;; file: memory.asm; This program illustrates how to use the string instructionsglobal _asm_copy, _asm_find, _asm_strlen, _asm_strcpysegment .text; function _asm_copy; copies blocks of memory; C prototype; void asm_copy( void * dest, const void * src, unsigned sz);; parameters:;   dest - pointer to buffer to copy to;   src  - pointer to buffer to copy from;   sz   - number of bytes to copy; next, some helpful symbols are defined%define dest [ebp+8]%define src  [ebp+12]%define sz   [ebp+16]_asm_copy:        enter   0, 0        push    esi        push    edi        mov     esi, src         ; esi = address of buffer to copy from        mov     edi, dest        ; edi = address of buffer to copy to        mov     ecx, sz          ; ecx = number of bytes to copy        cld                     ; clear direction flag         rep     movsb           ; execute movsb ECX times        pop     edi        pop     esi        leave        ret; function _asm_find; searches memory for a given byte; void * asm_find( const void * src, char target, unsigned sz);; parameters:;   src    - pointer to buffer to search;   target - byte value to search for;   sz     - number of bytes in buffer; return value:;   if target is found, pointer to first occurrence of target in buffer;   is returned;   else;     NULL is returned; NOTE: target is a byte value, but is pushed on stack as a dword value.;       The byte value is stored in the lower 8-bits.; %define src    [ebp+8]%define target [ebp+12]%define sz     [ebp+16]_asm_find:        enter   0,0        push    edi        mov     eax, target      ; al has value to search for        mov     edi, src        mov     ecx, sz        cld        repne   scasb           ; scan until ECX == 0 or [ES:EDI] == AL        je      found_it        ; if zero flag set, then found value        mov     eax, 0          ; if not found, return NULL pointer        jmp     short quitfound_it:        mov     eax, edi                  dec     eax              ; if found return (DI - 1)quit:        pop     edi        leave        ret; function _asm_strlen; returns the size of a string; unsigned asm_strlen( const char * );; parameter:;   src - pointer to string; return value:;   number of chars in string (not counting, ending 0) (in EAX)%define src [ebp + 8]_asm_strlen:        enter   0,0        push    edi        mov     edi, src        ; edi = pointer to string        mov     ecx, 0FFFFFFFFh ; use largest possible ECX        xor     al,al           ; al = 0        cld        repnz   scasb           ; scan for terminating 0;; repnz will go one step too far, so length is FFFFFFFE - ECX,; not FFFFFFFF - ECX;        mov     eax,0FFFFFFFEh        sub     eax, ecx          ; length = 0FFFFFFFEh - ecx        pop     edi        leave        ret; function _asm_strcpy; copies a string; void asm_strcpy( char * dest, const char * src);; parameters:;   dest - pointer to string to copy to;   src  - pointer to string to copy from; %define dest [ebp + 8]%define src  [ebp + 12]_asm_strcpy:        enter   0,0        push    esi        push    edi        mov     edi, dest        mov     esi, src        cldcpy_loop:        lodsb                   ; load AL & inc si        stosb                   ; store AL & inc di        or      al, al          ; set condition flags        jnz     cpy_loop        ; if not past terminating 0, continue        pop     edi        pop     esi        leave        ret