程序集8086循环问题

Question

The pseudocode is the following:

read c        //a double digit number
for(i=1,n,i++)
{ if (n%i==0)
     print i;}

In assembly I have written it as:

mov bx,ax   ;  ax was the number  ex.0020, storing a copy in bx.

mov cx,1    ; the start of the for loop
.forloop:
mov ax,bx   ; resetting ax to be the number(needed for the next iterations)
div cx
cmp ah,0    ; checking if the remainder is 0 
jne .ifinstr
add cl 48    ;adding so my number would be displayed later as decimal
mov dl,cl   ;printing the remainder
mov ah,2
int 21h
sub cl,48   ;converting it back to hexa
.ifinstr:
inc cx      ;the loop goes on
cmp cx,bx
jle .forloop

I've checked by tracing its steps. The first iteration goes well, then, at the second one, it makes ax=the initial number and cx=2 as it should, but at 'div cx' it jumps somwhere unknown to me and it doesn't stop anywhere. It does:

push ax
mov al,12
nop
push 9
.
.

Any idea why it does that?

Answer 1

try to do mov dx,0 just before div instruction. Basically every time you come after jump, there may be some data in dx register, so you can just move zero in dx or XOR dx,dx. This is to be done, because otherwise division will be considered differently. See this: Unsigned divide.

Algorithm:

when operand is a byte:
AL = AX / operand
AH = remainder (modulus) 

when operand is a word:
AX = (DX AX) / operand
DX = remainder (modulus) 

Example:

MOV AX, 203 ; AX = 00CBh MOV BL, 4 DIV BL ; AL = 50 (32h), AH = 3 RET