Editing Atari BASIC (section)

===Embedded machine language===
Atari BASIC can call machine code subroutines stored in strings or <code>POKE</code>ed into memory. The 256 byte area starting at address 1536 ($0600) is often used for this purpose.

Machine code is invoked with the <code>USR</code> function. The first parameter is the address of the subroutine and the following values are parameters. If the code is stored in a string named <code>ROUTINE$</code> it can be called with two parameters as {{code|2=cbmbas|1=ANSWER=USR(ADR(ROUTINE$),VAR1,VAR2)}}.

Parameters are pushed onto the hardware stack as 16-bit integers in the order specified in the <code>USR</code> call in low byte, high byte order. A final byte is pushed indicating the number of arguments. The machine language code must remove these values before returning via the <code>RTS</code> instruction. A 16-bit value can be returned to BASIC by placing it in addresses 212 and 213 ($D4 and $D5).