原创 A look at the semiconductor revolution (Part 2)

[Continued from A look at the semiconductor revolution (Part 1)]

The market for computers remained relatively small till the PDP-8 brought prices to a more reasonable level, but the match of minis and ICs caused costs to plummet. By the late 1960s everyone was building computers. Xerox. Raytheon (their 704 was possibly the ugliest computer ever built). Interdata. Multidata. Computer Automation. General Automation. Varian. SDS. Xerox. A complete list would fill a page. Minis created a new niche: the embedded system, though that name didn't surface for many years. Labs found that a small machine was perfect for controlling instrumentation, and you'd often find a rack with a built-in mini that was part of an experimenter's equipment.

The PDP-8/E was typical. Introduced in 1970, this 12bit machine cost $6,500 ($38k today). Instead of hundreds of flip chips, the machine used a few large PCBs with gobs of ICs to cut down on interconnects. Circuit density was just awful compared with today's densities. The technology of the time was small scale ICs that contained a couple of flip flops or a few gates, and medium scale integration. An example of the latter is the 74181 ALU, which performed simple math and logic on a pair of four bit operands. Amazingly, TI still sells the military version of this part. It was used in many minicomputers, such as Data General's Nova line and DEC's seminal PDP-11.

The PDP-11 debuted in 1970 for about $11k with 4k words of core memory. Those who wanted a hard disk shelled out more: a 256KW disk with controller ran an extra $14k ($82k today). Today's $100 terabyte drive would have cost the best part of $100 million.

Experienced programmers were immediately smitten with the PDP-11's rich set of addressing modes and completely orthogonal instruction set. Most prior, and too many subsequent, instruction set architectures were constrained by the costs and complexity of the hardware, and were awkward and full of special cases. A decade later IBM incensed many by selecting the 8088, whose instruction set was a mess, over the orthogonal 68000 which in many ways imitated the PDP-11. Around 1990 I traded a case of beer for a PDP-11/70, but eventually was unable to even give it away.

Minicomputers were used in embedded systems even into the 1980s. We put a PDP-11 in a steel mill in 1983. It was sealed in an explosion-proof cabinet and interacted with Z80 processors. The installers had for reasons unknown left a hole in the top of the cabinet. A window in the steel door let operators see the machine's controls and displays. I got a panicked 3 a.m. call one morning—someone had cut a water line in the ceiling. Not only were the computer's lights showing through the window—so was the water level. All of the electronics were submerged. I immediately told them the warranty was void, but over the course of weeks they dried out the boards and got it working again.

I mentioned Data General: they were probably the second most successful mini vendor. Their Nova was a 16bit design introduced a year before the PDP-11, and it was a pretty typical machine in that the instruction set was designed to keep the hardware costs down. A bare-bones unit with no memory ran about $4k—lots less than DEC's offerings. In fact, early versions used a single 74181 ALU with data fed through it a nibble at a time. The circuit boards were 15" x 15", just enormous, populated with a sea of mostly 14- and 16-pin DIP packages. The boards were typically two layers, and often had hand-strung wires where the layout people couldn't get a track across the board. The Nova was a 16bit machine, but peculiar as it could only address 32 KB. Bit 15, if set, meant the data was an indirect address (in modern parlance, a pointer). It was possible to cause the thing to indirect forever.

Before minis, few computers had a production run of even 100 (IBM's 360 was a notable exception). Some minicomputers, though, had were manufactured in the tens of thousands. Those quantities would look laughable when the microprocessor started the modern era of electronics.