88,89



     Why the minus one in the calculation? The first byte of the screen
     is the first byte in the loop. If we add the total size, we will go one
     byte past the end of the soreen, so we subtract one from the total.
     Here's how to arrive at the value for the total amount ot memory
     located for screen use, display list and Text window:

               Total memory allocation for the screen

                      Screen display       Display List
     -----------------------------------------------------------
          Text    unused  bytes  screen    unused  used
     GR  window   always  cond.   use      bytes   bytes   Total
     -----------------------------------------------------------
     0    ...      none   none    960      none      32      992
     1    160      none     80    400      none      34      674
     2    160      none     40    200      none      24      424
     3    160      none     40    200      none      34      434
     4    160      none     80    400      none      54      694
     5    160      none    160    800      none      54     1174
     6    160      none    320   1600      none      94     2174
     7    160      none    640   3200        96      94     4190
     8    160        16   1280   6400        80     176     8112

     The number of bytes from RAMTOP (location 106; $6A) is counted
     from the left text window column towards the total column.
     MEMTOP (741, 742; $2E5, $2E6) points to one byte below
     RAMTOP * 256 minus the number of bytes in the total column. If
     16 is added to the GRAPHICS mode (no text window), then the
     conditional unused bytes are added to the total. Then the bytes
     normally added for the text window become unused, and the
     Display List expands slightly. (See COMPUTE!, September 1981.)

     When you normally PRINT CHR$(125) (clear screen), Atari sends
     zeroes to the memory starting at locations 88 and 89. It continues to
     do this until it reaches one byte less than the contents of RAMTQP
     (location 106; $6A). Here is a potential source of conflict with your
     program, however: CHR$(125)--CLEAR SCREEN--and any
     GRAPHICS command actually continue to clear the first 64 ($40)
     bytes above RAMTOP!

     It would have no effect on BASIC since BASIC is a ROM
     cartridge. The OS Source Listing seems to indicate that it ends at
     RAMTOP, but Atari assumed that there would be nothing after
     RAMTOP, so no checks were provided. Don't reserve any data
     within 64 bytes of RAMTOP or else it will be eaten by the CLEAR
     SCREEN routine, or avoid using a CLEAR SCREEN or a
     GRAPHICS command. Scrolling the text window also clears 800
     bytes of memory above RAMTOP.

     You can use this to clear other areas of memory by POKEing the