −11 branding as PDP-11 compatibility was no longer a major concern. The line expanded to both high-end mainframes like the VAX 9000 as well as to the workstation-scale systems like the VAXstation series. The VAX family ultimately contained ten distinct designs and over 100 individual models in total. All of them were compatible with each other and normally ran the VAX/VMS operating system.
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 instruction set architecture, and instructions for complex operations such as queue insertion or deletion, number formatting, and polynomial evaluation.
The name "VAX" originated as an acronym for virtual address extension, both because the VAX was seen as a 32-bit extension of the 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, orthogonal, and "compiler-friendly". 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. On most architectures, it is up to the compiler to produce instructions to save out the needed data, typically using the call stack for temporary storage. On the VAX, with 16 registers, this might require 16 instructions to save the data and another 16 to restore it. Using the mask, a single 16-bit value performs the same operations internally in hardware, saving time and memory.
Hub AI
VAX AI simulator
(@VAX_simulator)
VAX
VAX (an acronym for virtual address extension) is a series of computers featuring a 32-bit instruction set architecture (ISA) and virtual memory that was developed and sold by Digital Equipment Corporation (DEC) in the late 20th century. The VAX-11/780, introduced October 25, 1977, was the first of a range of popular and influential computers implementing the VAX ISA. The VAX family was a huge success for DEC, with the last members arriving in the early 1990s. The VAX was succeeded by the DEC Alpha, which included several features from VAX machines to make porting from the VAX easier.
VAX was designed by Digital Equipment Corporation (DEC) as a successor to the 16-bit PDP-11, one of the most successful minicomputers in history with approximately 600,000 units sold. The system was designed to offer backward compatibility with the PDP-11 while extending the memory to a full 32-bit implementation and adding demand paged virtual memory. The name VAX refers to its virtual address extension concept that allowed programs to make use of this newly available memory while still being compatible with unmodified user mode PDP-11 code. The name "VAX-11", used on early models, was chosen to highlight this capability. The VAX ISA is considered a complex instruction set computer (CISC) design.
DEC quickly dropped the −11 branding as PDP-11 compatibility was no longer a major concern. The line expanded to both high-end mainframes like the VAX 9000 as well as to the workstation-scale systems like the VAXstation series. The VAX family ultimately contained ten distinct designs and over 100 individual models in total. All of them were compatible with each other and normally ran the VAX/VMS operating system.
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 instruction set architecture, and instructions for complex operations such as queue insertion or deletion, number formatting, and polynomial evaluation.
The name "VAX" originated as an acronym for virtual address extension, both because the VAX was seen as a 32-bit extension of the 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, orthogonal, and "compiler-friendly". 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. On most architectures, it is up to the compiler to produce instructions to save out the needed data, typically using the call stack for temporary storage. On the VAX, with 16 registers, this might require 16 instructions to save the data and another 16 to restore it. Using the mask, a single 16-bit value performs the same operations internally in hardware, saving time and memory.