1STACK(5) File Formats Manual STACK(5)
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6 stack - 2.10BSD PDP-11 C stack frame conventions
7
9 The standard C stack frame layout:
10 ------------------
12 |...nth argument | push arguments in reverse order
13 ------------------
14 |second argument |
15 ------------------
16 | first argument |
17 ------------------ JSR PC,*$_FOO
18 | return address |
19 ------------------ JSR R5,CSV
20 | old R5 value | <-----
21 ------------------ |
22 |previous overlay| |
23 | number | |
24 ------------------ |
25 | r4 | |
26 ------------------ |
27 | r3 | |
28 ------------------ |
29 | r2 | |
30 ------------------ |
31 | first local var| | This is the top of the stack
32 ------------------ | when the called routine ``starts''
33 | routine | |
34 | allocates | |
35 | storage | | SUB $n,SP
36 | temporary | |
37 ------------------ |
38 | push arguments | |
39 | of next routine| |
40 ------------------ | JSR PC,*$_BAR
41 | return address | |
42 ------------------ | JSR R5,CSV
43 | old R5 value---+-------
44 ------------------ ^
45 |previous overlay| |
46 | number | |
47 ------------------ |
48 | r4/43/r2/... | |
49 ------------------
50 | and so on..... |
51 The stack pushes downward through memory addresses. Overlay numbers
54 saved in non-overlaid objects are always zero, but the simplification
55 of not having to maintain two different stack frame formats more than
56 outweighs the extra few micro seconds (less than four) necessary to
57 save the zero ...
58 Functions returning integers leave their return value in R0; functions
60 returning floating constants use FR0; functions returning longs leave
61 return values in R1/R0 (R0 high word, R1 low); functions returning
62 structures leave a pointer to bss storage (one chunk of which is allo‐
63 cated for each such routine) in R0, and the caller will copy from that
64 bss storage to the local destination.
65 Local variables are allocated in such a way that they are referred to
67 as ``-N(R5)'', arguments are referred to as ``+N(R5)''; arguments start
68 at 4(R5), the first integer local declared will be at -10(R5).
69 The SP normally points at the first word available for parameter push‐
71 ing. A function taking only single word as a parameter can be called
72 simply by moving the parameter into (SP) and calling the function,
73 without having to clean the parameter off the stack on return. Any
74 parameters passed after the first (actually "Nth") must be pushed
75 before the call and cleaned off afterwards. If the function has no
76 local variables and calls no functions, it will allocate no stack and
77 the word labelled ``first local var'' will be unused.
78 It is important to note that routines know how many arguments they pass
80 to a function, and will adjust the stack accordingly after a function
81 returns.
82
84 This stack frame format is the same as that used by overlaid objects in
85 2.9BSD.
86
88 John F. Woods, MIT Concouse Computer Center
893rd Berkeley Distribution STACK(5)