Fixed! Here it is (with lots of comments):
.model small
.stack 100h
.data
input db 30 dup ('$')
n dw ?
count dw ?
output db 30 dup ('$')
msj db 13,10,'The number is = $'
.code
main proc
;INITIALIZE DATA SEGMENT.
mov ax , @data
mov ds , ax
;CAPTURE NUMBER CHAR BY CHAR. NOTICE CHR(13) WILL BE
;STORED IN STRING AND COUNTED.
mov bx , offset input
mov count , 0
l1:
mov ah , 1
int 21h ;CAPTURE ONE CHAR FROM KEYBOARD.
mov [bx] , al ;STORE CHAR IN STRING.
inc bx
inc count
cmp al , 13
jne l1 ;IF CHAR IS NOT "ENTER", REPEAT.
dec count ;NECESSARY BECAUSE CHR(13) WAS COUNTED.
;CONVERT STRING TO NUMBER.
mov bx , offset input ;BX POINTS TO THE FIRST CHAR.
add bx, count ;NOW BX POINTS ONE CHAR AFTER THE LAST ONE.
mov bp, 0 ;BP WILL BE THE NUMBER CONVERTED FROM STRING.
mov cx, 0 ;PROCESS STARTS WITH 10^0.
l2:
;GET CURRENT POWER OF 10.
cmp cx, 0
je first_time ;FIRST TIME IS 1, BECAUSE 10^0 = 1.
mov ax, 10
mul cx ;CX * 10. NEXT TIME=100, NEXT TIME=1000...
mov cx, ax ;CX == 10^CX.
jmp l22 ;SKIP THE "FIRST TIME" BLOCK.
first_time:
mov cx, 1 ;FIRST TIME 10^0 = 1.
l22:
;CONVERT CURRENT CHAR TO NUMBER.
dec bx ;BX POINTS TO CURRENT CHAR.
mov al , [bx] ;AL = CURRENT CHAR.
sub al , 48 ;CONVERT CHAR TO NUMBER.
;MULTIPLY CURRENT NUMBER BY CURRENT POWER OF 10.
mov ah, 0 ;CLEAR AH TO USE AX.
mul cx ;AX * CX = DX:AX. LET'S IGNORE DX.
add bp , ax ;STORE RESULT IN BP.
;CHECK IF THERE ARE MORE CHARS.
dec count
cmp count , 0
jne l2
;EXTRACT DIGITS FROM NUMBER ONE BY ONE BY DIVIDING THEM BY 10 AND
;STORING THE REMAINDERS INTO STACK. WE USE THE STACK BECAUSE THE
;STACK STORES DATA IN REVERSE ORDER.
mov count, 0
mov ax, bp ;AX = NUMBER TO PROCESS.
l3:
mov dx, 0 ;CLEAR DX. NECESSARY FOR DIV.
mov cx, 10 ;WILL DIVIDE BY 10.
div cx ;DX:AX / 10. RESULT: AX=QUOTIENT, DX=REMAINDER.
add dl,48 ;CONVERT DIGIT TO CHAR.
push dx ;STORE DIGIT IN STACK.
inc count
cmp ax, 0
jne l3 ;IF QUOTIENT != 0, REPEAT.
;EXTRACT CHARS FROM STACK IN REVERSE ORDER TO BUILD THE NUMBER IN STRING.
mov bx, offset output
l4:
pop dx ;RETRIEVE ONE CHAR.
mov [bx], dl ;STORE CHAR IN STRING.
inc bx ;POSITION FOR NEXT CHAR.
dec count
jnz l4 ;IF ( COUNT != 0 ) REPEAT.
;DISPLAY MESSAGE.
mov dx , offset msj
mov ah, 9
int 21h
;DISPLAY NUMBER CONVERTED TO STRING.
mov dx , offset output
mov ah, 9
int 21h
;WAIT FOR A KEY.
mov ah,7
int 21h
;FINISH PROGRAM.
mov ax, 4c00h
int 21h
main endp
end main
