Question

I have a trouble with simple assembler program.I don’t know what i why happens but there is problem.
Task is : Compute scalar product of two arrays in assembler (emu 8086)
Here is my code

    ; multi-segment executable file template.

data segment
; add your data here!
n1 db 1,2,3
n2 db 4,5,6
i db ?
j db ?

k dw ?
pkey db "press any key...$"

ends

stack segment
dw 128 dup(0)
ends

code segment
start:
; set segment registers:
mov ax, data
mov ds, ax
mov es, ax

mov al,00h
mov ah,02h

mov ch,00h
mov cl,03h

mov bx,offset n1
ciklus:

mov dl,[bx]
;add dl,30h


mov j,0d

add j,dl
int 21h
mov al,j
mov ah,0h


inc bx
inc bx
inc bx
mov dl,[bx]
;add dl,30h

mov i,0d
add i,dl
mul i

mov k,ax


mov al,00h
mov ah,02h
int 21h



dec bx
dec bx
dec bx

inc bx



loop ciklus

lea dx, pkey
mov ah, 9
int 21h        ; output string at ds:dx

; wait for any key....    
mov ah, 1
int 21h

mov ax, 4c00h ; exit to operating system.
int 21h    

ends

end start ; set entry point and stop the assembler.

So when i run program step by step i see strange values in ax. I want
1×4 2×5 3×6 = > 4 10 18 to save in some variable and print to screen .

If someone can help me i will be grateful.
P.S this i’m noob in assembler programming so don’t be angry of my stupid question.

Answer 1

After the ciklus: label, you are adding 0x30 to dl, then adding dl to j, so j >= 0x30 at this point. After the interrupt you set al to j, and ah to 0x0, so ax >= 0x30. Later you add 0x30 to dl, then add dl to i.

So mul i is effectively: [dx:ax] = i * ax, which is going to be at least (0x30 * 0x30), i.e., ax >= 2304, dx = 0. Larger, depending on the contents of [bx], i, j.

After the mul you overwrite the ax register with the value: 0x200 (512), anyway, so the results are lost. In short, you need to look at saving your running ax [ah:al] register before clobbering it for use as the interrupt service code.

P.S. You might consider using the si and di registers to save working values.