MAC/65 was an assembler written by Stephen D. Lawrow and sold by Optimized Systems Software for the Atari 8-bit family of microcomputers. It was a direct descendent of OSS's EASMD. MAC/65 was first released on disk in 1982. A cartridge version came later.
MAC/65 was notable for combining a line editor, assembler, and debugger into a single package, while being much faster than both Atari's similarly structured Assembler Editor cartridge and the standalone Atari Macro Assembler. It was used to write numerous commercial games and applications, and the majority of assembly language listings in ANALOG Computing were written using MAC/65.
Much like Atari BASIC, source code in MAC/65 used line numbers and was tokenized as it was entered. This allowed immediate reporting of syntax errors, shorter assembly times, and smaller data sizes (important when both the source and object code were in memory). Source files could be saved and loaded in either tokenized format or as text files.
Unlike the Atari Assembler Editor, MAC/65 provided true macro processing and conditional assembly.
The cartridge version added 65C02 op-code support as well as a condensed version of Dunion's Debugging Tool (DDT) by Jim Dunion, which was originally sold through the Atari Program Exchange. DDT replaced the BUG/65 debugger which shipped with the disk version of MAC/65.