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

NAME
     stack -  PDP-11 C stack frame conventions

DESCRIPTION
     There are two standard C stack frame layouts.  For non-
     overlaid programs:

          ------------------
          |...nth argument |              push arguments in reverse order
          ------------------
          |second argument |
          ------------------
          | first argument |
          ------------------              JSR PC,*$_FOO
          | return address |
          ------------------              JSR R5,CSV
          |  old R5 value  | <-----
          ------------------      |
          |       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---+-------
          ------------------      ^
          | r4/43/r2/...   |      |
          ------------------
          | and so on..... |

     For overlaid programs:

          ------------------
          |...nth argument |              push arguments in reverse order
          ------------------
          |second argument |
          ------------------
          | first argument |
          ------------------              JSR PC,*$_FOO
          | return address |

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

          ------------------              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..... |

     Functions returning integers leave their return value in R0;
     functions returning floating constants use FR0;  functions
     returning longs leave return values in R1/R0; 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).  SP always points to the
     next available stack word.  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

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

     accordingly after a function returns.

NOTE
     Newer systems implement the c-save by JSR R0,CSV, which is a
     more bullet-proof version of the old call sequence.  The
     stack winds up looking the same.

AUTHOR
     John F. Woods, MIT Concouse Computer Center

Printed 5/30/83                                                 3