The computer and its operating system
An emulator reproduces Atari hardware. It also needs an operating system and, for BASIC programs, an interpreter. Original ROMs or modern replacements such as Altirra's systems can be used. Unusual programs calling ROM routines directly may behave differently with replacements.
Altirra: configuration and debugger
Avery Lee's Altirra is an Atari emulator for Windows. It includes independently written OS and BASIC replacements, supports disk, cassette and cartridge images, and loads binary programs directly. Its debugger analyses memory, registers and running code.
Atari800
Atari800 is open source and runs on many platforms, including Linux. It emulates various computer models, video standards and firmware versions, including Altirra replacements. The interface depends on the build; popular releases use SDL.
Media and SIO transfers
Emulators load XEX programs, ATR disk images, CAR cartridges and CAS tapes. SIO acceleration shortens disk operations but may affect loaders sensitive to transfer timing. Write-protecting an image preserves the original media copy.