743,744



     at the next byte above MEMTOP.

     The screen handler will only OPEN the S: device if no RAM is
     needed below this value (i.e. there is enough free RAM below
     here to accommodate the requested GRAPHICS mode change).
     Memory above this address is used for the display list and the
     screen display RAM. Also, if a screen mode change would
     extend the screen mode memory below APPMHI (locations 14,
     15: $E, $F), then the screen is set back for GR.0, MEMTOP is
     updated, and an error is returned to the user. Otherwise the
     mode change will take place and MEMTOP will be updated.

     Space saved by moving MEMTOP is below the display list. Be 
     careful not to overwrite it if you change GRAPHICS modes in
     mid-program. When using memory below MEMTOP for storage, 
     make sure to set APPMHI above your data to avoid having the
     screen data descend into it and destroy it.

743,744        2E7,2E8        MEMLO
     Pointer to the bottom of free memory, initialized to 1792 ($700)
     and updated by the presence of DOS or any other low-memory
     application program. It is used by the OS; the BASIC pointer to
     the bottom of free memory is at locations 128, 129 ($80, $81). The
     value in MEMLO is never altered by the OS after powerup.

     This is the address of the first free location in RAM available for
     program use. Set after all FMS buffers have been allocated (see
     locations 1801 and 1802; $709 and $70A). The address of the last
     sector buffer is incremented by 128 (the buffer size in bytes) and
     the value placed in MEMLO. The value updates on powerup or
     when RESET is pressed. This value is passed back to locations 
     128, 129 ($80, $81) on the execution of the BASIC NEW
     command, but not RUN, LOAD or RESET.

     If you are reserving space for your own device driver(s) or
     reserving buffer space, you load your routine into the address
     specified by MEMLO, add the size of your routine to the MEMLO
     value, and POKE the new value plus one back into MEMLO.

     When you don't have DOS or any other application program
     using low-memory resident, MEMLO points to 1792 ($700. With
     DOS 2.0 present, MEMLO points to 7420 ($1CFC). If you change
     the buffer defaults mentioned earlier, you will raise or lower this
     latter value by 128 ($80) bytes for every buffer added or deleted,
     respectively. When you boot up the 850 Interface with or without
     disk, you add another 1728 ($6C0) bytes to the value in MEMLO.

     You can alter MEMLO to protect an area of memory below your
     program. This is an alternative to protecting an area above
     RAMTOP (location 106; $6A) and avoids the problem of the
     CLEAR SCREEN routine destroying data. However, unless you