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.. _pyboard_tutorial_assembler:
Inline assembler
================
Here you will learn how to write inline assembler in MicroPython.
**Note**: this is an advanced tutorial, intended for those who already
know a bit about microcontrollers and assembly language.
MicroPython includes an inline assembler. It allows you to write
assembly routines as a Python function, and you can call them as you would
a normal Python function.
Returning a value
-----------------
Inline assembler functions are denoted by a special function decorator.
Let's start with the simplest example::
@micropython.asm_thumb
def fun():
movw(r0, 42)
You can enter this in a script or at the REPL. This function takes no
arguments and returns the number 42. ``r0`` is a register, and the value
in this register when the function returns is the value that is returned.
MicroPython always interprets the ``r0`` as an integer, and converts it to an
integer object for the caller.
If you run ``print(fun())`` you will see it print out 42.
Accessing peripherals
---------------------
For something a bit more complicated, let's turn on an LED::
@micropython.asm_thumb
def led_on():
movwt(r0, stm.GPIOA)
movw(r1, 1 << 13)
strh(r1, [r0, stm.GPIO_BSRRL])
This code uses a few new concepts:
- ``stm`` is a module which provides a set of constants for easy
access to the registers of the pyboard's microcontroller. Try
running ``import stm`` and then ``help(stm)`` at the REPL. It will
give you a list of all the available constants.
- ``stm.GPIOA`` is the address in memory of the GPIOA peripheral.
On the pyboard, the red LED is on port A, pin PA13.
- ``movwt`` moves a 32-bit number into a register. It is a convenience
function that turns into 2 thumb instructions: ``movw`` followed by ``movt``.
The ``movt`` also shifts the immediate value right by 16 bits.
- ``strh`` stores a half-word (16 bits). The instruction above stores
the lower 16-bits of ``r1`` into the memory location ``r0 + stm.GPIO_BSRRL``.
This has the effect of setting high all those pins on port A for which
the corresponding bit in ``r0`` is set. In our example above, the 13th
bit in ``r0`` is set, so PA13 is pulled high. This turns on the red LED.
Accepting arguments
-------------------
Inline assembler functions can accept up to 4 arguments. If they are
used, they must be named ``r0``, ``r1``, ``r2`` and ``r3`` to reflect the registers
and the calling conventions.
Here is a function that adds its arguments::
@micropython.asm_thumb
def asm_add(r0, r1):
add(r0, r0, r1)
This performs the computation ``r0 = r0 + r1``. Since the result is put
in ``r0``, that is what is returned. Try ``asm_add(1, 2)``, it should return
3.
Loops
-----
We can assign labels with ``label(my_label)``, and branch to them using
``b(my_label)``, or a conditional branch like ``bgt(my_label)``.
The following example flashes the green LED. It flashes it ``r0`` times. ::
@micropython.asm_thumb
def flash_led(r0):
# get the GPIOA address in r1
movwt(r1, stm.GPIOA)
# get the bit mask for PA14 (the pin LED #2 is on)
movw(r2, 1 << 14)
b(loop_entry)
label(loop1)
# turn LED on
strh(r2, [r1, stm.GPIO_BSRRL])
# delay for a bit
movwt(r4, 5599900)
label(delay_on)
sub(r4, r4, 1)
cmp(r4, 0)
bgt(delay_on)
# turn LED off
strh(r2, [r1, stm.GPIO_BSRRH])
# delay for a bit
movwt(r4, 5599900)
label(delay_off)
sub(r4, r4, 1)
cmp(r4, 0)
bgt(delay_off)
# loop r0 times
sub(r0, r0, 1)
label(loop_entry)
cmp(r0, 0)
bgt(loop1)
Further reading
---------------
For further information about supported instructions of the inline assembler,
see the :ref:`reference documentation <asm_thumb2_index>`.
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