7.1.8) Why do some programs run only on the XL/XE (not the 400/800)?

Section started by Konrad M. Kokoszkiewicz. Software designed for the Atari XL/XE won't work on the 400/800 if: 1) It uses shadow RAM at $C000-$CFFF (4KiB) or $D800-$FFFF (10KiB). In other words, it requires 64KiB RAM. 2) It uses RAM expansions at $4000-$7FFF controlled by PORTB $D301. In other words, it requires at least 128KiB total RAM, 130XE compatible. 3) It uses XL OS vectors (routines) not present in the 400/800 OS. Some of these correspond to XL/XE specific hardware, such as the [HELP] key or the PBI/ECI interface. 4) Rather than using documented OS vectors, it "illegally" uses OS routines directly for routines that were located at different memory addresses in the 400/800 OS. 5) It uses the International Character Set.