Here is the famous line 2238

UNIX.c

2230  /* 
2231  * If the new process paused because it was 
2232  * swapped out, set the stack level to the last call 3333  * to savu(u_ssav). This means that the return 
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  } 
Ref: UNIX™ 6 Edition Kernel Source Code, © Western Electric Company