The computer that changed everything

April 7th, we celebrated the 50th anniversary of computers that made one of the first revolutions in the enterprise computing market. This model was not the first, other computers were already on the market, but it was the heroine of today's article who turned the idea of ​​“computers for business” upside down. I think everyone has already guessed that we are talking about the legendary IBM / 360, which largely laid down the approaches that became the basis of modern computers, both personal and "large", without which we would not have seen all the wonders of modern IT.

The first question to answer is: why was IBM / 360 a turning point for the market? Having discarded various reasons, of which there are many, it’s worth immediately mentioning the main thing - the right approach to architecture and design allowed IBM to make the new model available (relatively, of course). This is what allowed smart machines to step out of government and university computing centers in the field of business, and private business was happy to learn a new, incredibly convenient tool.

I will tell you about some of the innovations that IBM implemented in System / 360, and you can easily compare them with what we use now. Personally, I was surprised when I first found out how much we owe System / 360.

The first IBM innovation so far used was the announcement of a line of computers that differed in price, size and performance, but used a common set of commands (except for a few models for specific markets). This allowed companies to acquire a simpler model, and as requirements grow, to “upgrade” the hardware, without the need to rewrite already debugged software.

The first announcement promised 6 IBM / 360 models and 40 peripherals. Models 30, 40, 50, 60, 62, and 70 were announced. The first three were to replace the “bottom” line of the IBM 1400 series and were sold until 1965. The older models were developed to replace the IBM 7000 series, but did not go on sale, as they were replaced by models 65 and 75, released in late 1965 and early 1966, respectively.



Over time, many other interesting variations appeared. For example, the budget 20 model, which had only 4K base memory, 8 16-bit registers (and not 16 32-bit ones like the rest of the models) and a reduced set of instructions. Another budget model, numbered 22, was essentially a redesigned 30 model with slower I / O ports and memory limits.

Of course, non-budget segments also developed. For example, in model 67 IBM first implemented the technology of dynamic address translation (DAT or dynamic address translation), which is now known to us under the name of “virtual memory”. DAT, in turn, made it possible to implement work with time sharing.



In models 65 and then 67, support for two processors was implemented, and “dual-core” modifications of these systems were supplied to the market.

IBM System / 360 was the first to use microcode technology. In conventional architecture, a high-level language program translates into a series of processor instructions that the latter executes. Actions when executing commands are implemented in hardware and cannot be changed. In the case of using microcode, it is he who determines how certain commands will be executed, matching “lower level” atomic operations with machine instructions. By changing the microcode, it was possible to change how machine instructions are executed, which in turn made it possible to fix any errors, which was impossible when implementing machine instructions “in hardware”. In turn, the use of microcode made it possible to complicate the set of machine instructions and provide more opportunities for developers.

The drawback of the microcode approach is the slower operation of the computer, therefore, in the older System / 360 models IBM already used a “hardware” implementation that excluded the microcode.

Since backward compatibility was very important for IBM customers who had already invested a lot of money in developing software for their previous computers, System / 360 had support for previous-generation computer emulation. So, for example, the 30th model could emulate the IBM 1400 system, and the 65th - the IBM 7094. For this, a complex combination of hardware, microcodes and a virtualization program was used, which allowed the old code to work in the new system. In the first models, to run the program in virtualization mode, the computer had to be stopped and restarted. Later, in the 85 model and System / 370, similar programs could already be launched by the operating system and work simultaneously with "native" applications.

What else should we be grateful to System / 360 for?

- nine-track magnetic tape, which has become practically the standard for storing digital information;
- code table EBCDIC;
- 8-bit bytes. Now this may seem surprising, but during the development of System / 360, for financial reasons, they wanted to limit the byte to 4 or 6 bits. We also considered a variant of bytes with variable length and bit addressing as in IBM 7030;
- byte memory addressing;
- 32 bit words;
- IBM architecture for fractional numbers (in fact, the standard for 20 years);
- The hexadecimal constants used in the System / 360 documentation replaced the octal ones used before.

Of course, the next generation of computers replaced the System / 360. System / 370, System / 390 and System z. Many other companies built their computers based on the System / 360 architecture. Among them are Hitachi, UNIVAC, Amdahl, the Soviet series of EU computers and several others. System / 360 also became the basis of System / 4 Pi - computers protected from radiation and mechanical influences for use in aviation and space technology.

Now in the world there are several more System / 360 computers that are not disassembled for parts, but, unfortunately, none of them work. The costs of maintaining such a large computer in working condition are too high.

If this topic will be interesting to readers, in the next article I will continue the story about this revolutionary computer.