An add-on card for the Apple II+ and Apple //e that added 64k of RAM and the ability to display text at twice the horizontal resolution.

Really, all the card did was add 64k of RAM. The 80 column part more or less came for free along with a little glue logic. The Apple II video framebuffer lived in main memory. Adding 64k increased the standard 64k to 128k and therefore increased the usable video memory by twice as much. The glue logic interleaved the framebuffer portion of memory to allow for this goofiness.

By some freak accident, software developers discovered that the 80 column card could be fooled into displaying graphics at twice the horizontal rez. In addition, due to the Apple's weird NTSC color scheme, the color palette was increased as well. Thus was born double hi-res graphics.

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