Wow... just like when I'm sleepy I find a comment like this as I read lol

2230:         /*
2231:          * If the new process paused because it was
2232:          * swapped out, set the stack level to the last call
2233:          * to savu(u_ssav).  This means that the return
2234:          * which is executed immediately after the call to aretu
2235:          * actually returns from the last routine which did
2236:          * the savu.
2237:          *
2238:          * You are not expected to understand this.
2239:          */
2240:         if(rp->p_flag&SSWAP) {
2241:                 rp->p_flag =& ~SSWAP;
2242:                 aretu(u.u_ssav);
2243:         }