button_asm.py

# (c) 2024 David Whale - 20/03/2024
# ARM thumb assembly language that reads button A

from microbit import *
from micropython import const

REG_GPIO_P0     = const(0x50000000)
REG_GPIO_IN     = const(REG_GPIO_P0 + 0x510)
BUTTON_A_PIN    = const(14) # P0_14 of the nRF5x GPIO block

@micropython.asm_thumb
def button_a_pressed():
    movwt(r0, REG_GPIO_IN)   # r0 = REG_GPIO_IN   GPIO input register address
    ldr(r0, [r0, 0])         # r0 = mem[r0]       read GPIO pin states
    movw(r1, BUTTON_A_PIN)   # r1 = BUTTON_A_PIN  bit number we are interested in
    lsr(r0, r1)              # r0 = r0 >> r1      right shift our bit down to bit 0
    mov(r1, 1)               # r1 = 1
    and_(r0, r1)             # r0 = r0 & r1       mask out all other bits except bit 0
    eor(r0,r1)               # r0 = r0 ^ r1       invert the bit, low=pressed, high=released
    # value in r0 at the end, is the value returned back to python
    
while True:
    if button_a_pressed():
        display.show(Image.YES)
    else:
        display.show(Image.NO)

@carlosperate has some nice MicroPython examples here: https://gist.github.com/microbit-carlos?direction=asc&sort=created

The MicroPython thumb assembly instructions are documented here: https://docs.micropython.org/en/latest/reference/asm_thumb2_index.html

The simulator at python.microbit.org won't run the assembly instructions, but it does flash and run fine on the micro:bit even though there are lots of red marks over it all!

MakeCode shims mentioned here, that can call assembly language code from javascript code: https://forum.makecode.com/t/using-assembler-routines-in-both-js-and-c/2155

MakeCode extensions here (neopixel is a good example as it has assembly code in it as well): https://makecode.com/extensions

.asm files can't be added at present: microsoft/pxt-microbit#4426

This shows the output/binary mode, which also shows the hex codes that represent each assembly instruction, as they would be assembled into memory by an assembler tool.

The red underlining in the python.microbit.org editor for the asm lines can be removed with this fix:

microbit-foundation/python-editor-v3#774 (comment)

42 in ASCII is a '*', this prints ABC then *BC, the assembly program has stored a 42 in the first memory location of the buffer data. id(b) is the bytearray address, the third longword32 of that memory holds the address of the actual buffer data.

# pyright: reportGeneralTypeIssues=false, reportUndefinedVariable=false
from machine import mem8, mem32

@micropython.asm_thumb
def ultimate_answer(r0):
    mov(r1,42)
    strb(r1,[r0,0])

b = bytearray(b'ABC')
print("before", b)
ITEMS_ADDR = mem32[id(b)+(4*3)]

ultimate_answer(ITEMS_ADDR)
print("after", b)