I'm new to assembly language, and I have a problem. I'm trying to make a basic assembler program with TASM(DOS) that turns on some LEDs according to the result of an operation. All my operations work, except for division. When I do division, only the first LED turns on, which means that the result is 1 in binary. If I put in 4 for the first number and 2 for the second number the result should be 2. The second led should be on not the first one. Why is my division not working properly?
This is the code:
.model small
.stack 64
.data
Numero1 db 0
Numero2 db 0
VariableEspecial db 0
Mensaje1 db 10,13, "Introduce la primera cifra: -> ", '$'
Mensaje2 db 10,13, "Introduce la segunda cifra: -> ", '$'
Mensaje4 db 10,13, "Otra operacion.. 1 - Si, 2 - No, -> ", '$'
Mensaje7 db 10,13, "Escoge operacion: 1 - Suma, 2 - Resta, 3 - Multiplicacion, 4 - Division, -> ", '$'
.code
begin proc far
mov ax, @data
mov ds, ax
Inicio:
mov ah, 09
lea dx, Mensaje1
int 21h
mov ah, 01
int 21h
sub al, 30h
mov Numero1, al
mov ah, 09
lea dx, Mensaje2
int 21h
mov ah, 01
int 21h
sub al, 30h
mov Numero2, al
mov ah, 09
lea dx, Mensaje7
int 21h
mov ah, 01
int 21h
cmp al, 31h
je Suma
cmp al, 32h
je Resta
cmp al, 33h
je Multiplicacion
cmp al, 34h
je Division
Suma:
mov al, Numero1
add al, Numero2
jmp Leds
Resta:
mov al, Numero1
sub al, Numero2
jmp Leds
; ------------------------
Inicio2:
jmp Inicio
; ------------------------
Multiplicacion:
mov al, Numero1
mul Numero2
jmp Leds
Division:
mov al, Numero1
div Numero2
jmp Leds
Leds:
add al, 30h
cmp al, 31h
je UnLed
cmp al, 32h
je DosLed
cmp al, 33h
je TresLed
cmp al, 34h
je CuatroLed
cmp al, 35h
je CincoLed
cmp al, 36h
je SeisLed
cmp al, 37h
je SieteLed
cmp al, 38h
je OchoLed
cmp al, 39h
je NueveLed
;-----------------
Inicio3:
jmp Inicio2
;-----------------
UnLed:
mov dx, 0378H
mov al, 00000001b
out dx, al
jmp FinalizarOperacion
DosLed:
mov dx, 0378H
mov al, 00000010b
out dx, al
jmp FinalizarOperacion
TresLed:
mov dx, 0378H
mov al, 00000011b
out dx, al
jmp FinalizarOperacion
CuatroLed:
mov dx, 0378H
mov al, 00000100b
out dx, al
jmp FinalizarOperacion
CincoLed:
mov dx, 0378H
mov al, 00000101b
out dx, al
jmp FinalizarOperacion
SeisLed:
mov dx, 0378H
mov al, 00000110b
Out dx, al
jmp FinalizarOperacion
SieteLed:
mov dx, 0378H
mov al, 00000111b
out dx, al
jmp FinalizarOperacion
OchoLed:
mov dx, 0378H
mov al, 00001000b
out dx, al
jmp FinalizarOperacion
NueveLed:
mov dx, 0378H
mov al, 00001001b
out dx, al
jmp FinalizarOperacion
FinalizarOperacion:
mov ah, 09
lea dx, Mensaje4
int 21h
mov ah, 01
int 21h
cmp al, 31h
mov dx, 0378H
mov al, 00000000b
out dx, al
je Inicio3
cmp al, 32h
mov dx, 0378H
mov al, 00000000b
out dx, al
je Cerrar
Cerrar:
mov ah, 4ch
int 21h
begin endp
end
There is an issue with the way you are doing division. You are trying to divide the 8-bit number in AL(Numero1) by an 8-bit value (Numero2).
The thing you have to be careful with is that the DIV instruction divides all of AX by the 8-bit(byte) operand. You set AL to Numero1 but you didn't make sure that the AH register was 0. By not doing this you are dividing AX which likely has junk in AH making your division operation work with the wrong number.
The Instruction Set Reference says this about DIV
Unsigned divide AX by r/m8, with result stored in AL = Quotient, AH = Remainder.
You need to explicitly set AH to 0 in your case. You can do that with mov ah, 0, or use the preferred alternative xor ah, ah. Both set AH to zero.
Your division code looks like this:
Division:
mov al, Numero1
div Numero2
jmp Leds
It should look something like:
Division:
xor ah, ah
mov al, Numero1
div Numero2
jmp Leds
This ensures that the division is 00000000:AL(Numero1) divided by Numero2