Can someone point me to a good description of gcodes for SD Card printing?

3D Printing
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1

Can someone point me to a good description of gcodes for SD Card printing? I want to add SD Card upload and printing to #GcodeSimulator but my Reprap does not have SD card support to try it out.For example its unclear to me what M27 returns

2

FWIW Octoprint has code to integrate with the SD Card. I’ve only messed with getting the file list and managing files myself so I’m not 100% clear on M27.

I believe M27 returns the byte position in the print but I’m not 100% sure of that.

http://reprap.org/wiki/G-code

3

Would it be useful to look at the source for Marlin or Repetier fw and reverse engineer from there?