4.7.7Examples

Next is an example that shows how an assembly routine can be interfaced to a C program. (Note that this program does not use the assembly skeleton program (Figure 1.7) or the driver.c module.)

7int n, sum;

8

9 printf (”Sum integers up to : ”);

10scanf(”%d”, &n);

11calc sum(n, &sum);

12printf (”Sum is %d\n”, sum);

13return 0;

14}

main5.c

sub5.asm

1; subroutine _calc_sum

2; finds the sum of the integers 1 through n

3; Parameters:

5; sump - pointer to int to store sum into (at [ebp + 12])

6; pseudo C code:

7; void calc_sum( int n, int * sump )

8; {

9; int i, sum = 0;

10; for( i=1; i <= n; i++ )

11; sum += i;

12; *sump = sum;

13; }

14

15segment .text

16global _calc_sum

17;

Why is line 22 of sub5.asm so important? Because the C calling convention requires the value of EBX to be unmodified by the function call. If this is not done, it is very likely that the program will not work correctly.

Line 25 demonstrates how the dump stack macro works. Recall that the first parameter is just a numeric label, and the second and third parameters determine how many double words to display below and above EBP respectively. Figure 4.13 shows an example run of the program. For this dump, one can see that the address of the dword to store the sum is BFFFFB80 (at EBP + 12); the number to sum up to is 0000000A (at EBP + 8); the return address for the routine is 08048501 (at EBP + 4); the saved EBP value is

BFFFFB88 (at EBP); the value of the local variable is 0 at (EBP - 4); and

finally the saved EBX value is 4010648C (at EBP - 8).

The calc sum function could be rewritten to return the sum as its return value instead of using a pointer parameter. Since the sum is an integral value, the sum should be left in the EAX register. Line 11 of the main5.c

file would be changed to:

sum = calc sum(n);

Also, the prototype of calc sum would need be altered. Below is the modi-

fied assembly code:

sub6.asm

1; subroutine _calc_sum

2; finds the sum of the integers 1 through n

3; Parameters:

5; Return value:

6; value of sum

7; pseudo C code:

8; int calc_sum( int n )

9; {

10; int i, sum = 0;

11; for( i=1; i <= n; i++ )

12; sum += i;

13; return sum;

14; }

15segment .text

16global _calc_sum

17;

18; local variable:

19; sum at [ebp-4]

20_calc_sum: