i need to move some picture by keyboard. Picture must be stopped if it comes to sreen edge. I've tried smth like this(example):
(...)
.data
(...)
;buffer for picture
pict db 0,0,1,1,0,0
db 0,0,1,1,0,0
db 1,1,1,1,1,1
db 1,1,1,1,1,1
db 0,0,1,1,0,0
db 0,0,1,1,0,0
(...)
.code
;there should be setting of 13h mode, changing vga palette and then drawing that picture in cycle
(...)
xor ah,ah
int 16h
;there should be checking if any button is pressed
(...)
;for example, if 'w' is pressed:
cmp al,'w'
jne @f
.if (PICTURE_POSITION >= 320) ;if there's one pixel left at least
sub PICTURE_POSITION,320
.endif
jmp short @000 ;otherwise
(...)
;then if 's' is pressed:
cmp al,'s'
jne @f ;otherwise
.if (PICTURE_POSITION < 320 * 200 - PICTURE_HEIGHT * 320) ;if there's one pixel left at least
sub PICTURE_POSITION,320
.endif
jmp short @001 ;otherwise
(...)
;that code works, but if i add code for left and right edges...
cmp al,'a'
jne @f
mov ax,PICURE_POSITION
cwd
mov bx,320 ;we need to find vertical component of picture's postion, it should be (0/320), (320/320), (640/320) and so on
div bx
mul bx
;for example: if picture's postion is 645 - it is located on third row, so if we want to stop it in the right place,
;we need to check if its position is bigger than (645/320)*320=640 and if it is, we move picture left
.if PICTURE_POSITION>ax
dec mach_pos
.endif
so there's problem: initially all border checking works fine, but when picture have passed some rows down and i try to move it left or right, my program just stucks hopelessly,
and all my next actions is ignored. I think if it worth to try this on real machine (i've tried it on dosbox so far) but what if i'll make my pc to be knelt?
And one silly question: what does happen when some bytes being drawn reach upper left screen edge? Do they damage data which's located before 0A000h?
deleted
if a segment register is set to A000h, you can only access A0000 to AFFFF physical
if you keep going, it "segment wraps" back to A0000 :biggrin: