The new graphics cards were known as Enhanced Graphics Adapter (EGA) cards. They had more on-board video memory than the earlier Color Graphics Adapter (CGA) cards and could display 16 colors at once, instead of four. For Carmack, the extra memory had two important consequences. First, while intended for a single relatively high-resolution screen image, the card's memory could hold several video screens' worth of low-resolution images, typically 300 by 200 pixels, simultaneously, good enough for video games. By pointing to different video memory addresses, the card could switch which image was being sent to the screen at around 60 times a second, allowing smooth animation without annoying flicker. Second, the card could move data around in its video memory much faster than image data could be copied from the PC's main memory to the card, eliminating a major graphics performance bottleneck.

Carmack wrote a so-called graphics display engine that exploited both properties to the full by using a technique that had been originally developed in the 1970s for scrolling over large images, such as satellite photographs. First, he assembled a complete screen in video memory, tile by tile—plus a border one tile wide [see illustration, "Scrolling With the Action" ]. If the player moved one pixel in any direction, the display engine moved the origin of the image it sent to the screen by one pixel in the corresponding direction. No new tiles had to be drawn. When the player's movements finally pushed the screen image to the outer edge of a border, the engine still did not redraw most of the screen. Instead, it copied most of the existing image—the part that would remain constant—into another portion of video memory. Then it added the new tiles and moved the origin of the screen display so that it pointed to the new image .

Scrolling With the Action: For two-dimensional scrolling in his PC game, programmer John Carmack cheated a little by not always redrawing the background. He built the background of graphical tiles stored in video memory [left] but only sent part of the image to the screen [top left, inside orange border]. As the play character [yellow circle] moved, the background sent to the screen was adjusted to include tiles outside the border [see top right]. New background elements would be needed only after a shift of one tile width. Then, most of the background was copied to another region of video memory [see bottom right], and the screen image centered in the new background.

In short, rather than having the PC redraw tens of thousands of pixels every time the player moved, the engine usually had to change only a single memory address—the one that indicated the origin of the screen image—or, at worst, draw a relatively thin strip of pixels for the new tiles. So the PC's CPU was left with plenty of time for other tasks, such as drawing and animating the game's moving platforms, hostile characters, and the other active elements with which the player interacted.

Hall and Carmack knocked up a Mario clone for the PC, which they dubbed Dangerous Dave in Copyright Infringement. But Softdisk, their employer, had no interest in publishing what were then high-end EGA games, preferring to stick with the market for CGA applications. So the nascent Id Software company went into moonlight overdrive, using the technology to create its own side-scrolling PC game called Commander Keen. When it came time to release the game, they hooked up with game publisher Scott Miller, who urged them to go with a distribution plan that was as novel as their technology: shareware.

In the 1980s, hackers started making their programs available through shareware, which relied on an honor code: try it and if you like it, pay me. But it had been used only for utilitarian programs like file tools or word processors. The next frontier, Miller suggested, was games. Instead of giving away the entire game, he said, why not give out only the first portion, then make the player buy the rest? Id agreed to let Miller's company, Apogee, release the game. Prior to Commander Keen, Apogee's most popular shareware game had sold a few thousand copies. Within months of Keen's release in December 1990, the game had sold 30 000 copies. For the burgeoning world of PC games, Miller recalls, "it was a little atom bomb."