HelenOS sources

X_SIGN            306 uspace/drv/hid/ps2mouse/ps2mouse.c 		    ((packet[0] & X_SIGN) ? 0xff00 : 0) | packet[1];
X_SIGN            374 uspace/drv/hid/ps2mouse/ps2mouse.c 		    ((packet[0] & X_SIGN) ? 0xff00 : 0) | packet[1];
HelenOS homepage, sources at GitHub