History of General Purpose CPUs - 1960s: The Computer Revolution and CISC

1960s: The Computer Revolution and CISC

One major problem with early computers was that a program for one would not work on others. Computer companies found that their customers had little reason to remain loyal to a particular brand, as the next computer they purchased would be incompatible anyway. At that point, price and performance were usually the only concerns.

In 1962, IBM tried a new approach to designing computers. The plan was to make an entire family of computers that could all run the same software, but with different performances, and at different prices. As users' requirements grew they could move up to larger computers, and still keep all of their investment in programs, data and storage media.

In order to do this they designed a single reference computer called the System/360 (or S/360). The System/360 was a virtual computer, a reference instruction set and capabilities that all machines in the family would support. In order to provide different classes of machines, each computer in the family would use more or less hardware emulation, and more or less microprogram emulation, to create a machine capable of running the entire System/360 instruction set.

For instance a low-end machine could include a very simple processor for low cost. However this would require the use of a larger microcode emulator to provide the rest of the instruction set, which would slow it down. A high-end machine would use a much more complex processor that could directly process more of the System/360 design, thus running a much simpler and faster emulator.

IBM chose to make the reference instruction set quite complex, and very capable. This was a conscious choice. Even though the computer was complex, its "control store" containing the microprogram would stay relatively small, and could be made with very fast memory. Another important effect was that a single instruction could describe quite a complex sequence of operations. Thus the computers would generally have to fetch fewer instructions from the main memory, which could be made slower, smaller and less expensive for a given combination of speed and price.

As the S/360 was to be a successor to both scientific machines like the 7090 and data processing machines like the 1401, it needed a design that could reasonably support all forms of processing. Hence the instruction set was designed to manipulate not just simple binary numbers, but text, scientific floating-point (similar to the numbers used in a calculator), and the binary coded decimal arithmetic needed by accounting systems.

Almost all following computers included these innovations in some form. This basic set of features is now called a "Complex Instruction Set Computer," or CISC (pronounced "sisk"), a term not invented until many years later, when RISC (Reduced Instruction Set Computer) began to get market share.

In many CISCs, an instruction could access either registers or memory, usually in several different ways. This made the CISCs easier to program, because a programmer could remember just thirty to a hundred instructions, and a set of three to ten addressing modes rather than thousands of distinct instructions. This was called an "orthogonal instruction set." The PDP-11 and Motorola 68000 architecture are examples of nearly orthogonal instruction sets.

There was also the BUNCH (Burroughs, UNIVAC, NCR, Control Data Corporation, and Honeywell) that competed against IBM at this time; however, IBM dominated the era with S/360.

The Burroughs Corporation (which later merged with Sperry/Univac to become Unisys) offered an alternative to S/360 with their B5000 series machines. In 1961, the B5000 had virtual memory, symmetric multiprocessing, a multi-programming operating system (Master Control Program or MCP), written in ALGOL 60, and the industry's first recursive-descent compilers as early as 1963.