54277



     Now you will have to make each display instruction for each line
     into a Load Memory Scan (LMS) instruction. To direct each LMS
     to the proper screen RAM for that line, you will have to increment
     each memory location by the total length of the line. For
     example, if you want to scroll a 256-byte horizontal screen, each
     LMS instruction will have to point to a location in memory 256
     bytes above the last one. Of course, you will have to implement
     error-trapping routines so that your screen does not extend
     beyond your desired boundaries.

     Coarse scrolling, one byte at a time, can be done without setting
     the HSCROL register by the method described above. For
     smooth scrolling, you will have to use this register. See De Re
     Atari.

54277          D405           VSCROL
     (W) Vertical scroll enable, POKE VSCROL with from zero to 16
     scan lines, depending on the GRAPHICS mode of the screen for
     the number of scan lines to scroll. Vertical fine scrolls can be
     used only if BIT 5 of the display list instruction has been set.

     Coarse scrolling can be done without using this register, simply
     by moving the top of the screen address (as defined by the DL
     LMS instruction) up or down one mode line (plus or minus 40 or
     20 bytes, depending on the GRAPHICS mode). The top of the
     screen address can be found by:

     10  DLIST = PEEK(560) + PEEK(561) * 2
        56
     20  SCRNLO = DLIST + 4: SCRNHI = DLIS
        T + 5: REM LSB/MSB OF SCREEN ADDRE
        SS
     25  PRINT "SCREEN ADDRESS = " PEEK(SC
        RNLO) + PEEK(SCRNHI) * 256

     You could then add a routine to this for a coarse - scroll vertically
     through the memory with a joystick, such as:

     30  LOBYTE = 0: HIBYTE = 0
     40  IF STICK(0) = 14 THEN LOBYTE = LO
        BYTE + 40:GOTO 100
     50  IF STICK(0) = 13 THEN LOBYTE = LO
        BYTE - 40
     60  IF LOBYTE < 0 THEN LOBYTE = LOBYT
        E + 256: HIBYTE = HIBYTE - 1
     70  IF HIBYTE < 0 THEN HIBYTE = 0
     80  GOTO 200
     100 IF LOBYTE 255 THEN LOBYTE = LOB
         YTE - 256
     110 HIBYTE = HIBYTE + 1