问题
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 told us to try and implement delay in the code(ah=86h|int 15h), but when i tried to use it the location and colors of some pixels got changed, and i don't understand why
code: (just a piece of the code, there is some more there also get's ruined after the delay)
mov cx, 20
add [y], 2
mov dx, [y]
paint1RowOf10:
mov bh, 0h
mov bx, cx
add [x], 1
mov cx, [x]
add [y], 4
mov dx, [y]
mov ax, [red]
mov ah,0ch
int 10h
; here is a delay part, it waits for 1 seconde
; from the help of assembly:
; INT 15h / AH = 86h - BIOS wait function.
;CX:DX = interval in microseconds (these are notes from my teacher)
mov ah, 86h
mov cx, 1
mov dx, 2
int 15h
mov cx, bx
loop paint1RowOf10
This is the result http://prntscr.com/9a8lpw can you tell me why does this happen? because as far as i see, the pixels are supposed to go in one line, and not change colors.
回答1:
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.
回答2:
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.
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.
来源：https://stackoverflow.com/questions/34089884/problems-with-bios-delay-function-int-15h-ah-86h