I began, then, by reading doc and building some tools for microPython. It has a makefile driven build system and can be built for using a gnu arm-eabi toolchain. I have a windows laptop, and figured cygwin might be a useful option to host the tool chain.

I found some hints on how to build such a toolchain on cygwin, and followed the steps given there.

At the cygwin prompt, I changed the current directory to ports/bare-arm and issued make. I found that the build system expected the toolchain prefix to be arm-none-eabi-, rather than the arm-eabi- prefix I had used in building it. I remedied this by altering micropython/ports/bare-arm/Makefile so:

<<<< CROSS_COMPILE = arm-none-eabi-
----
>>>> CROSS_COMPILE = arm-eabi-

I then re-issued make and got

% make
...
LINK build/firmware.elf
   text    data     bss     dec     hex filename
  66440       0     412   66852   10524 build/firmware.elf

The binary built to a 66 k bytes text segment. This might have been better, but but I thought I would actually see what we've got.

I couldn't find any doc to explain the port -- nor any doc to explain design of the base code. However, it looks like it targets an STM32F405. It looks like mpconfigport.h is responsible defining what optional features to include, and that #defines <option> (0) for most <options>.

Looking at main, there appears to be no REPL or file system, or serial I/O; there is a local definition of a do_str() function, and that is called in main to execute some Python strings.

Even without a REPL the, size of the text segment of the bare-arm port is 66K bytes.

This port does have an REPL, by default calling pyexec_friendly_repl() in main.c.

Following a similar procedure (fixing up the Makefile to say CROSS_COMPILE = arm-eabi-) and issuing make in the ports/minimal sub-directory I got a 70K text segment in my binary.

This build, choked for me -- but I could see some size info (317K):

LINK mpy-cross
   text    data     bss     dec     hex filename
 317729    2800    1248  321777   4e8f1 mpy-cross
make[1]: Leaving directory '/cygdrive/c/Users/smart/Documents/WORK/es/ees/micropython/mpy-cross'
MPY modules/dht.py
Traceback (most recent call last):
  File "modules/dht.py", line 1
SyntaxError: invalid syntax
make: *** [../../py/mkrules.mk:118: build-PYBV10/frozen_mpy/dht.mpy] Error 1

It seems this is an issue with cloning the repository without symlinks (I had used TortoseGit from an Explorer shell):

Just want to add, if you enable symlinks -c core.symlinks=true while cloning the repo I'm able to successfully compile.

Using cygwin git to clone the repository, and also installing an arm-none-eabi- tool chain from launchpad to /usr/local on my cygwin installation, the stm32 port builds OK

LINK build-PYBV10/firmware.elf
   text    data     bss     dec     hex filename
 334488      32   27876  362396   5879c build-PYBV10/firmware.elf