Commander Keen’s Adaptive Tile Refresh (ATR) is a crucial piece of technology that powered the first trilogy of the Commander Keen series. The Enhanced Graphics Adapter (EGA) card, with its four planes of graphics data storage, was the key to running the game at its best. The EGA CRTC (screen controller) read four bytes in parallel, allowing for higher bandwidth necessary to keep up with the screen. ATR solved the problem of bandwidth by building a virtual screen in VRAM and using smooth scrolling techniques. The second trilogy of the game used a different technique called “drifting” to achieve smooth scrolling without repeating tiles. John Carmack, one of the developers, explained that the screen would wrap around at the edge of the VRAM, eliminating the need for a full screen redraw. This technique worked well for most cases, but there were compatibility issues with certain Super VGA cards that had more memory. In those cases, copying the whole screen was necessary. Overall, ATR and drifting were clever solutions that allowed for smoother scrolling in the game.
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