.TH STACK 5
.UC
.SH NAME
stack \-  PDP-11 C stack frame conventions
.SH DESCRIPTION
There are two standard C stack frame layouts.  For nonoverlaid
programs:
.RS
.nf
.ta 8 16 24 32 40 48 56 64

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

.fi
.RE
For overlaid programs:
.RS
.nf
.ta 8 16 24 32 40 48 56 64

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

.fi
.RE
.DT
.PP
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.
.PP
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) in a nonoverlaid
program, \-12(R5) in an overlaid one.  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.
.PP
It is important to note that routines know how many arguments they pass to a
function, and will adjust the stack accordingly after a function returns.
.SH 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.
.SH AUTHOR
John F. Woods, MIT Concouse Computer Center