7. PDP-11 — Defining Normal
In 1970, DEC launched a machine that would change everything. The PDP-11 was a radical break. It was a 16-bit machine with an orthogonal instruction set that made it far easier to program than anything before it. It had eight general-purpose registers (later models had more). It introduced the Unibus, a standardized bus that made adding peripherals simple. And the design scaled up from a tiny 4K embedded controller to a system with 8 megabytes that could rival mainframes — a scale factor of 2,000.
The design was so elegant that every major microprocessor architecture that came after — the Intel x86, the Motorola 68000 — was visibly shaped by it. The 68000's register set, the x86's addressing modes, the very idea of what a CPU should look like: the PDP-11 defined it.
Where Unix Grew Up
In 1970, Ken Thompson and Dennis Ritchie at Bell Labs ported their experimental operating system — Unix — to the PDP-11/20. It was the first machine Unix ran on as a named operating system. Over the next decade, Unix and the PDP-11 evolved together. The C programming language was designed specifically to be efficient on the PDP-11's architecture. The abstract machine that C targets, to this day, is a PDP-11 in its fundamental assumptions.
The PDP-11's influence is even visible in the syntax of C itself. The close relationship between arrays and pointers in C — the fact that array[i] is equivalent to *(array + i) — is not an accident of language design. It reflects the PDP-11's architecture, where memory addressing was uniform and efficient. C was not designed for an abstract machine; it was designed for the PDP-11.
The Unix source code for the PDP-11 contains one of the most famous comments in programming history. In the I/O driver for the 11/20, Ken Thompson wrote: "You are not expected to understand this". It was a joke about the complexity of the interrupt system, but it became legendary.
When you open a terminal on a modern Linux or macOS system, you are in a direct line of descent from a programmer sitting at a PDP-11 in 1971. The file system hierarchy, the pipe operator, the shell itself — all of it was invented on the PDP-11.
The Road Not Taken — RSX-11
But Unix was not the only operating system on the PDP-11, and in fact it was DEC's least favorite. The company's own RSX-11M was a groundbreaking multi-user, real-time OS that feels utterly different from Unix. But its integrated development environment, its seamless multitasking, its approach to resource management — RSX was an alternative vision of computing that could have won. Easily.
Here is where the story takes a strange turn. When Microsoft decided to break with the MS-DOS past in the late 1980s, Dave Cutler, the lead architect of RSX-11M (and later of DEC's VMS), was tasked with building a new operating system from scratch. That became Windows NT. Its kernel design, its I/O model, and its structured exception handling all bear the marks of the PDP-11 RSX heritage. So: the other half of the modern desktop, the non-Unix half, also traces back to the PDP-11.
Over 600,000 PDP-11s were sold. They were used in laboratories, factories, universities, telephone exchanges, and spacecraft. The top-of-the-line PDP-11/70 pushed performance further with a second MASSBUS interface, capable of transferring data at 4 megabytes per second — breathtaking speed in 1975. The architecture remained in production for over 25 years. When you use any modern computer — PC, Mac, or Linux workstation — you are running on decisions made by the PDP-11's designers in the late 1960s.