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The Second Generation

The transistor was invented during 1947 by John Bardeen, W. H. Brittain, and William Shockley at Bell Labs, and publicly announced on June 30, 1948. Although transistors did consume some power in order to work, they didn't require heater filaments like vacuum tubes, and so their power consumption was far lower. This also meant that they could have a much longer lifetime, and hence higher reliability. And they were considerably smaller.

All these advantages meant that transistors were a revolutionary development in electronics, but that revolution did take several years to come to fruition.

One major reason for this is that the initial kind of transistor that could be produced was the point-contact transistor, which had to be made by hand with difficulty. The diode in a crystal radio was a crystal of galena (lead sulfide), a semiconductor, which was clamped in place by an assembly of conductive metal, making one ohmic contact (a contact where nothing special happens, which behaves like a plain electrical resistance), with the other contact made by a thin wire, called a "cat's whisker", which touched the crystal at a point, making a rectifying contact.

A point-contact transistor used the metal Germanium instead of the mineral galena, it involved one ohmic contact with the Germanium, and two gold or tungsten wires making rectifying contacts that were closely adjacent. These were bipolar transistors; the ohmic contact provided the base of the transistor, and the two rectifying contacts provided the emitter and collector.

As transistors were at first expensive to manufacture, the royalties charged by Bell for their manufacture initially reflected that; but they waived their royalties on transistors used for making hearing aids, to honor Alexander Graham Bell, the inventor of the telephone, who had dealt with a hearing impairment for much of his life. This led to one early Austrian transistorized computer, Mailufterl (the name, referring to a gentle breeze which takes place in Austria during the month of May, being a play on the name of the American early experimental computer with core memory and transistor logic, Whirlwind I), being built out of the rejects from an Austrian hearing-aid factory!

After new designs for transistors, such as mesa transistors, and especially the planar transistor (which could be modified to become an integrated circuit instead) came along, it became practical to use them in computers, which required thousands of logic gates. It is possible to design logic circuits so that not all logic gates require even one transistor or vacuum tube by performing most logic functions with diodes, but a computer would still require many transistors or vacuum tubes even when designed that way.

The first computer built using transistors that we will discuss (well, at least at length, having already mentioned two others) is closely related to the vacuum tube computers from IBM with which we ended the previous page.

As we noted, the IBM 704 was an early machine to use core memory, and also an early machine to have hardware floating-point. Its successor, the IBM 709, had an expanded instruction set, but only a relatively few of them were made.

The IBM 7090 was a re-implementation of the 709, having exactly the same instruction set, but built using transistors.

For many years, the IBM 7090 was widely used by those who needed a powerful scientific computer.


This IBM photograph shows two NASA scientists, Dr. Helmut Hoelzer on the left, seated, and Dr. Rudolph Hoelker on the right, standing, at the console of an IBM 7090 computer.

It is a famous photograph that you may have seen before elsewhere. The very first IBM 7090 that IBM delivered was for NASA at the Marshall Space Flight Center, it arrived in December, 1959; this model was announced in December, 1958, twelve months previously.

The photograph below, also by IBM, shows the IBM 7094 II computer.

Above a front panel similar to that of the IBM 7090, there is a small box added with four rows of fifteen lights. These exhibit the distinguishing feature of the IBM 7094. The 7090 had three index registers, called A, B, and C, and there was a three-bit tag field in the instruction field to indicate which one was used. If more than one was specified, the bits of the index registers specified would be ORed before this value was subtracted from the address.

The 7094, instead, had seven index registers. But it also had "Multiple Tag Mode" to allow the computer to switch to just using the original three index registers for strict compatibility with the 7090 if desired.

Behind the computer, on the left, is a 1301 disk drive. It is around the same size, with a similar set of platters, as the disk drive for the RAMAC, but that disk drive had only one arm with read/write heads, which was positioned to the surface to use. This one, instead, has one arm with read/write heads for each pair of surfaces, connected together in combs, as on modern hard disk drives.

