I want to try out the "M99 Xmm Ymm Zmm Emm" g-code command and see how backlash can be reduced or eliminated via software. (Yes, I'm trying everything I can in the hardware department too).

However, my firmware (Solidoodle 3 Marlin firmware, Printrboard Rev E motherboard) appears to not support that command, or it does something else.

While M99 is meant to set the backlash compensation values in mm, M98 prints out the current values.
Typing "M98" in Manual Control of Repetier-Host I get "N13 M98 *0" (13 and 0 change when I try again).

If this means my firmware doesn't support hysteresis, please tell me how I can modify the firmware source to enable it.
If I'm doing it wrong, please tell me what I'm doing wrong.