Creative Prediction with Neural Networks

A course in ML/AI for creative expression

Creative Computing on the BBC Microbit with Bare-Metal ARM Assembly

Charles Martin - The Australian National University

Ngunnawal & Ngambri Country

Idea:

Teaching second-year computing by coding in ARM assembly on real hardware.

Goals:

to build and cement mental models for computer organisation and code execution,
have the transformational experience of running code right on the metal with no guard rails!

tl;dr: Can students do it? yes!

BBC micro:bit v2

Bare-metal ARM Assembly - minimal example

    .thumb
    .syntax unified
        .global __reset

    .section .vectors
    .word __stack
    .word __reset

    .text
    .thumb_func
__reset:
    mov r0, #0
loop:
    add r0, #1
    nop
    b loop

Build and flash with this .ld file and this Makefile.

COMP2300 template

.syntax unified
.global main

.type main, %function
main:
  nop
  ldr r0, =hello

  b main
.size main, .-main

.data
hello:
.word 0x424242

Short startup.S program to load static .data to RAM and setup interrupt vector

Toolchain

Visual Studio Code
COMP2300 extension
- Cortex Debug Extension
COMP2300 toolchain
- xPack OpenOCD
- xPack GNU Arm Embedded GCC (arm-none-eabi) - gcc, gdb, ld, objcopy, as
- COMP2300 discoserver: ARM m4 CPU emulator (by Benjamin Grey)
- make
baremetal microbit vscode template
baremetal example

Context: Computer Organisation and Program Execution

COMP2300/COMP6300
2nd year computing course, obligatory for all undergraduate computing majors, equivalent to “intro to computer systems”.
Since 2020: >400 students at start of semester, 325 passed in 2022.
Lectures and labs available online

Digital Logic, CPU architecture (high level)
Machine instructions, ALU operations
Memory and memory instructions
Conditional execution and control flow
Functions
Toolchains and Compilers
Data structures in ASM
Interrupts, context switches, asynchronism
Networks
Operating Systems
CPU architectures
Revision

Learning Activities

Labs: core of the course starts with mov r0, #1, ends with implementing context switching between two programs (YourNameOS 1.0).

Assignments: two open-ended, creative programming tasks in ARM assembly.

Assignment 1: Lightshow

use the LEDs to create a light show that changes over time

can use the speaker to create sound

input peripherals not allowed!

Lightshow Scaffolding

Need:

ALU operations: add, sub
memory operations: ldr, str
conditional branching: ble etc
control structions: if/then/while/for in ASM (Blinky)

Helps:

functions
shifting operations
reading comprehension of the nrf52833 manual

Assignment 2: Digital Pet

must use the LED display to show a digital pet

must use (at least one) data structure in memory to store the “state” of the digital pet

must use interrupts to detect interactions with the microbit’s buttons

can use sound and any peripheral available on the microbit

Digital Pet Scaffolding:

Need:

functions bl, bx lr
memory directives, labels, ldr= pseudo-instruction
calling convention
interrupts (Buttons and Interrupts)
arrays/structs in ASM

Helps:

SysTick and timers
high comprehension of the nrf52833 manual
patience

IO and Networks

How can computers connect to each other? How can we interpret voltage on a wire?

Interrupts leads to parallel or serial data transmission.

Construct basic UART (RS232/Serial) from first principles and using on-chip peripherals

Serial Output

MIDI

Serial Experiments Charles

It's a UNIX system! I know this!

Lightning intro to operating systems. What’s the connection to ARM?

context switch, threads, scheduling, privilege, supervisor calls, exception model…

YourOS 1.0

Challenge Lab: build a handcrafted context switch, then change between two programs (blinking one LED vs another)

.data
process_table:
.word 0 @ index of currently-operating process
.word 0x20008000 @ stack pointer 1
.word 0x20007000 @ stack pointer 2

Reflections

What can go wrong?

What can go right?

What can go wrong? Surprisingly little!

Don’t tell the students “be careful”!

1-2 instances of students breaking microbit
2 instances of bricking the microbit

Turns out: turning the GPIO on/off as fast as possible (1 cycle loop) messes up the USB connection: saved by special firmware prepared by microbit developers for us.

What can go right?

Assessments students want to do (crazy concept)

“This is the best course I’ve ever taken”

“I thought assembly would be boring, but it was really fun!”

“I feel like I learnt a lot from taking this course.”