Cannot get the greatest common divisor - masm32

Question

This is what I have so far, it will display the fraction but nothing else:
INCLUDELIB c:\masm32\lib\user32.lib
INCLUDELIB c:\masm32\lib\kernel32.lib
.data
str1 BYTE "Enter the numerator: ",0
str2 BYTE "Enter the denominator: ",0
str3 BYTE "/",0
str4 BYTE "The fraction you entered is: ",0
str5 BYTE "Simplest form of your fraction: ",0 ;msg9
str6 BYTE "You can't store a 0 in the denominator",0dh, 0ah, 0
str7 BYTE " ",0dh, 0ah, 0
num1 SDWORD 0 ;n1
num2 SDWORD 0 ;n2
tempN SDWORD 0
tempD SDWORD 0
temp3 SDWORD 0
tempR SDWORD 0
.code
main PROC
call Clrscr
mov eax,0
mov ebx,0
mov ecx,0
call fractionPrompt
call gcd
call displayFraction
exit
main ENDP
fractionPrompt PROC
mov edx, OFFSET str1 ;prompt for numerator
J1:
call writestring
call readDec
mov num1,eax
jmp J2
J2:
mov edx,OFFSET str2 ;Prompt for denominator
call writestring
call readDec
mov num2,eax
cmp eax,0
je reenter
jne OP1
reenter:
mov edx,OFFSET str7
call writestring
mov edx,OFFSET str6
call writestring
OP1:
mov edx, OFFSET str7
call writestring
mov edx,OFFSET str4
call writestring
mov eax,num1
call writeDec
mov tempN,eax
mov edx, OFFSET str3
call writestring ;/
mov eax,num2
mov tempD,eax
call writeDec
ret
fractionPrompt ENDP
gcd PROC
mov eax,tempN
cdq
mov ebx,tempD
mov edx,0
div ebx
cmp edx,0
jne J1
je J2
J1:
mov eax,tempD
mov edx,0
dec temp3
div temp3
mov temp3,edx
cmp edx,0
je J2
loop J1
J2:
mov eax,tempN
neg eax
cdq
mov edx,0
div temp3
cmp edx,0
je J3
dec temp3
loop J2
J3:
mov eax,tempN
cdq
div temp3
mov tempN,eax
call writeDec
mov eax,tempD
cdq
div temp3
mov tempD,eax
call writeDec
ret
gcd ENDP
displayFraction PROC
mov edx,OFFSET str5
call writestring
mov tempN,eax
call writeDec
mov edx, OFFSET str3
call writestring
mov ebx,tempD
call writeDec
ret
displayFraction ENDP
END main

Answer 1

Your code is so erroneous that it isn't worth to correct it, respectively, I tried and failed. Take a look at this working example (Masm32 Version 11):
include \masm32\include\masm32rt.inc
.data
num SDWORD 15 ; example numerator
denom SDWORD 12 ; example denominator
gcd SDWORD 0
.code
get_gcd PROC arg1:DWORD, arg2:DWORD
mov eax, arg1
mov ecx, arg2
test ecx, ecx
jz _Ret
##:
cdq
idiv ecx
test edx, edx
jz _Ret
mov eax, ecx
mov ecx, edx
jmp #B
_Ret:
mov eax, ecx
cdq ; EAX = abs(EAX)
add eax, edx
xor eax, edx
ret
get_gcd ENDP
main PROC
printf ("Fraction: %i / %i\n",num,denom)
invoke get_gcd, num, denom
mov gcd, eax
printf ("GCD: %i\n",eax)
mov eax, num
cdq
idiv [gcd]
mov num, eax
mov eax, denom
cdq
idiv [gcd]
mov denom, eax
printf ("Reduced: %i / %i\n",num,denom)
invoke ExitProcess, 0
main ENDP
END main