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NASM Forum => Programming with NASM => Topic started by: bilsch01 on December 05, 2019, 05:53:42 AM

Title: need help scrolling text mode screen DOWN
Post by: bilsch01 on December 05, 2019, 05:53:42 AM

My program uses 4000 bytes at segment 0xb800 for text mode display. The bottom 3 lines of the screen (160 bytes of memory per line) are static display and are not to be moved. Bytes 0 thru 3519 contain part of a text document and are the area to be moved up/down.

1. The code below moves the displayed document text up one line. It works fine:

mov ax,0xb800
mov ds,ax
mov es,ax
mov cx,1680
mov si,160
mov di,0
rep movsw

2. I tried the code below to move the displayed document text DOWN  one line:

mov ax,0xb800
mov ds,ax
mov es,ax
mov cx,1680
mov si,3200
mov di,3360
rep movsw

It moves the line beginning at b800:3200 down one line on the screen but the lines above it are not moved down. It is unfortunate that the static display gets overwritten, I can handle that later. I tried some variations of the code but I wasn't able to get the entire document area to move down one line. Can someone here tell me how to move the document area down one line.
TIA.   Bill S.

Title: Re: need help scrolling text mode screen DOWN
Post by: fredericopissarra on December 05, 2019, 11:34:45 AM

The problem is overlapping data...

Try to point ES:DI to the end of the last line and DS:SI to the end of the line above, CX to 80*24, change the DF to 1 and do REP/MOVSW:

Code: [Select]

; OBS: destroys ax,cx, si and di.
scrolldown1line:
  push ds
  push es
  mov ax,0xb8000
  mov ds,ax
  mov es,ax
  mov di,160*25-2
  mov si,160*24-2
  mov cx,80*24
  std       ; to move backwards
  rep movsw
  cld
  pop es
  pop ds
  ret