Page "Applesoft BASIC" Paragraph 28

* Applesoft could be extended by two means: the ampersand (&) command and the USR () function.

These were two functions that called machine-language functions stored in memory.

Routines that needed to be as fast or required direct access to arbitrary functions or data in memory could thus be called from a higher-level interpreted BASIC program.

"&" was effectively a shorthand for CALL, with an address that would be predefined.

Additionally, these lower-level functions could be programmed to read the data immediately following the "&" or " USR " command, allowing a BASIC program to pass parameters to the functions.