The microprocessor was not directly derived from the mainframe computer. Between the mainframe and the micro lies the minicomputer. Mainframes were very expensive indeed and only large institutions could afford a mainframe computer during the 1950s and 1960s. Advances in semiconductor technology and manufacturing techniques allowed computer companies to build cut-down mainframes, called minicomputers, in the 1960s.

Minicomputers were affordable at the departmental level rather than the individual level; that is, a department of computer science in the 1960s could afford its own minicomputer. A whole generation of students in the 1960s and 70s learned computer science from PDP-11s and NOVAs. Some of these minicomputers were used in real-time applications (i.e., applications in which the computer has to respond to changes in its inputs within a specified time).

The importance of the minicomputer in the history of computer architecture is that it provided a template for its successor, the microprocessor. That is, microprocessor architectures looked much more like minicomputer architectures than mainframes.

One of the first minicomputers was Digital Equipment Corporation’s PDP-5, which was introduced in 1964. This was followed by the PDP-8 in 1966, which had a 12-bit wordlength and could address 2¹² = 4096 memory locations. By using two 3-bit segment registers, the address space could be increased to two sets of 2⁸ = 8 pages of 4 K words. One segment register was used to access program space and one was used to access data space. Later, the Intel 8086 family was to adopt a similar form of segmented addressing.

The PDP-8 had a single 12-bit accumulator and provided a very basic instruction set including addition, subtraction, data movement, Boolean operations and shifts. Program control instructions were of the form “skip on condition”, where “condition” was limited to negative, positive, zero, and not zero accumulator (plus two skip operations dependent on the state of the accumulator’s link bit – a carry bit). An important feature of the PDP-8 was its eight 12-bit index registers that were located in memory. These registers were autoincrementing so that the contents of an index register were incremented whenever it was used to access data.

The principal limitations of the PDP-8 were its lack of registers, its lack of hardware support for a stack pointer, its lack of byte-oriented instructions, and its lack of interrupt prioritization. The PDP-8’s relatively primitive architecture led DEC to design a more sophisticated successor, the PDP-11, in 1969. Even the PDP-11 would be regarded as a very basic machine by today’s standards.

Because of its byte-oriented 16-bit architecture, the PDP-11 was better suited to text processing applications than the PDP-8. The PDP-11 has eight 16-bit general-purpose registers, R0 to R7 and looked very much like the second-generation microprocessors that would appear in under a decade. An important feature of the PDP-11 was the UNIBUS, a data highway used by the CPU to communicate with memory and peripherals. The UNIBUS used an asynchronous protocol to allow data transfers to take place at a rate determined by the characteristics of the bus slave being accessed. A later model, the PDP-11/70, implemented a dedicated bus between its cache and CPU because the UNIBUS was too slow for memory traffic. Adding a dedicated bus to get round technology problems when a standard bus began to grow obsolete was also a feature of the development of the IBM PC.

Digital built on their success with the PDP-11 series and introduced their VAX architecture in 1978 with the VAX-11/780. The VAX had a 32-bit architecture and was available in a range of models. Microprogramming was used to implement its control unit. The VAX dominated the minicomputer world in the 1980s – it was sometimes called a superminicomputer. The VAX range was replaced by the 64-bit Alpha architecture (a high performance microprocessor) in 1991.

It would be wrong to suggest that DEC was the only minicomputer manufacturer. There were several other major players in the minicomputer market. For example, Data General produced its Nova range of computers in the late 1960s and announced its Eclipse series of minicomputers in 1974. Hewlett-Packard’s HP2100 series was another significant range of minicomputers – not least because of its use of microprogramming.

 As a sign of the times, the reader might be interested in the definition of a minicomputer by Kraft and Toy in 1970:

 A minicomputer is a small, general-purpose, stored program digital computer that:

1.   Has a word length ranging from 8 bits to 32 bits.
2.   Provides at least 4096 words of program memory.
3.   Sells for under $20,000 in a minimum usable configuration, consisting of a teletypewriter, a power supply, and a front panel to be used as a programmer’s console.