Rewind to the release of the first major game system with interchangeable cartridges, the Atari VCS (a.k.a. Atari 2600) in 1977. Now instead of buying that game system, imagine you wanted a general purpose PC that could create displays of the same color and resolution as the Atari. What would the capabilities of that mythical 1977 PC need to be?
For starters, you'd need a 160x192 pixel display with a byte per pixel. Well, technically you'd need 7-bits, as the 2600 can only display 128 colors, but a byte per pixel is simpler to deal with. That works out to 30,720 bytes for the display. Sounds simple enough, but there's a major roadblock: 4K of RAM in 1977 cost roughly $125. To get enough memory for our 2600-equivalent display, ignoring everything else, would have been over $900.
For comparison, the retail price of the Atari 2600 was $200.
How did Atari's engineers do it? By cheating. Well, cheating is too strong of a word. Instead of building a financially unrealistic 30K frame buffer, they created an elaborate, specialized illusion. They built a video system--a monochrome background and two single-color sprites--that was only large enough for a single horizontal line. To get more complex displays, game code wrote and rewrote that data for each line on the TV screen. That let the Atari 2600 ship with 128bytes of RAM instead of the 30K of our fantasy system.