From: terje.mathisen@nospicedham.tmsw.no   
      
   bilsch01 wrote:   
   > 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.   
   >   
      
   This is the classic overlapping move problem. :-)   
      
   You have recognized it partially, in that you are pointing at hte end of   
   the buffer areas instead of the beginning, but you have failed to issue   
   a STD (Set Direction bit, vs CLD which is the default).   
      
   However, the real issue here is the fact that you should NOT move text   
   around on a classic PC screen at all! Instead you maintain an off-screen   
   buffer and then you use REP MOVSW to copy the relevant parts into the   
   screen memory.   
      
   This is because the screen buffer is/was 3-10 times slower than normal   
   RAM, so it was far faster to do all updates off-screen and then copy in   
   the finished screen.   
      
   In my own code I used a list of line buffers for that off-screen   
   display, so that I could do partial scrolling with just a few pointer   
   updates, before using REP MOVSW on individual lines to copy them to the   
   correct screen location.   
      
   If you have the original CGA screen then you also need to worry about   
   screen flicker if you write to the CGA memory while the screen is being   
   refreshed! You can copy at least 1-4 lines (afair) of data in the   
   vertical retrace interval and a few characters in each horizontal   
   retrace if you need to.   
      
   Terje   
      
   --   
   -    
   "almost all programming can be viewed as an exercise in caching"   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)