The +D's DOS was named G+DOS, and was compatible with the DISCiPLE's DOS, GDOS. SAM DOS for MGT's SAM Coupé was backwards-compatible with GDOS and G+DOS. "The Complete +D Disassembly" by Rudy Biesma documents the "G+DOS system 2a" version.Rudy Biesma's homepage An enhanced version called Beta DOS fixed bugs and added features.
