Problems with BIOS delay function (INT 15h / AH = 86h)

Question

I began studying assembly this year in my school, and we just started learning about pixels.

Our teacher gave us some code and told us to mess around with it, he also to

Answer 1

Try using a debugger to make sure all the registers still have the values you expect, after the int 10h system/BIOS call (whatever it is). I haven't checked docs, but it may clobber some other registers?

Or maybe your problem is:

mov ax, [red]
mov ah,0ch 
int 10h

Remember that ah is the high half of ax. So if you modify ah right after loading something into ax, it's YOUR code that modified your color value.

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Also, code like

add [y], 2
mov dx, [y]

is horrible. Try to keep variables in registers when you can. If you need to spill some, try to avoid doing it with variables that change every time through the loop.

mov dx, [y] 
add dx, 2 
mov [y], dx

Would be somewhat better, because dx would come from the first load, not a chain of read-modify-write -> load. It is more instructions, which is why keeping important loop variables in memory is bad.

Answer 2

There is a weird fact, about int 15h ah=86h you also need to set al=0, unless it get an erratic behavior

Delay Code:

mov al, 0 mov ah, 86h mov cx, 1 mov dx, 2 int 15h

I discovered it with try-and-error method and I realize when al=0 it works.