litwr (litwr) wrote,

Emotional stories about processors for first computers: part 8 (DEC VAX)

Processor for DEC VAX

The VAX-11 systems were quite popular in the 80's, especially in higher education. They could emulate the popular PDP-11. Now it is difficult to understand some of the concepts described in books from those years, without knowing the features of the architecture of those systems.

The VAX-11 was more expensive than the PDP-11. However it was more oriented towards universal programming than the PDP-11. Additionally the VAX-11 was significantly cheaper than the IBM/370 systems.

The V-11 processor that was produced by the mid-80s for the VAX architecture, before that time processor assemblies were the only option.

The VAX-11 architecture is 32-bit, it uses 16 registers, among which, like the PDP-11, there is a program counter. It assumes the use of two stacks, one of which is used to store frames of subroutines. In addition one of the registers is assigned to work with the arguments of called functions. Thus, 3 of 16 registers are allocated for stacks.

The instruction system of the VAX-11 cannot fail to amaze with its vastness and the presence of very rare and often unique commands. For example it has commands for working with bit fields, for working with several types of queues, for calculating the CRC, for multiplying decimal strings, etc. Many instructions have both three-address variants (like the ARM) and two-address variants (like the x86), but there are also four-address instructions, for example, the extended division – EDIV. Of course, there is support for working with floating point numbers.

However the VAX-11 is a very slow system for its class and price. Even the super-simple 6502 at 4 MHz could outrun the slowest family member VAX-11/730. The fastest VAX-11 systems – huge cabinets and “whole furniture sets” – was at the same level of speed as the first PC AT's. When the 80286 appeared it became clear that the days of the VAX-11 were numbered and even the slowdown of the development of systems based on the 80286 could not change anything fundamentally. The straightforward people from Acorn having made the ARM in 1985 without hiding anything, said that the ARM is much cheaper and much faster. The VAX-11 however remained relevant until the early 90's, while still having some advantages over the PC, in particular faster systems for working with disks.

There were known compatibility issues among the cheapest VAX-11s. In particular, it was problematic to port Unix to the first VAX-11/730's due to the peculiarities of the implementation of privileged instructions on them.

The VAX is probably the last mass computer system in which the convenience of working in assembly language was considered more important than its performance. In a sense this approach has moved to modern popular scripting languages.

The VAX-11/785 is also a computer (1984) – the fastest among the VAX-11 series, with its processor speed comparable to the IBM PC AT or ARM Evaluation System

Surprisingly there is very little literature available on the VAX systems in open access, as if there is some strange law of oblivion. Several episodes close to politics and correlated with the history of the USSR have been associated with the history of this architecture. It is possible that the actual rejection of the development of the PDP-11 architecture was caused by its low cost and the success of its cloning in the Soviet Union. The cloning of the VAX cost a higher order of magnitude in resources and led to a dead end. Interest in the VAX was created using for example, hoaxes like the famous Kremvax on April 1 1984, in which the then USSR leader Konstantin Chernenko offered to drink vodka on the occasion of connecting to the Usenet network. Another joke was that some VAX-11 chips were impressed with a message in broken Russian about how good the VAX was.

Some first models of the VAX were cloned in the USSR by the end of the 80's, but such clones were produced in very little numbers and they almost did not find a use.

The latest VAX-11 models were made in the mid-80s. Of course, you need to keep in mind that with the VAX-11 the history of VAX computers in DEC did not end there, they were replaced by the VAX 8000 models. In parallel, the development of the MicroVax, VAXstation and VAXserver lines was underway. The VAX 8000 was replaced by the cheaper and somewhat faster VAX 6000. Subsequently in the early 90's MicroVax was replaced by the VAX 4000 models. The VAX processors from the early 90's showed performance at the level of the 80486, but had slightly higher clock speeds. I can assume that the 80486DX4 at 100 MHz and the first Pentium processors began to overtake the best VAX 7000 models in performance. After that, DEC had to abandon support for the VAX instruction system and switch to emulating it in the DEC Alpha systems. There were also VAX 9000 supercomputers and multiprocessor variants, such as the VAX 7000, but these were very expensive systems. We can also mention the high-reliability VAXft systems, in which the processor functions were duplicated, which allowed overcoming any failure of one of the processors. After the V11, there were the CVAX, Rigel, NVAX, and NVAX+ processors.

Several VAX systems are available for use over the network and this distinguishes them favorably from the IBM/370 systems with which they competed.

Edited by Ralph Kernbach and Richard BN

Tags: 80286, acorn, arm, computer, cpu, dec, hardware, history, ibm/370, intel, pc at, pdp-11, processor, v-11, vax-11, x86

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