🚀 go-pugleaf

The XL's do have 64K, but the 'extra' 16 K is hidden beneath the
cartridge, and as you rightly guessed, is not usually available, in
fact hardly ever available. OSS (optimised software systems) does make
bank switched cartridges for the Atari; called 'Supercartridges', their
BASIC XL, ACTION!, and MAC65 are in those cartridges, which basically
are 16K carts. which appear to the machine to be 8k carts, and which
bank switch to give you an extra 4K. They also sell DOS XL, a true
double density DOS which can use some of the memory that exists in parallel
with the OS to give you some more (beyond the supercart); DOS XL can be
used with any XL machine to get this extra memory, not just with supercarts,
though they work together to give even more.
   
          -Ravi