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The Minicomputer

A transistorized computer, the PDP-1, was first delivered to Bolt, Beranek, and Newman in November, 1960, made by an up-and-coming computer company, the Digital Equipment Corporation. It had an 18-bit word. Another specimen of this model was sold to MIT, and some students there occasionally used it to play Spacewar.

The next model of computer sold by DEC was the PDP-4. It also had an 18 bit word, but it was not compatible with the PDP-1. It had a simpler design; for example, the opcode field in each instruction was five bits long in the PDP-1, but four bits long in the PDP-4, so the latter had about half as many instructions that involved working with data at an address in memory. The instruction set was even more constrained because it included two versions of most binary arithmetic instructions, one that used one's complement arithmetic and one that used two's complement arithmetic.

They then made an even simpler computer, initially envisaged for industrial process control applications, although from the start it was suitable for general-purpose use, the PDP-5. This computer had a word length of only 12 bits. It used memory location 0 to store its program counter.

They later made a large-scale computer, the PDP-6, with a 36-bit word and hardware floating-point, and a new model of computer compatible with the PDP-4, the PDP-7.

And then DEC made history with the PDP-8 computer. An early publicity photo from DEC of that computer is shown at right. In a small configuration, it could sit on a table top, despite still being made from discrete transistors, placed on circuit cards referred to as FLIP CHIP modules. It was similar to the PDP-5, but with some minor incompatibilities; for one thing, it had a real program counter, and so it moved the interrupt save locations one position earlier in memory; also, the PDP-8 allowed some relatively less useful combinations of operate instructions which the PDP-5 did not; however, nearly all the combinations likely to actually be useful were also allowed on the PDP-5. This was not a serious problem, as few PDP-5 computers were sold, and only a limited amount of software was developed for them.

A later photo, in color, but less detailed, is shown at left. Incidentally, both of these photographs are of prototypes, instead of the actual production PDP-8. In the case of the color photograph, the only distinguishing factor is that all the switches at the bottom of the front panel are of white plastic, rather than mostly being in alternating groups of three white switches and three brown switches. In the case of the black-and-white image, this is also true, but in addition, the three lights indicating the data field are below the three lights indicating the instruction field, instead of being to their left. As well, there was a space between the light for the Link (or carry bit) and the twelve lights for the accumulator contents, while on the production units and the later prototype, that light is directly adjacent to the lights for the accumulator, with all thirteen lights having uniform spacing. (On the PDP-5, the light for the Link was instead with a group of other status lights to the right of the lights for the registers.)

No, I am not kidding. Below is a detail from a significantly more recent color photograph which shows how the front panel looked on a production PDP-8. In another part of the photograph from which this detail is taken, not shown, there was also a prototype of the later PDP-8/S model (all of its switches were white, when again, on the production version of the PDP-8/S, the switches alternated in color).

The original PDP-8 sold for $18,000. It was introduced on March 22, 1965. It is considered to have begun the era of minicomputers. There were computers before that weren't giant mainframes that filled whole rooms; the Bendix G-15 filled one corner of a room, being a bit larger than a refrigerator, despite being made with vacuum tubes; the Recomp II was a box that sat on the floor beside a desk, being about as high as the desk and half as wide.

In 1961, the Packard-Bell pb205 computer was not that much bulkier than a PDP-8 would later be. However, to make it affordable, it used magnetostrictive delay lines instead of core as memory; by then, most computers did use core memory, and unwillingness to give up the convenience that offered may have limited its success. Also, the price was $40,000.


The later PDP-8/S, announced on August 23, 1966, set new milestones in the minicomputer era. It sold for under $10,000, and it could be delivered from stock at the factory - and each regional office would even have one in stock as well, for those who were truly in a hurry. The advertisement shown at right advised prospective customers of this new situation. And it was much more compact than the original PDP-8. However, it achieved its low cost by using much slower core memory, and a serial arithmetic unit, so its lesser performance limited its popularity.

On the previous page, where the impact of the integrated circuit on computers was discussed, a number of minicomputers were shown as examples of computers using integrated circuits. The PDP-8 and the PDP-8/S stand alone as minicomputers made with discrete transistors; once integrated circuits became generally available, thanks to monolithic integrated circuits reaching affordable prices, several companies entered the field to compete with DEC, and, of course, DEC also made use of the new technology.

