CS 2130

1
CS 2130

• Lecture 7
• Macro Review
• Stack Frames

2
Questions?
3
Macros Revisited

• Write a macro to automatically check for return
  values from printf

4
Recall

• printf("some stuff d f", i, x)
• Problem We're ignoring the return value which
  could indicate a problem

5
We should write...

• if(printf("some stuff d f", i, x)
• (void)fprintf(stderr,"Yikes!!!")
• exit(EXIT_FAILURE)
• Which, of course, is a pain in the neck!

6
First Approach

• define PRINTF( stuff ) if(printf stuff
• \
• (void)fprintf(stderr,"Yikes!!!") \
• exit(EXIT_FAILURE) \
• and we would use it like this
• PRINTF( "some stuff d f", i, x )
• OK?

7
Not exactly...

• define PRINTF( stuff ) if(printf stuff
• \
• (void)fprintf(stderr,"Yikes!!!") \
• exit(EXIT_FAILURE) \
• This is the fix
• PRINTF( ( "some stuff d f", i, x ) )

8
Questions?
9
Stack Frames
10
Stack Frames

• A language supporting subroutines must provide a
  place to store the data values representing the
  variables of the subroutine as well as an address
  to return to once execution of the subroutine is
  complete.
• Languages that don't support recursion can simply
  have a static "activation record" for each
  subroutine.
• Languages that support recursion must have
  allocate storage for each call to the subroutine
• This is normally implemented using a stack which
  may be known as the activation stack

11
Buzzwords

• Stack pointer a variable usually maintained in a
  dedicated register which points to the top of the
  stack. Note In reality stacks typically grow
  down from high memory.
• Activation record or stack frame The area of the
  stack which is storing the activation information
  (parameters, local variables and return address,
  etc.) for a given module or function.
• Frame pointer a variable also maintained in a
  dedicated register which indicates a known point
  in the stack frame from whence the location of
  parameters and variables may be located.

12
Key concept

• Converting a C program into assembly language
  requires that a certain known sequence of
  operations or at least a certain ordering of data
  will occur in order to call a function.
• Thus when the function code is compiled it will
  know for certain exactly where to find it's
  parameters, etc.

13
Basic Idea

• Caller
• Pushing arguments on stack
• Push fp onto stack
• MOV fp, sp
• JALR Rd, Rs
• (Rd?PC4 PC ? Rs)

• Callee
• Put return address on stack if necessary
• Code may move sp if necessary
• fp is fixed.

14
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

Starting out assuming that values of 15 and 10
have been read into x and y. The stack frame for
main is on the stack. The frame pointer (fp) is
pointing at a known place in the frame while the
stack pointer (sp) indicates the top of the stack
15
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
Calling sequence for first call to gcd starts by
evaluating and pushing argument values onto stack
16
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
17
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
Frame pointer value is pushed onto stack
providing a control link from new frame back to
old
18
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
19
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
Pushing return address onto stack completes stack
frame and function execution may start by
jumping to code
20
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
21
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
22
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
u 10
Starting second call to gcd
23
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
u 10
v 5
24
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
u 10
v 5
control link (fp)
25
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
u 10
v 5
control link (fp)
return address
26
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
u 10
v 5
control link (fp)
return address
27
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
u 10
v 5
control link (fp)
return address
28
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
u 10
v 5
control link (fp)
return address
u 5
Starting third call to gcd
29
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
u 10
v 5
control link (fp)
return address
u 5
v 0
30
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
u 10
v 5
control link (fp)
return address
u 5
v 0
control link (fp)
31
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
u 10
v 5
control link (fp)
return address
u 5
v 0
control link (fp)
return address
32
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
u 10
v 5
control link (fp)
return address
u 5
v 0
control link (fp)
return address
33
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
u 10
v 5
control link (fp)
return address
u 5
v 0
control link (fp)
return address
34
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
u 10
v 5
control link (fp)
return address
u 5
v 0
control link (fp)
return address
35
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
u 10
v 5
Put the return address into a register
control link (fp)
return address
u 5
v 0
control link (fp)
return address
36
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
u 10
v 5
control link (fp)
return address
u 5
v 0
control link (fp)
37
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
u 10
v 5
control link (fp)
return address
u 5
v 0
control link (fp)
We can move the SP and jump to the return address
38
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
u 10
v 5
control link (fp)
return address
Put the return address into a register
39
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
u 10
v 5
control link (fp)
40
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
u 10
v 5
control link (fp)
41
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
42
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
return address
Put the return address into a register
43
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
44
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

u 15
v 10
control link (fp)
45
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

46
Euclid's Algorithm
Global/static area x 15 y 10
main
return address

• int x,y
• int gcd(int u, int v)
• if(v 0)
• return u
• else
• return gcd(v,u v)
• int main()
• scanf("dd", x, y)
• printf
• ("d\n",gcd(x,y))

5
47
Questions?
48
(No Transcript)