756



     One problem with POKEing 756 with 226 is that there is no blank
     space character in the second set: you get a screen full of hearts.
     You have two choices: you can change the color of register zero
     to the same as the background and lose those characters which
     use register zero--the control characters--but get your blanks
     (and you still have registers one, two and three left). Or you can
     redefine your own set with a blank character in it. The latter is
     obviously more work. See "Ask The Readers," COMPUTE!, July
     1982.

     It is seldom mentioned in the manuals, but you cannot set 756 to
     225 ($El) or any other odd number. Doing so will only give you
     screen garbage. The page number 756 points to must be evenly
     divisible by two.

     When you create your own character set and store it in memory,
     you need to reserve at least 1K for a full character set (1024 bytes
     --$400 or four pages), and you must begin on a page boundary.
     In hex these are the numbers ending with $XX00 such as $C000
     or $600 because you store the pointer to your set here in 756; it
     can only hold the MSB of the address and assumes that the LSB is
     always zero--or rather a page boundary. You can reserve
     memory by:

     POKE 106,PEEK(106)-4 (or any multiple of four)

     And do a GRAPHICS command immediately after to have your
     new memory value accepted by the computer. If you are using
     only one half of the entire set, for GR.1 or GR.2, you need only
     reserve 512 bytes, and it may begin on a .5K boundary (like
     $E200; these are hexadecimal memory locations that end in
     $X200). If you plan to switch to different character sets, you will
     need to reserve the full 1K or more, according to the number of
     different character sets you need to display. RAM for half-K sets
     can be reserved by:

     POKE 106,PEEK(106)-2 (or a multiple of two)

     The location for your set will then begin at PEEK(106)*256.
     Because BASIC cannot always handle setting up a display list for
     GR.7 and GR.8 when you modify location 106 by less than 4K (16
     pages), you may find you must use PEEK(106)-16. See location
     88,89 ($58,$59) and 54279 ($D407) for information regarding
     screen use and reserving memory.

     Make sure you don't have your character set overlap with your
     player/missile graphics. Be very careful when using altered
     character sets in high memory. Changing GRAPHICS modes, a
     CLEAR command, or scrolling the text window all clear memory
     past the screen display. When you scroll the text window, you