litwr (litwr) wrote,

Emotional stories about processors for first computers: part 8 (DEC VAX-11)

Processor for DEC VAX-11

VAX-11 systems were quite popular in the 80s, especially in higher education. Now it is difficult to understand some of the concepts described in the books of those years, without knowing the features of the architecture of these systems. VAX-11s were more expensive than PDP-11s, but more oriented towards universal programming and still significantly cheaper than IBM/370 systems. For the VAX architecture, the V-11 processor was made by the mid-80s, and before that time, processor assemblies were used.

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 VAX-11 cannot fail to amaze with its vastness and the presence of very rare and often unique commands, for example, for working with bit fields or several types of queues, for calculating the CRC, multiplying decimal strings, ... Many instructions have both three-address variants (like ARM) and two-address variants (like 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.

But 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-11/30, and the fastest VAX-11 systems – huge cabinets and “whole furniture sets” are at the same level of speed as the first PC ATs. When the 80286 appeared, it became clear that the days of the VAX-11 were numbered and even slowdown with the development of systems based on 80286 could not change anything fundamentally. The straightforward men from Acorn, having made ARM in 1985, without hiding anything, said that ARM is much cheaper and much faster. VAX-11, however, remained relevant until the early 90s, while still having some advantages over the PC, in particular, faster systems for working with disks.

VAX-11 is probably the last mass computer system, in which the convenience of working in assembly language was considered more important than its speed. 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, with its processor speed comparable to the IBM PC AT or ARM Evaluation System

Surprisingly, there is very little literature available on VAX-11 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 are 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. And the cloning of the VAX-11 cost an order of magnitude more resources and led to a dead end. Interest in VAX-11 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-11 is.

Some models of VAX-11 were cloned in the USSR by the end of the 80's, but such clones were produced very little and they almost did not find a use.

Several VAX-11 systems are available for use over the network. And this distinguishes them favorably from the IBM/370 systems with which they competed.

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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