64 KB and BASIC without a cartridge
The 800XL has a full 64 KB RAM on its motherboard, without needing the 1064 module used by the 600XL. Atari BASIC is in ROM. The cartridge slot is on top, with two controller ports on the right.
Built-in tests check memory, keyboard, display and sound. HELP calls up guidance in programs that support it.
Running older software
Holding OPTION at power-on disables built-in BASIC, making its address space available to other programs. Many disk programs require this startup method. Older 400/800 titles may also need an earlier operating system if they use fixed addresses of ROM routines.
A BASIC program cannot use all 64 KB: screen and system data occupy some memory, and ROM overlays part of the address space.
SIO and PBI
SIO connects the 800XL to a disk drive, cassette recorder or printer. PBI at the rear exposes bus signals to external expansions.
The XE series' ECI differs from PBI. TURBO FREEZER, for example, therefore has XL and XE adapters, and some 800XL machines require an additional interface power connection. Video comes from the DIN monitor socket or RF.