While the C64 had the advertised of RAM installed, only about was available for BASIC programs. The Plus/4's BASIC V3.5 made available, aided by its memory map that swapped the ROMs in and out of memory as needed, and that placed the memory mapped I/O registers, which all 6502-based computers have to use, at the top of memory ($FD00), while in the C64 they had been located at the much lower address $D000. On the C64, a program was able to swap out the ROMs and the I/O registers manually and thus gain access to the full , but this was not compatible with BASIC on that machine; on the Plus/4, on the other hand, most of the ROM area was automatically switched out when not needed, rendering the RAM existing at the same addresses accessible for BASIC programs.