5.2.5Example

This section contains an assembly source file with several functions that implement array operations using string instructions. Many of the functions duplicate familiar C library functions.

memory.asm

1global _asm_copy, _asm_find, _asm_strlen, _asm_strcpy

2

3segment .text

4; function _asm_copy

5; copies blocks of memory

6; C prototype

7; void asm_copy( void * dest, const void * src, unsigned sz);

8; parameters:

9; dest - pointer to buffer to copy to

12

13 ; next, some helpful symbols are defined

14

15%define dest [ebp+8]

16%define src [ebp+12]

36; function _asm_find

37; searches memory for a given byte

38; void * asm_find( const void * src, char target, unsigned sz);

39; parameters:

43; return value:

44; if target is found, pointer to first occurrence of target in buffer

45; is returned

46 ; else

47 ; NULL is returned

48; NOTE: target is a byte value, but is pushed on stack as a dword value.

49; The byte value is stored in the lower 8-bits.

50;

78; function _asm_strlen

79; returns the size of a string

80; unsigned asm_strlen( const char * );

81; parameter:

82; src - pointer to string

83; return value:

84; number of chars in string (not counting, ending 0) (in EAX)

85

86%define src [ebp + 8]

87_asm_strlen:

97

98;

99; repnz will go one step too far, so length is FFFFFFFE - ECX,

100; not FFFFFFFF - ECX

101;

104

109; function _asm_strcpy

110; copies a string

111; void asm_strcpy( char * dest, const char * src);

112; parameters:

113; dest - pointer to string to copy to

114; src - pointer to string to copy from

115;

116%define dest [ebp + 8]

117%define src [ebp + 12]

118_asm_strcpy:

125cld

126cpy_loop:

11

12int main()

13{

14char st1 [STR SIZE] = ”test string”;

15char st2 [STR SIZE];

16char st ;

17 char ch;

37

38return 0;

39}

memex.c