Super Expander

It was designed to provide several extensions to the BASIC interpreter on the computer, mostly to help with programming graphics and sound.

To be fair, the decision by Commodore to recycle the old BASIC, and the fact that it could fit in just 16 KB ROM (including the KERNAL), helped keep the VIC-20's price to a minimum and so contributed to its huge success.

To use VIC-20's graphics and sound programmers had to "PEEK and POKE" bytes directly from/to the VIC-20's graphics/sound hardware, the 6560 Video Interface Chip (VIC).

Programmers could mitigate these problems by using machine code, to an extent, but this in itself was a tedious process with a rather steep learning curve.

This was a complex and long-winded process; implementing it in a BASIC program was virtually useless due to the execution time required to draw anything.

It allowed the programmer to draw points, lines, ellipses and arcs, and to paint enclosed regions, with one-line statements.

Even so, the Super Expander provided a command to play simple tones on the VIC-20's four voice channels, and to control the volume.

Each of the VIC-20's four voice channels could play their own scores simultaneously, giving harmonious effects which could be striking by the standards of the time.