DEC then implemented this architecture with integrated circuits, providing two models, the full-featured PDP-8/I, which we will see in its two radically different enclosure styles further down on this page, and the less-expensive PDP-8/L for which some options were not available for expansion: the PDP-8/L had the same simple box style as the PDP-11/20 and the later PDP-8/e.

A revised integrated-circuit model included some modifications to the optional feature, the Extended Arithmetic Element, which provided hardware multiplication. This was the PDP-8/e. Introduced in the summer of 1970, its price was initially $6,500, and that price was later reduced to $4,995. DEC encouraged its sale to schools and colleges. Before there were microcomputers, a group called the "People's Computer Company" encouraged individuals to attempt to purchase one if they could afford it; they had a magazine that featured game programs written in BASIC.

Other companies besides DEC made minicomputers.

The Honeywell 316 computer, first made available in 1969, was a minicomputer that followed in the architectural tradition of the Computer Control Company (3c) DDP-116 from 1964; its predecessor, the DDP-516, may have been too large to have been termed a minicomputer, but it was also constructed from integrated circuits. The Hewlett Packard 2116, from 1967, was the first in a line of minicomputers from that company, but I've just recently learned that it was influenced by a computer made by a company acquired by HP: the DSI 1000; but that computer had a 12-bit word length.

I've actually managed to find a picture of this obscure computer. It had a serial ALU, like the PDP-8/S or the Honeywell 112, but in addition, its memory, containing 2,048 words of 12 bits each, consisted of acoustic delay lines made of glass, rather than being of random-access core memory.

On the left is a picture of the HP 2115 itself; this member of the HP 211x series had a front panel similar to that of the top-of-the-line 2116, but it was a compact unit that did not take up a substantial portion of a rack mount enclosure.

Both the Honeywell 316 and the Hewlett-Packard 2116 had 16 bit words; their basic architecture was similar to that of the PDP-8, the PDP-4, or the PDP-1, in that instructions did calculations between an accumulator and one memory location, the memory location was indicated by a short address which included one bit to indicate whether it referred to a location on the same page of memory as the current instruction or a location on the globally shared page zero of memory, and there was also an indirect bit in instructions to allow these short addresses to point to an address that took up a whole word (whether of 12, 16, or 18 bits) to allow broader access to memory. Some of the larger computers of this group also had an index register, and a bit to indicate if its contents would be added to the address before use.


When DEC decided to make its own minicomputer in the popular 16 bit word length, however, rather than designing something similar to the Honeywell 316 and the Hewlett-Packard 2114 with the PDP-8 and the PDP-4 as sources of inspiration, it did something quite different.

The first PDP-11/20 computers were delivered in the spring of 1970.

This computer's instruction word consisted of a four-bit opcode field, followed by two operand fields, each six bits long, consisting of three bits to indicate an addressing mode, and three bits to indicate a register.

If the addressing mode for either or both operands was indexed addressing, for each operand in that mode, a sixteen-bit address was appended to the instruction. The register field was used to indicate a register to use as an index register for the instruction.

So instructions could be 16, 32, or 48 bits in length, and they could be register-to-register, memory-to-register, register-to-memory, or memory-to-memory.

This was more than a little reminiscent of the IBM System/360 computer.

In one important respect, however, the PDP-11 was very unlike the System/360. The System/360 included instructions that worked with packed decimal numbers, and instructions to convert directly between them and character strings. So decimal and binary numbers in memory were organized the same way strings of digits in text were organized - with the most significant part in the lowest memory address. This is known as the "big-endian" numeric representation.

The Honeywell 316 computer, as one example, had instructions to perform a 32-bit two's complement binary addition. It was not as fancy as a System/360 mainframe, and so to make things simple, it picked up the least significant 16 bits of a 32-bit number from one word, then performing that part of the addition and saving the carry for later use, and then picked up the most significant bits of the 32-bit number from the next word. (Actually, it seems my memory is playing tricks on me, and the H-316 was consistently big-endian. However, there were other 16-bit minis that did do what is described here.)

It addressed memory as 16-bit words, not as 8-bit bytes. When character data was packed into 16-bit words, the first of two characters would be in the left, or most significant, half of the word.

So if you put the character string "ABCD" into such a computer, and read it out as a 32-bit integer, that integer would be composed of the ASCII codes for the letters in this order: C, D, A, and B. At the time, this was not much of a concern, but it seemed inelegant.

