im making a space invaders project to school and im trying to check if the aliens got to the button of the screen. i used the int 10h d (it takes the color of a pixel on the screen) on my code and it stuck every thing, i tried to understand what happened and came to the conclusion that the line "mov ah,0Dh" along with the "int 10h" line makes it stuck. does anybody has any idea what i can do in order to make it work?
(this is the code)
proc checkAliensDown
pusha
push ax
mov cx,320 ;loop counter
checkPixel:
push cx
mov bh,0h
mov cx,[x]
mov dx,140
mov ah,0Dh
int 10h
;the output of the int 10h goes into al
cmp al,10 ;check if the color of the pixel is green
jne notDown ;if not jumps to notDown
mov [lost],1 ;if it is green moves to lost 1
jmp endProc ;if it found that the pixel is green it jumps to the end of the proc
notDown:
inc [x] ;increasing x var in order to check all the pixels in the row, if one of them is green the player lost
pop cx
loop checkPixel
endProc:
pop ax
popa
ret
endp checkAliensDown
i tried to push and pop ah and it didn't work
Random data on the stack is used as the return address, and that crashes the program.
The push cx right below checkPixel is still on the stack when your code jumps to endProc (in case the green alien got to the bottom of the screen). Simply add a pop cx to remove it:
mov [lost],1 ;if it is green moves to lost 1
pop cx
jmp endProc
Alternatively, use another register to control the loop:
mov SI,320 ;loop counter
checkPixel:
mov bh,0h
mov cx,[x]
mov dx,140
mov ah,0Dh
int 10h
cmp al,10 ;check if the color of the pixel is green
jne notDown ;if not jumps to notDown
mov [lost],1 ;if it is green moves to lost 1
jmp endProc
notDown:
inc [x]
dec SI
jnz checkPixel
endProc:
Tip: the push ax that follows pusha is quite useless. AX is already preserved through pusha. Same applies to the pop ax right before popa.
i tried to push and pop ah and it didn't work
It is normal not being able to push or pop a byte-sized register. In x86-16 the stack operates in word-sized chunks.
Your checkAliensDown proc depends on the x variable to contain 0 beforehand. I would suggest you add that to the proc:
proc checkAliensDown
pusha
mov word [x], 0
The best solution for this 8086 task where pusha and popa are not valid instructions:
proc checkAliensDown
push ax
push bx
push cx
push dx
mov [lost], 1 ; Guarantees LOST contains a sensible value upon return
mov dx, 140 ; Y
xor cx, cx ; X
mov bh, 0 ; Page
checkPixel:
mov ah, 0Dh ; BIOS.ReadPixel
int 10h ; -> AL
cmp al, 10 ; Is it green?
je endProc ; Jump one time, but have the majority case fall through
inc cx
cmp cx, 320 ; Use CX for both the X coordinate and the loop counter
jb checkPixel
mov [lost], cl ; CL=0
endProc:
pop dx
pop cx
pop bx
pop ax
ret
endp checkAliensDown