106



106            6A             RAMTOP
     RAM size, defined by powerup as passed from TRAMSZ (location
     6), given in the total number of available pages (one page equals
     256 bytes, so PEEK(106) * 256 will tell you where the Atari thinks
     the last usable address --byte-- of RAM is). MEMIOP (741,
     742; $2E5. $2E6) may not extend below this value. In a 48K Atari,
     RAMTOP is initialized to 160 ($A0), which points to location
     40960 ($A000). The user's highest address will be one byte less
     than this value.

     This is initially the same value as in location 740. PEEK(740) / 4 or
     PEEK(106) / 4 gives the number of 1K blocks. You can fool the
     computer into thinking you have less memory than you actually
     have, thus reserving a relatively safe area for data (for your new
     character set or player/missile characters, for example) or
     machine language subroutines by:

     POKE(106), PEEK(106) - # of pages you want to reserve.

     The value here is the number of memory pages (256-byte blocks)
     present. This is useful to know when changing GR.7 and GR.8
     screen RAM. If you are reserving memory for PM graphics,
     POKE 54279, PEEK(106) - # of pages you are reserving before
     you actually POKE 106 with that value. To test to see if you have
     exceeded your memory by reserving too much memory space,
     you can use:

     10   SIZE = (PEEK(106) - # of pages)
        * 256
     20   IF SIZE < = PEEK(144) + PEEK(145
        ) * 256 THEN PRINT "TOO MUCH MEMOR
        Y USED"

     If you move RAMTOP to reserve memory, always issue a
     GRAPHICS command (even issuing one to the same GRAPHICS
     mode you are in will work) immediately so that the display list
     and data are moved beneath the new RAMTOP.

     You should note that a GRAPHICS command and a CLEAR
     command (or PRINT CHR$(125)) actually clear the first 64 bytes
     above RAMTOP (see location 88; $58 for further discussion).
     Scrolling the text window of a GRAPHICS mode clears up to 800
     ($320) bytes above RAMTOP (the text window scroll actually
     scrolls an entire GR.0 screen-worth of data, so the unseen 20
     lines * 40 bytes equals 800 bytes). PM graphics may be safe
     (unless you scroll the text window) since the first 384 or 768 bytes
     (double or single line resolution, respectively) are unused.
     However, you should take both of these effects into account when
     writing your programs.