The Atari 7800 differs from the 2600 in several key areas. It features a full Atari SALLY 6502 processor whereas the 2600 VCS has a stripped down 6507 processor running at a slower speed. It has additional RAM (Random Access Memory) and the ability to access more cartridge data at one time than the 2600. The most substantial difference, however, is a graphics architecture which differs markedly from either the Atari 2600 VCS or Atari’s 8-bit line of computers.
The 7800's compatibility with the Atari 2600 is made possible by including many of the same chips used in the Atari 2600. When operating in “2600” mode to play Atari 2600 titles, the 7800 uses a Television Interface Adapter (TIA) chip to generate graphics and sound. The processor is slowed to 1.19 MHz, enabling the 7800 to mirror the performance of the 2600s stripped-down 6507 processor. RAM is limited to 128 bytes found in the RIOT and game data is accessed in 4K blocks.
When in “7800” mode (signified by the appearance of the full screen Atari logo), the graphics are generated entirely by the MARIA graphics processing unit, all system RAM is available and game data is accessed in larger 48K blocks. The system’s SALLY 6502 runs at its normal 1.79 MHz instead of the reduced speed of 2600 mode. The 2600 chips are used in 7800 mode to generate sound as well as switch and controller interfaces.
Atari 7800 does not suport Backward Compitablity for Atari 5200.