[Microbit-Python] This happens when your code is "too big" (more than 8kB)

[Jeff Young]

Hi James.

 

Very interesting reading.

 

With reference to another thread I want to understand how you are able to load such large files onto the micro:bit.  I’m not able to load files which are well under 4kb of code ( ondisk size ) and around 120 lines. ( no comments ).  Does you test code actually run ?

 

Thanks

 

Jeff

 

From: Microbit [mailto:microbit-bounces+jeff=perfectmotion.co.uk at python.org] On Behalf Of James Sheppard
Sent: 01 July 2016 10:16
To: microbit at python.org
Subject: [Microbit-Python] This happens when your code is "too big" (more than 8kB)

 

Hello everyone

 

I thought people might be interested to know what happens when the MicroPython code being flashed onto a micro:bit is more than 8kB.

 

This analysis was done on a hex file generated by the MicroPython Editor on https://www.microbit.co.uk/ bulked out with ~200 lines of comments. 

 

If you’ve never opened up a .hex file to take a look, now could be the right time…

 

(You might find  <https://en.wikipedia.org/wiki/Intel_HEX> https://en.wikipedia.org/wiki/Intel_HEX useful to refer to.)

 

The first line in the file looks like this:

 

:020000040000FA

 

Which sets the base address of data to be flashed to 0x0 0000.

 

Lines then follow that start with

 

:10XXXX00...

 

which indicates the data to be written to 0x0 XXXX

 

Keep going, and eventually you’ll see

 

:020000040001F9

 

which sets the base address to 0x1 0000.

 

Then more data.

 

Then

 

:020000040002F8

 

which sets the base address to 0x2 0000.

 

Then more data.

 

Then

 

:020000040003F7

 

(0x3 0000)

 

and yet more data. So far this has all been MicroPython data (over 200kB). Your code is next.

 

You’ll see

 

:020000040003F7

 

again, which sets the base address to 0x3 0000 again (i.e. no change). User code starts at 0x3 E000 with the bytes 4D 50 54 32 (“MPT2”) then the script itself.

 

:10E000004D505432...

 

If there's more than 8kB of user code, you’ll see the address overflow from 0x3 FFF0 to 0x3 0000

 

:10FFF000...

:10000000...

 

During flashing of the hex file to the micro:bit, this causes the MicroPython hex at these addresses to be overwritten - who knows what’s just been trampled over!

 

Cheers,

James

 

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