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Digital Systems

Copyright © 2017–2023 J. M. Spivey
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The micro:bit (or electronic teabag[1])

Digital systems is a course about how computers work, from logic gates and latches at the bottom, to concurrent processes at the top (running under a simple operating system), with time spent on machine-level programming in the middle. For an easy start, we will begin in the middle with small programs written in machine code.

  • In Hilary Term, we will study low-level programming with the help of a tiny computer, the BBC micro:bit, which includes an ARM-based microcontroller as its processor. Starting in assembly language and moving on to C, we will learn about the instructions that make up machine code, including arithmetic, branching, memory access, and subroutines. We will also (vitally for embedded systems) learn about controlling I/O devices, such as the buttons and lights on the micro:bit and the serial interface that allows it to talk to a host computer. Once a program reaches a certain degree of complexity, it is no longer sufficient to wait in a tight loop for an event to happen, and we will study the hardware and software aspects of solutions to this problem: using interrupts to respond to external events, and an operating system to structure the program as a family of concurrent processes, each responsible for one part of the task.
  • In Trinity Term, we will study the elements of computer hardware, building up from gates and latches to architectural elements such as registers, adders and decoders, and finally a paper model of a processor able to execute a selection of ARM instructions, supported by an architectural simulator.

The V2 micro:bit

In the course, we will be avoiding the latest "V2" version of the micro:bit and using the older "V1" version, which is slower and has less memory, but has a simpler processor that (vitally for us) is easier to understand. I will be handing out the remainder of my small stock of V1 boards for use in the labs; but if you buy your own board, it's likely to be a V2 and therefore incompatible. For simplicity, the lab software we will use this year supports the V1 board only. A version that supports both V1 and V2 is under development, and will appear with the Bare Metal micro:bit book.


Hilary Term


  • 16 lectures: TBA.

Although some elements of the lectures will be automatically recorded, the dynamic blend of slides, calculations on the whiteboard, software demonstrations, computer hardware shown via live video and online instrumentation, and problems solved by the lecturer and class working together means that not all elements will be captured, and the recordings may therefore be almost useless. In addition, the content of each lecture may change in response to questions from the audience. Participants who cannot attend lectures in person may find it best to follow the course by reading the extensive lecture notes available on this site.

Lab sessions

In the Thom Building, Department of Engineering Science. In addition to the official lab times noted above, the lecturer will be available at the same time and in the same place in weeks 3 and 4 to help those who want to get an early start with assembly language programming.

  • Informal sessions: TBA, weeks 3 and 4.
  • Official sessions: TBA, weeks 5–8.

There will be no lab sessions in Trinity Term.


Your college will organise tutorials in this subject, as in others. To support them, there are four problem sheets this term, two on machine-level programming, one on I/O devices, and one on embedded operating systems.

Trinity Term


  • 8 lectures, TBA.


Two more tutorials are suggested, with a problem sheet on digital design, and another on processor architecture.


Information about the course is provided on several other pages.

And last of all:

  • There's a website for my book Bare Metal micro:bit, containing expanded versions of the lab materials for the course, with explanations appended.



  1. So called because the board dangles from its USB cable like a teabag dangling from its string.