On the right are Hypertape drives. Very few of these were sold by IBM to outside customers, despite the improved performance they offered.

Although the first 7090 was delivered in December, 1959, and the first STRETCH was delivered to the Los Alamos National Laboratory in 1961, the type of transistor circuitry used in the 7090 was originally developed for the STRETCH, pictured below:

It was intended to perform 50-100 times faster than the IBM 704, but did not meet that goal, and so the price of the computer to the customers which had pre-ordered them was decreased, and no more were made.

This image of a 7090 computer shows that its circuitry is within the same style of cabinet as the ones behind the console of the STRETCH in the imagle above:

One of the earliest computers built from transistors was the Control Data 1604, shown below. This was a computer with a 48-bit word length, intended for advanced scientific calculations.

Control Data was a company formed by Seymour Cray and William Norris, after leaving Univac, to which Engineering Research Associates, founded by William Norris, where Cray and others designed the 1103 computer, had been sold.

Later, Control Data introduced the Control Data 6600 computer. It was announced on August 23, 1963, and was for a time the fastest computer in the world. Earlier computers had been using a simple form of pipelining to achieve greater execution speed: while the computer was executing one instruction, it could decode the next instruction, and fetch the instruction after that from memory.

The Control Data 6600 used a technique called "scoreboarding" which allowed it to split the execution of instructions into multiple parts, and execute parts of several instructions at once, while reducing delays due to the fact that instructions that depend on the result of a previous instruction must wait for it. Scoreboarding, however, was only a partial solution to that issue; modern out-of-order execution, which addresses this issue as much as it is possible to do so, came later, with the Tomasulo algorithm developed for the IBM System/360 Model 91 computer, which we will discuss on the next page.

The Control Data 6600 is pictured below:

This is not to say it is defective; the need for register renaming can be avoided simply by using a larger register file - the IBM 360 had four floating-point registers, while RISC processors typically have banks of at least 32 registers. The scoreboard of the Control Data 6600 is eminently suitable for dealing with the remaining use case for out-of-order execution that a larger register file can't solve, cache misses.

That is not to say the CDC 6600 had a cache; it explicitly transferred data from a slower large memory to its smaller main memory with specialized instructions. One advantage it had in providing high performance with a simple design was that it was a new design from scratch, whereas IBM sought to provide high performance and strict compatibility with their existing System/360 line of mainframes, which is what made both the Tomasulo algorithm and cache memory necessities for them.

One of the factors contributing to the high speed of the Control Data 6600 was that it made use of Silicon transistors, which had only recently become available from Fairchild Instruments; previously, only Germanium transistors were available.

The first model of the Digital Equipment Corporation's PDP-10 computer, the version with the KA-10 chassis, used discrete transistors in its circuits, just as its predecessor, the PDP-6, had done. It is pictured below:

Here is another image, in color, focusing in on its attractive front panel:

The PDP-10 was first installed in September of 1967, according to the book Computer Engineering by Ken Olsen, but a contemporary source, the Computer Census in Computers and Automation gives the date as July 1967. Either way, this makes it one of the later examples of a computer made with discrete transistors.

Researchers at MIT created the first time-sharing system on a computer in 1961, using a modified IBM 709 computer; this system was then moved to a modified IBM 7090 and then an IBM 7094 as work on this experimental time-sharing operating system, CTSS, continued. While the MIT computers were used remotely from other post-secondary institutions, CTSS only ran on two machines at MIT at most at any one time.

A brochure for the PDP-10 notes that its predecessor, the PDP-6, was the first computer, in 1964, that offered time-sharing with both the hardware and the software supplied by the computer maker.

The software offered with the PDP-10 was sufficiently well-liked that many users had fond memories of that machine.

Just as transistors replaced vacuum tubes, integrated circuits then replaced discrete transistors.


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