STACK(5)							File Formats 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 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 arguments they pass to a function, and will adjust the stack accordingly after a func-
       tion 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

3rd Berkeley Distribution														  STACK(5)