New G-code for firmware self-awareness

[clefranc]

Hi @alexrj,
I'm currently experimenting on giving Marlin some awareness of its print environment. I've introduced 3 new G-code commands, namely M2, M3 and M4:

 * M2   - Host printing started
 * M3   - Host printing ended
 * M4   - Object information and status: E<extrusion needed> in mm, P<percent completed>

I need these commands to be included automatically at the beginning and end to the slicer G-code. Right now I'm using the M2 and M3 commands using host scripts start/end code.

The M4 command is a little problematic for the host, because only the slicer knows valuable information about the print. I think it must be included in the G-code generated by the slicer (at least the E parameter).

So, in brief, what I'm looking for is:

• M2 at the top of the G-code
• M4 at the top with E parameter (filament needed in mm)
• M3 at the bottom.

Do you think you can produce a branch with these new commands (added as an option)?

See Wiki

Thanks