40 years of IBM PC: the computer that unintentionally ushered in the IT revolution

The IBM Personal Computer in its original configuration, Model 5150, will celebrate its 40th birthday today, August 12, 2021. At the time, IBM hadn’t planned it as a long-term product category; the New York headquarters were annoyed by the burgeoning competition from Commodore, Tandy, Apple and Co. The small computers were about to become serious competitors in the offices. Apple’s concept in particular seemed dangerous, people heard threatening news about the Apple III, and so the IBM managers wanted to spoil the business of the rebellious Steve Jobs in particular. Otherwise they had nothing to do with the PC.

So IBM hired a development team to put something together from the grab box that they could counter to Apple. The expandability of the Apple II with its many slots – for which, among other things, a small start-up called Microsoft Z80 cards was developing at the time – had proven to be a successful concept, so the IBM PC had to come up with something like that too.

You could have connected to the EC bus from Kontron, which is already well-known in Europe, but why? Its 64- or 96-pin connectors were too expensive and, moreover, they wanted to choose a bus assignment that just happened to fit the board layout. A reasonable timing of the bus signals? Incidentally, the main thing is that it works – later with the IBM AT you would even have needed signals with negative transit times so that they arrive on time.

The IBM PC with Intel’s 8088 processor was also not cheap: the basic configuration with 16 KByte RAM cost around 1500 US dollars without a monitor, and at least a keyboard was included. With 64 Kbytes of RAM, 160 Kbytes of floppy disk drive and monochrome monitor, the price rose to more than 3,000 US dollars. The high price and the inadequate graphics capabilities also meant that hardly any private user turned to a PC.

Any halfway talented hobbyist could have conjured up better things, but IBM didn’t want it at all. This was also reflected in the budget: At 36 million US dollars, the marketing campaign was more expensive than the development. The product should be exactly like this, so as not to cannibalize your own larger computers. Those in charge didn’t care that the processor manufacturer had reserved certain areas for future expansions – a company like IBM doesn’t have to adhere to it if the product is not intended for the future anyway.

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Unexpectedly open

Another important recipe for success from Apple was to publish all essential construction details. So IBM also showed a previously unfamiliar openness and documented the complete hardware including all circuit diagrams, interfaces and commented BIOS in detail in the Technical Reference – where else can you find that today?

As a quite desired side effect, IBM was able to involve as many replica companies as possible in Apple’s dismantling – and exactly this calculation worked, IBM only half-heartedly did something against the countless, mostly illegal replicas. It wasn’t until 15 years later that the company came up with the idea of ​​mercilessly cashing in with some trivial patents, for example for the ingenious floppy detection.

In the Far East they often just copied one-to-one at that time, in the USA they were a bit more careful and at least reprogrammed the BIOS. Less than half a year after the IBM PC Model 5150 was presented in August 1981, Compaq was founded, which had already anchored compatibility in its name.

In Germany, the c’t had a hand in the game when the PC arrived: at the end of 1983, the three-man editorial team brought the c’t86. 8086-CPU, i.e. 16-bit bus width, processor clock 5 to 8 MHz, 20-bit address space; 256 KByte RAM, expandable up to 1 MByte, universal ECB plug-in card system – that was still a blast back then. Either CP / M-86 or PCDOS, which was later made “compatible” on the software side, served as the operating system.