Given the following C function, implement the equivalent assembly language function, int
returnLength(char *myArray, int *length) { int i; i = 0; while (myarray[i] !=0) { myarray[i] -
myarray[i) & 0x0F; i+ +;} *length = i; return(value);}
Solution
;Assume that all general purpose registers (rax, rbx, rcx and rdx) are saved before calling the
following assembly language procedure. Also, the following procedure assumes that the pointer
(address) to "myArray" is contained inside the register, "rdx" and pointer (address) to the
parameter "length" is contained inside the register, "rcx".
;The return value (length of the string) will be available inside the register, "eax", after the
procedure returns.
; Procedure to return the length of a string
ReturnLength PROC NEAR PUBLIC
mov DWORD PTR[rcx], 0 ; Initialize length with 0
jmp LOOP2
LOOP1:
mov eax, DWORD PTR [rcx]
movsx rbx, eax
add rbx, rdx
movzx eax, BYTE PTR [rbx] ; eax will contain the character at ith location
and eax, 15 ; and the character value with 0x0F
mov BYTE PTR [rbx], al ; store the result back at the same location

add DWORD PTR [ecx], 1 ; increment i (length) by 1
LOOP2: ; Checks the while loop and if is not end of the string,
then jums to LOOP1.
mov eax, DWORD PTR [rcx] ; assign i to eax
movsx rbx, eax
add rbx, rdx
movzx eax, BYTE PTR [rbx] ; Get the character at the ith location
test al, al ; Check whether it is NULL character
jne LOOP1 ; If not end of string jump to LOOP1 to peroform the operations
given in while loop
mov eax, DWORD PTR [rcx] ; store the final length in eax and return
ret
ReturnLength ENDP