Machine code, I believe, is as low-level as one can go. It is directly understood by the processor and executed without an interpreter. The processor is built with a certain set of instructions which define the machine code for a given architecture (x86, PPC, ARM, etc.).
Everything from drawing letters on the screen to calculating pi is, at its most base level, run in machine code. Drawing letters on the screen is just a matter of creating some machine code which computes letter height, color, and pixel locations and plots them on a screen using a video output device.
