756



     don't simply scroll the four lines; you actually scroll a full 24 (20
     additional lines * 40 bytes equals 800 bytes scrolled past
     memory)! This messes up the memory past the window display
     address, so position your character sets below all possible
     interference (or don't scroll or clear the screen).

     You can create and store as many character sets as your memory
     will allow. You switch back and forth between them and the ROM
     set by simply POKEing the MSB of the address into 756. Of
     course, you can display only one set at a time unless you use an
     altered display list and DLI to call up other sets. There are no
     restrictions outside of memory requirements on using altered
     character sets with P/M graphics as long as the areas reserved for
     them do not overlap.

     A GRAPHICS command such as GR.0, RESET or a DOS call
     restores the character set pointer to the ROM location, so you
     must always POKE it again with the correct location of your new
     set after any such command. A useful place to store these sets is
     one page after the end of RAM, assuming you've gone back to
     location 106 ($6A) and subtracted the correct number of pages
     from the value it holds (by POKE 106,PEEK(106) minus the
     number of pages to be reserved; see above). Then you can reset
     the character set location by simply using POKE
     756,PEEK(106)+1 (the plus one simply makes sure you start at
     the first byte of your set).

     A full character set requires 1024 bytes (1K: four pages) be
     reserved for it. Why? Because there are 128 characters, each
     represented by eight bytes, so 128 * eight equals 1024. If you are
     using a graphics mode that uses only half the character set, you
     need only reserve 512 bytes (64 * eight equals 512). Remember to
     begin either one on a page boundary (1K boundary for full sets or
     .5K for half sets). By switching back and forth between two
     character sets, you could create the illusion of animation.

     Many magazines have published good utilities to aid in the
     design of altered character sets, such as the January 1982
     Creative Computing, and SuperFont in COMPUTE!, January
     1982. I suggest that you examine The Next Step from Online,
     Instedit from APX, or FontEdit from the Code Works for very
     useful set generators. One potentially useful way to alter just a
     few of the characters is to duplicate the block of memory which
     holds the ROM set by moving it byte by byte into RAM. A BASIC
     FOR-NEXT loop can accomplish this, although it's very slow. For
     example:

     5 CH=57344
     10 START=PEEK(106)-4:PLACE=START*256:
        POKE 106,PEEK(106)-5:GRAPHICS 0: RE