54280



     the screen display RAM cannot cross a 4K boundary without an
     LMS instruction to point to the proper byte(s). Due to problems
     that arise when moving the GR.7 and GR.8 screens and data less
     than 4K, you should never reserve less than 16 pages above
     RAMTOP in these modes. If you are reserving more, add the
     pages in blocks of 4K (16 pages).
     See COMPUTE!, September 1981, for a discussion of the
     problems of positioning P/M graphics in memory, and using P/M
     graphics for animation.
     See De Re Atari, COMPUTE!, June 1982, and Creative
     Computing, April 1982, for a discussion of using string
     manipulation with P/M graphics. See Your Atari 400/800 for a
     general discussion of P/M graphics. Most of the popular
     magazines have also carried articles on simplifying P/M
     graphics.

54280          D408           ....
     Unused.

54281          D409           CHBASE
     (W) Character base address; the location of the start of the
     character set, either the standard Atari set or a user-designed set.
     The default is 224 ($E0), which points to the start of the Atari
     ROM character set. Iridis, a short-lived disk -and- documentation
     magazine, produced a good utility called FontEdit to aid in the
     design of altered character sets. Online Systems' program The
     Next Step is also very useful for this purpose, as is COMPUTE!'s
     "SuperFont," January 1982. Uses shadow register 756 ($2F4).
     Normally, this points to location 57344 or 57856 ($E000 or $E200)
     depending on your choice of characters used in which text mode.
     GRAPHICS mode zero uses the entire 128-character set; GR.1
     and GR.2 use only half the set (64 characters). You POKE a
     different number into the shadow register at 756 ($2F4) to point to
     your own character set in RAM. This must be an even number
     that points to a page in memory that is evenly divisible by two. In
     GR.1 and GR.2 this number is 224 (pointing to $E000), giving
     you uppercase, punctuation and numbers. POKEing the shadow
     or this location (in machine language) with 226 will give you
     lowercase and control characters.
     See the information about the ROM character set at 57344
     ($E000).

54282          D40A           WSYNC
     (W) Wait for horizontal synchronization. Allows the OS to
     synchronize the vertical TV display by causing the 6502 to halt
     and restart seven machine cycles before the beginning of the