GoKawiilTransistors, logic gates, flip-flops, full adders, registers, an accumulator, memory, and blinking LEDs. The PDP-8 is a great machine to learn how traditional digital electronics and computer architecture works.
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There is something deeply satisfying about looking at an old machine and realizing that, underneath all the blinking lights, switches, registers, and instructions, it is still possible to fully grasp the fundamental building blocks of a traditional computer.
If you're not new here, you probably know that, in our school, we often look back in time to understand how technology evolved. Studying the early days of computing allows us to take advantage of a type of simplicity that can only be found in older and more rudimentary systems. Even though modern technology is extremely complex, computers are, ultimately, just a very large collection of tiny circuits that manipulate bits. Retro architectures are a great way to fully grok the basic concepts of digital circuits and computers.
Computers are huge abstraction machines. You can start by understanding how electricity works and how it behaves, then we proceed to understand how transistors can behave like switches, then how a handful of transistors can form logic gates, then how logic gates can form adders, multiplexers, latches, and registers. We connect these circuits together to form an arithmetic & logic unit, then we add a control unit, some memory, and a clock, and suddenly we have a simple but working CPU.
A textbook CMOS 1-bit full adder circuit example
Note: This blog post assumes you're familiar with terms like program counter, stack, flip-flops, registers, full-adders, and ALU. That being said, if you are looking for a set of comprehensive, in-depth lectures on digital electronics & computer architecture, you should visit our This blog post assumes you're familiar with terms like program counter, stack, flip-flops, registers, full-adders, and ALU. That being said, if you are looking for a set of comprehensive, in-depth lectures on digital electronics & computer architecture, you should visit our courses page.
A great example of computer that is simple enough for us to dissect how basic digital circuits work is the DEC PDP-8. Introduced in 1965, the PDP-8 is a classic, textbook example of an accumulator-based architecture, and it was one of the most influential early minicomputers. More importantly for us, its architecture is small enough that we can understand a surprisingly large part of the machine without getting buried in complexity.
The PDP-8 is a 12-bit computer that has a tiny instruction set, a single main accumulator, a peculiar memory-addressing scheme, a one-bit link register, instructions that perform several jobs at once, and even a handful of registers that aren't actually registers at all.
And perhaps the strangest thing about it is that these aren't arbitrary historical oddities. They are the consequences of trying to build a useful computer out of expensive hardware.
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