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x86 Assembly: atoi Procedure
ASCII string to int - Convert Strings to Numbers.
Use MASM for Visual C++ Express Edition 2005 to compile this procedure.
Use the atoi_asm procedure to convert a C-string to an integer (ASCII to int). To convert a number to a C-string use itoa_asm (int to ASCII).
TITLE 'extern "C" int atoi_asm(const char *sz);'
.386P
.model FLAT
PUBLIC _atoi_asm
_TEXT SEGMENT
_atoi_asm PROC NEAR
mov edx, DWORD PTR [esp+4] ; sz
label1:
; skip white space
mov al, BYTE PTR [edx]
cmp al, 32 ; ' '
je SHORT label2
cmp al, 9 ; '\t'
je SHORT label2
cmp al, 13 ; '\r'
je SHORT label2
cmp al, 10 ; '\n'
jne SHORT label3
label2:
inc edx
jmp SHORT label1
label3:
xor ecx, ecx
mov cl, BYTE PTR [edx]
inc edx
push esi
cmp ecx, 45 ; '-'
mov esi, ecx
je SHORT label4
cmp ecx, 43 ; '+'
jne SHORT label5
label4:
xor ecx, ecx
mov cl, BYTE PTR [edx]
inc edx
label5:
xor eax, eax
cmp ecx, 48 ; '0'
jl SHORT label7
label6:
cmp ecx, 57 ; '9'
jg SHORT label7
lea eax, DWORD PTR [eax+eax*4]
lea eax, DWORD PTR [ecx+eax*2-48]
xor ecx, ecx
mov cl, BYTE PTR [edx]
inc edx
cmp ecx, 48 ; '0'
jge SHORT label6
label7:
cmp esi, 45 ; '-'
pop esi
jne SHORT label8
neg eax
label8:
ret 0
_atoi_asm ENDP
_TEXT ENDS
END
Use the atoui_asm procedure to convert a C-string to an unsigned integer (ASCII to uint). To convert a
positive number to a C-string, use uitoa_asm (uint to ASCII).
TITLE 'extern "C" unsigned int atoui_asm(char *sz);'
.386P
.model FLAT
PUBLIC _atoui_asm
_TEXT SEGMENT
_atoui_asm PROC NEAR
mov edx, DWORD PTR [esp+4] ; sz
label1:
; skip white space
mov al, BYTE PTR [edx]
cmp al, 32 ; ' '
je SHORT label2
cmp al, 9 ; '\t'
je SHORT label2
cmp al, 13 ; '\r'
je SHORT label2
cmp al, 10 ; '\n'
jne SHORT label3
label2:
inc edx
jmp SHORT label1
label3:
xor ecx, ecx
mov cl, BYTE PTR [edx]
inc edx
cmp ecx, 43 ; '+'
jne SHORT label4
xor ecx, ecx
mov cl, BYTE PTR [edx]
inc edx
label4:
xor eax, eax
cmp ecx, 48 ; '0'
jl SHORT label6
label5:
cmp ecx, 57 ; '9'
jg SHORT label6
lea eax, DWORD PTR [eax+eax*4]
lea eax, DWORD PTR [ecx+eax*2-48]
xor ecx, ecx
mov cl, BYTE PTR [edx]
inc edx
cmp ecx, 48 ; '0'
jge SHORT label5
label6:
ret 0
_atoui_asm ENDP
_TEXT ENDS
END
Convert hexadecimal string to unsigned integer with the hextoui_asm procedure. To create a hexadecimal string from a number use uitoa_asm.
TITLE 'extern "C" unsigned int hextoui_asm(const char *sz);'
.386P
.model FLAT
PUBLIC _hextoui_asm
_TEXT SEGMENT
_hextoui_asm PROC NEAR
mov edx, DWORD PTR [esp+4]
label1:
; skip whitespace
mov al, BYTE PTR [edx]
cmp al, 32 ; ' '
je SHORT label2
cmp al, 9 ; '\t'
je SHORT label2
cmp al, 13 ; '\r'
je SHORT label2
cmp al, 10 ; '\n'
jne SHORT label3
label2:
inc edx
jmp SHORT label1
label3:
xor ecx, ecx
mov cl, BYTE PTR [edx]
inc edx
cmp ecx, 43 ; '+'
jne SHORT label4
xor ecx, ecx
mov cl, BYTE PTR [edx]
inc edx
label4:
xor eax, eax
test ecx, ecx ; check for 0 terminating character
je SHORT label9
label5:
cmp ecx, 48 ; '0'
jl SHORT label6
cmp ecx, 57 ; '9'
jg SHORT label6
; ECX contains the character and EAX the return value
; left shift by 4 bits is the same as multiplying by 16
; then add ECX to EAX since ECX is a hexadecimal character
add eax, 0ffffffdH
shl eax, 4
add eax, ecx
jmp SHORT label8
label6:
cmp ecx, 97 ; 'a'
jl SHORT label7
cmp ecx, 102 ; 'f'
jg SHORT label7
sub ecx, 32 ; 'a' - 'A' = 32
label7:
cmp ecx, 65 ; 'A'
jl SHORT label8
cmp ecx, 70 ; 'F'
jg SHORT label8
shl eax, 4 ; multiply by 16
lea eax, DWORD PTR [eax+ecx-55] ; 10 - 'A' = -55
label8:
xor ecx, ecx
mov cl, BYTE PTR [edx]
inc edx
test ecx, ecx ; check for 0 terminating character
jne SHORT label5
label9:
ret 0
_hextoui_asm ENDP
_TEXT ENDS
END
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