Super Expander 64 adds functions for drawing graphics, using sprites, reading joysticks, playing audio, and other features.
The extra code is mapped into the "lower cartridge" 8 kilobytes area at $8000-$9FFF, reducing the 38,911 bytes for user programs by said 8K.
Besides a range of commands to initialize, position and move sprites (or Movable Object Blocks as Commodore called them; hardware-supported graphic elements that could move freely on the screen independently of other graphics and text on the screen), Super Expander had a built in tool to edit the pattern of 8 sprites (called upon with the SPRDEF command), either in high-res (24 × 21 pixels) or multicolor (12 wide × 21 pixels) mode.
Functions like RJOY, RPEN and RPOT would read the state of a connected joystick, light pen, or analog paddle.
In combination with the sprite-motion-related commands, it only took a single line of BASIC code to make a sprite move in the direction indicated by a connected joystick.