to the combination of an overly complex mechanical design and a very low-cost internal flex circuit system. Another major flaw of the controllers was that the design did not translate into a linear acceleration from the center through the arc of the stick travel. The controllers did, however, include a pause button, a unique feature at the time. Various third-party replacement joysticks were also released, including those made by Wico. Atari Inc. released the Pro-Line Trak-Ball controller for the system, which was used primarily for gaming titles such as Centipede and Missile Command. A paddle controller and an updated self-centering version of the original controller were also in development, but never made it to market. Games were shipped with plastic card overlays that snapped in over the keypad. The card would indicate which game functions, such as changing the view or vehicle speed, were assigned to each key. The primary controller was ranked the 10th worst video game controller by IGN editor Craig Harris. An editor for Next Generation said that their non-centering joysticks "rendered many games nearly unplayable". Internal differences from 8-bit computers David H. Ahl in 1983 described the Atari 5200 as "a 400 computer in disguise". Its internal design is a tweaked version of the Atari 8-bit family using the ANTIC, POKEY, and GTIA coprocessors. Software designed for one does not run on the other, but source code can be mechanically converted unless it uses computer-specific features. Antic magazine reported in 1984 that "the similarities grossly outweigh the differences, so that a 5200 program can be developed and almost entirely debugged [on an Atari 8-bit computer] before testing on a 5200". John J. Anderson of Creative Computing alluded to the incompatibility being intentional, caused by Atari's console division removing 8-bit compatibility to not lose control to the rival computer division. Besides the 5200's lack of a keyboard, the differences are: The Atari computer 10 KB operating system is replaced with a simpler 2 KB version, of which 1 KB is the built-in character set. Some hardware registers, such as those of the GTIA and POKEY chips, are at different memory locations. The purpose of some registers is slightly different on the 5200. The 5200's analog joysticks appear as pairs of paddles to the hardware, which requires different input handling from the digital joystick input on the Atari computers In 1987, Atari Corporation released the XE Game System