STACK(5)	    UNIX Programmer's Manual		 STACK(5)



NAME
     stack -  2.10BSD PDP-11 C stack frame conventions

DESCRIPTION
     The standard C stack frame layout:

	  ------------------
	  |...nth argument |		  push arguments in reverse order
	  ------------------
	  |second argument |
	  ------------------
	  | first argument |
	  ------------------		  JSR PC,*$_FOO
	  | return address |
	  ------------------		  JSR R5,CSV
	  |  old R5 value  | <-----
	  ------------------	  |
	  |previous overlay|	  |
	  |	number	   |	  |
	  ------------------	  |
	  |	  r4	   |	  |
	  ------------------	  |
	  |	  r3	   |	  |
	  ------------------	  |
	  |	  r2	   |	  |
	  ------------------	  |
	  | first local var|	  | This is the top of the stack
	  ------------------	  | when the called routine ``starts''
	  |	routine    |	  |
	  |    allocates   |	  |
	  |	storage    |	  |	  SUB $n,SP
	  |    temporary   |	  |
	  ------------------	  |
	  | push arguments |	  |
	  | of next routine|	  |
	  ------------------	  |	  JSR PC,*$_BAR
	  | return address |	  |
	  ------------------	  |	  JSR R5,CSV
	  | old R5 value---+-------
	  ------------------	  ^
	  |previous overlay|	  |
	  |	number	   |	  |
	  ------------------	  |
	  | r4/43/r2/...   |	  |
	  ------------------
	  | and so on..... |


     The stack pushes downward through memory addresses.  Overlay
     numbers saved in non-overlaid objects are always zero, but
     the simplification of not having to maintain two different
     stack frame formats more than outweighs the extra few micro



Printed 11/26/99						1






STACK(5)	    UNIX Programmer's Manual		 STACK(5)



     seconds (less than four) necessary to save the zero ...

     Functions returning integers leave their return value in R0;
     functions returning floating constants use FR0; functions
     returning longs leave return values in R1/R0 (R0 high word,
     R1 low); functions returning structures leave a pointer to
     bss storage (one chunk of which is allocated for each such
     routine) in R0, and the caller will copy from that bss
     storage to the local destination.

     Local variables are allocated in such a way that they are
     referred to as ``-N(R5)'', arguments are referred to as
     ``+N(R5)''; arguments start at 4(R5), the first integer
     local declared will be at -10(R5).

     The SP normally points at the first word available for
     parameter pushing.  A function taking only single word as a
     parameter can be called simply by moving the parameter into
     (SP) and calling the function, without having to clean the
     parameter off the stack on return.  Any parameters passed
     after the first (actually "Nth") must be pushed before the
     call and cleaned off afterwards.  If the function has no
     local variables and calls no functions, it will allocate no
     stack and the word labelled ``first local var'' will be
     unused.

     It is important to note that routines know how many argu-
     ments they pass to a function, and will adjust the stack
     accordingly after a function returns.

NOTE
     This stack frame format is the same as that used by overlaid
     objects in 2.9BSD.

AUTHOR
     John F. Woods, MIT Concouse Computer Center



















Printed 11/26/99						2