It Began with Babbage (40 page)

There still lay the problem of how this programlike flexibility could be attained using a regular, diode matrixlike circuit. The solution he arrived at was to use
two
diode matrices, one that would serve to store the control signals in the form of
microinstructions
, much as the computer's main memory stores a program's instructions. The execution of a microinstruction would cause a set of control signals to be issued in parallel.
²⁶
The second diode matrix, organized in tandem with the first, stored the addresses of the “next” microinstruction to select for execution and to control sequencing of the microinstructions. Analogous to a program stored in a computer's main memory as a sequence of instructions, the sequence of microinstructions (along with the addresses) stored in the two diode matrices constituted a
microprogram
. The diode matrices formed the “microprogram store” (or “control store”) collectively.
²⁷
The overall control unit came to be called a
microprogrammed control unit
(
Figure 12.3
).

Wilkes delivered his Manchester lecture in July 1951. In November 1952, the first manuscript devoted entirely to microprogramming was dispatched to the editor of the
Proceedings of the Cambridge Philosophical Society
and was published the following year.
²⁸
The article described in great detail the architecture of a microprogrammed control unit as it might be deployed in a parallel machine.

Wilkes, as remarked, was the quintessential empirical scientist–engineer. Design-as-theory, implementation-as-experiment, testing, and evaluation formed an inextricably entwined quadruple in his mind. Armed with a theory of microprogramming, he would want to see if it worked in practice. As it was, by about the time the EDSAC was operational, he was already ruminating on a machine that would succeed the EDSAC, and he had decided on a parallel machine.
²⁹

This successor, the EDSAC 2, begun in 1953 and fully operational in 1958, was this empirical test of his microprogramming idea.
³⁰
As such, it was a brilliant example in the history of computer design of a successful experimental corroboration of a new principle of design. Microprogramming was used to implement practically all aspects of control in this machine; every significant unit in the machine, including memory, input, and output, were driven from the microprogrammed control unit. As a test of the principles, it was a spectacular success.
³¹

However, the EDSAC 2 was more than an empirical confirmation of microprogramming principles. It was an experimental test bed for a number of ideas Wilkes had broached in his 1951 article in Manchester.
³²
They included the attractiveness of parallel computing (as the term was then understood). A parallel arithmetic unit, certainly, but also with access to main memory, and not in the bit-by-bit mode of the ultrasonic memory used in the EDSAC (or the EDSAC 1, as it began to be called, in deference to its successor) but in bit-parallel fashion. The Williams CRT invented by Frederic C. Williams and Tom Kilburn in Manchester was a possibility (see
Chapter 8
, Section XIII). Indeed, this memory device had found its way into some of the commercial computers that were being manufactured and marketed rapidly during the early 1950s.
³³
But, for Wilkes and his
Cambridge colleagues, it demanded too much “careful engineering” and “careful nursing” to guarantee acceptable performance.
³⁴

Besides, a new kind of memory had emerged in the United State. Forrester, at MIT, had developed a memory made of magnetizable ferrite “cores” shaped rather like doughnuts. The cores were arranged in a two-dimensional matrix, with each core representing a bit, and the corresponding bits of a set of
n
such matrices organized in parallel would constitute the
n
bits of a word of memory. If orthogonal wires were threaded through the central hole in a core and a current was sent through the wires, the core would be magnetized in one direction or the other, and these magnetic states would represent the binary digits 1 and 0. The cores were threaded with wires in such a fashion that the bits of word could be accessed, read out, and written into in parallel, and very rapidly. As a comparison, involving commercial computers during the early 1950s, the UNIVAC 1 (in 1951) had a delay line memory with an access time of 300 microseconds, the IBM 701 (in 1953) using the Williams tube had an access time of 30 microseconds, and the UNIVAC 1103 (in 1953) and the IBM 704 (in 1954) both used ferrite cores with access times of 10 microseconds.
³⁵

Wilkes, on his visit to America in summer and fall 1950 had witnessed the core memory being implemented on the Whirlwind at MIT—and as in the case of seeing the control matrix in the same machine, this made a strong impression on him.
³⁶
By summer 1953, they were receiving, in Cambridge, England, reports from Cambridge, Massachusetts, of the success of the core memory.
³⁷
In the EDSAC 2, not only was the read/write main memory implemented using ferrite core technology, but also the read-only microprogram store was a matrix of 1024 cores.
³⁸

This story of the “best way to design” a computer will be incomplete unless we contemplate the consequences of Wilkes's invention. As we have noted, creativity involves as much the future as the past; one may create something never
before
known, but one may also create something that influences what comes
afterward
.

The trouble with the latter—that is, the consequences of an act of creation—is that one never knows when that might happen. An idea or concept, a discovery or an invention may lie dormant, unattended, for years—until someone perceives its significance in some particular context. Until that moment, that act of creation is inconsequential.

