The cheaper of the first two models
The Atari 400 debuted alongside the more expensive 800. Both had a 6502 processor, the same graphics and sound chips, and SIO. The 400 stood out for its flat membrane keyboard. Early machines had 8 KB RAM, later increased to 16 KB. Limited memory restricted program length and software choice.
Memo Pad and the BASIC cartridge
Switching on without a cartridge or boot disk brings up Memo Pad: a screen where text can be typed but not saved. Programming requires an Atari BASIC cartridge. With it installed, the computer displays READY and waits for commands.
A factory-standard 400 connects to a television through RF. It lacks the separate monitor socket found on the 800.
Memory expansion and SIO devices
Disk drives, cassette recorders and printers connect through SIO. Atari also offered a 48 KB RAM upgrade. Installation required opening the case, replacing the memory board and soldering four connections. The system, screen and interpreter use some RAM, leaving less than 48 KB for a BASIC program.