ataplus
A disk-based extension to the original Atari BASIC, made by Shepardson Microsystems and distributed by Optimized Systems Software (OSS) in 1981. It modified some commands and made them faster, while bringing in 43 new instructions for graphics, games development and debugging.
Commands were implemented for accessing the Atari's "Player-Missile Graphics" (PMG) which had been ignored by the original Atari BASIC.
The OS/A+ disk should be placed in drive 1 then the machine should be powered on. At the "D1:" prompt, typing BASIC would load the extension. From DOS, the LOAD BINARY FILE menu option could be used for loading A+ from its disk.