Microprogramming, as a computer design principle, manifested something of this nature. During the 1950s, from the time when Wilkes first outlined the idea, not too many people were enticed by it as a way of designing a computer's control unit. Apart from papers emanating from the EDSAC 2 group, only seven or eight publications on the topic are on record for the entire decade.
³⁹

The 1960s showed a marked increase in interest in microprogramming: articles originating in Britain and the United States, in Italy, France, Russia, Japan, Australia, and
Germany were published. During that decade, some 44 publications were reported by Wilkes in his literature survey of 1969.
⁴⁰

The tipping point that transformed microprogramming from an experimental, exploratory technique belonging to the computer design laboratory into something like a computer design subparadigm was a decision made by IBM during the early 1960s—by which time they were as surely the undisputed leaders in the electronic computer industry as they had formerly been the leaders of the electromechanical punched-card data processing industry. The then-head of IBM's Hursley Laboratory in the United Kingdom drew corporate IBM's attention to the EDSAC 2. The result was the company's decision to use microprogramming as a key design philosophy in the IBM System/360 series of computers marketed during the early to mid 1960s.
⁴¹
The enormous technical and commercial success of the IBM 360 led to the large-scale adoption of microprogramming in commercial computers thereafter. When IBM talked, others listened! Later in this story we will visit this event in more detail because of the significance of the IBM 360 in another respect. As we will also see, microprogramming played a vital role.

The EDSAC 2 held its microprogram in read-only memory. In his Manchester article of 1951, tucked away at the very end, Wilkes envisioned the possibility of a read/write microprogram store that could be written into (thus erasing previous contents) as well as read from. This led to the intriguing possibility that, if the contents of the “erasable” microprogram memory could be changed, then one could design a computer
without a fixed order code
(in present-centered terms, instruction set). Instead, the programmer could design her own order code to meet her particular requirements, and change the contents of the microprogram store accordingly.
⁴²

What Wilkes called “erasable store” came to be known as
writable control store
(or writable control memory), and with the later development of semiconductor memories, the writable control store became a practical reality during the early 1970s.
⁴³
Wilkes's speculation of “a machine with no fixed order” became a practical possibility. A machine with microprogramming capability for a writable control store and no “fixed order” came to be called a
universal host machine
.
⁴⁴
During the 1970s, a number of interesting universal host machines were designed and built, both in universities and by companies, to explore their applications.
⁴⁵
Here was another consequence of Wilkes's invention that he
had
anticipated, albeit speculatively. In these machines, the computer
user
(not the designer) would microprogram the host to suit his or her requirements—thus, customize the host into a desired target machine. Programming could then dissolve into microprogramming.

  
1
. T. Kilburn. (1951). The new computing machine at the University of Manchester.
Nature, 168
, 95–96.

  
2
. S. H. Lavington. (1998).
A history of Manchester computers
(2nd ed., p. 25). Swindon, UK: The British Computer Society.

  
3
. S. Rosen. (1969). Electronic computers: A historical survey.
Computing Surveys, 1
, 7–36 (see especially p. 10).

  
4
. M. V. Wilkes. (1951).
The best way to design an automatic calculating machine
. Report of the Manchester University Computer Inaugural Conference, Manchester, July 1951. Reprinted in E. Mallach & N. Sondak. (Eds.). (1983).
Advances in microprogramming
(pp. 58–60). Dedham, MA: Artech House. See also M. V. Wilkes. (1986). The genesis of microprogramming.
Annals of the History of Computing, 8
, 116–126 (especially pp. 118–121). All citations refer to the Mallach-Sondak reprint.

  
5
. Recall that the EDSAC main memory was implemented as an ultrasonic storage device consisting of a bank of tanks filled with mercury. At one end of each tank, electrical pulses arriving at fixed time intervals were converted by quartz crystals into acoustic pulses that traveled along the length of the tank, were converted at the output end into electrical pulses, were amplified, and passed back to the input end for recycling. The pulses emerging at the output end could also be “read out” to other parts of the computer for processing. Thus, the train of pulses circulating through each tank represented information “stored” in the device, with each pulse (or absence thereof) representing a bit. In the EDSAC, there were 32 such memory tanks, each capable of storing 32 17-bit numbers. Reading out a particular number from a tank would entail a delay until the first bit of that number appeared at the output of a particular tank; there would be a further delay until the remaining 16 pulses (bits) appeared one by one. Hence the bit-serial nature of the EDSAC memory. For more on ultrasonic memories, see M. V. Wilkes. (1956).
Automatic digital computers
. London: Wiley.

  
6
. In a serial arithmetic unit, all operations are done one digit-pair at a time—that is, they follow the way humans normally perform arithmetic operations. In binary computers, the numbers are held as bit strings, so the operations are performed one bit-pair at a time. For an early discussion of serial arithmetic units, see Wilkes 1956, op cit.

  
7
. Wilkes, 1951, op cit., p. 58.

  
8
. Ibid.

  
9
. Ibid.

10
. M. V. Wilkes. (1985).
Memoirs of a computer pioneer
(pp. 164–165). Cambridge, MA: MIT Press.

11
. Wilkes, 1951, op cit., p. 58.

12
. In a parallel arithmetic unit, operations are performed in a bit-parallel manner. For example, an add operation on two 4-bit binary numbers would involve four independent and identical adding circuits (called
half-adders
), each of which would have as inputs one pair of the positionally corresponding bits of the two numbers. For an extensive description of various parallel arithmetic units of the immediate post-EDSAC era, see R. K. Richards. (1957).
Arithmetic operations in digital computers
(pp. 82–92). Princeton, NJ: van Nostrand.

13
. Wilkes, 1951, op cit., p. 59.