All you need to know about RAM and ROM from $0000-$ffff. Which parts are free, which are not and what's in there in that case. Remarks: The area from $D000-D7ff is the I/O area where the custom chips are located. The RAM under that area is not usable (so the Atari XL only has 62k RAM and not 64k in fact). The OS-ROM in that area contains the Self-Test and is mapped to $5000-$57ff if Self-Test is activated via PORTB.