The PDP-11 addressed memory in 8-bit bytes, as the IBM System/360 did. But it, too, was a small minicomputer intended to be much cheaper than the IBM System/360. So, like the Honeywell 316, when it worked with 32-bit integers, it put the least significant 16-bit word first, and the most significant 16-bit word second.

How to be as beautifully consistent as the System/360, instead of messy like the Honeywell 316?

Well, while much later packed decimal and string hardware became available as options for larger PDP-11 models, it didn't start out with them. So the idea came to them: why not, when packing two characters of text in a 16-bit word, place the first character in the least significant half of the word? And so give that byte the lower address, since here individual bytes were addressable.

So now the ASCII codes for "ABCD" would be found in a 32-bit number in the order D, C, B, and A, which was at least systematic.

The PDP-11 originated the idea of making a computer that was consistently little-endian. A floating-point hardware option made for it, however, put the most significant portions of floating-point numbers in words with lower addresses, thus marring that consistency. But it is still the PDP-11 that inspired many later designs, particularly microprocessors such as the Intel 8008, 8080, 8086, 80386, and so on, the MOS Technology 6502, and the National Semiconductor 16032 to be little-endian. In contrast, the Texas Instruments 9900, as well as the Motorola 6800 and 68000, were big-endian.



The Pedestal Enclosure

Below is a picture of the PDP-8/I, from an advertisement that ran in late 1967. The PDP-8/I actually became available around May of 1968, but the units in the photograph don't embody any noticeable differents from the production units, unlike some prototypes of the original PDP-8.

On the right of the photograph is an example of the rack mounted form of the PDP-8/I of which a great many were sold, and which is quite familiar.

But whatever posessed DEC to also offer the PDP-8/I in the form factor shown on the left?

One possibility is that the PDP-8/I simply would not fit in a box that looked like a pdp-8e or a PDP-8/L or a PDP-8/S, that simply extended directly behind the front panel. Inside the rack mount enclosure, the internals of the PDP-8/I were about as wide as the front panel was high, but they were considerably higher than the front panel was wide.

Note that the pedestal model on the left has a bit of extra width to the right of the front panel area, sufficient to accomodate this extra volume.


However, around that time, the PDP-8/I wasn't the only minicomputer to come in a pedestal model. The Data General Nova, although it normally came in a box with a black front panel - and which even used ordinary toggle switches - was also shown in its pedestal mount enclosure in many advertisements, in a top view such as is shown at right.

Edson de Castro previously worked for DEC before starting Data General; he is said to have left due to dissatisfaction over their plans for the PDP-11 minicomputer.

Is the idea of making minicomputers in pedestal enclosures a secret DEC plan that he walked away with? And if so, why wasn't there a lawsuit? Well, the answer is no; this was not DEC's idea. A third company was also making their minicomputers in pedestal enclosures around that time, and I believe it was the first.


At left we see an image of the humble Honeywell 316 minicomputer. It has an ordinary enough front panel, but with only one row of lights; the five push-buttons in the row below the main array of front-panel switches select which register is displayed.

A ruggedized version of the Honeywell 316 minicomputer was used as the Interface Message Processor at several nodes of the ARPAnet, the network of the Advanced Research Projects Agency (ARPA) of the U.S. Department of Defense which was the direct ancestor of the Internet, giving it some historical importance.

This image is from a brochure, as most of their advertisements at the time in which the 316 is pictured tended to use certain fancy zoom effects.

But below is an image from the same brochure of the Honeywell 316 computer in its pedestal form factor.

And why did Honeywell make the 316 in this form? Well, it was to satisfy the requirements of a particular customer.


The image at left is from the 1969 Christmas catalog of Neiman-Marcus. Neiman-Marcus is an American catalog sales company that has as its unique claim to fame that it offers exotic items of merchandise that other catalog sales outlets are far too pedestrian to include among their offerings.

Yes. The Honeywell 316 was also the notorious Neiman-Marcus "Kitchen Computer".


Data General offered the Supernova as well as the Nova in a pedestal enclosure; this may be partly due to the fact that the Supernova came out very shortly after the Nova, and was ver similar in shape and size - so the tooling was there, and it fit. But the PDP-8/I was DEC's first and only minicomputer in a pedestal enclosure, and the Honeywell 316 minicomputer was the same for Honeywell.




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