MacsBug

MacsBug is a low-level (assembly language/machine-level) debugger for pre-Mac OS X Macintosh operating systems. MacsBug is an acronym for Motorola Advanced Computer Systems Debugger, as opposed to Macintosh debugger (The Motorola 68000 Microprocessor is imprinted with the MACSS acronym). The original version was developed by Motorola as a general debugger for its 68000 systems — it was ported to the Mac as a programmer's tool early in the project's development.

MacsBug is invoked by hitting the Macintosh's "Programmer's Key" or, as it became later known, the "Interrupt Key" or by pressing "Command-Power". MacsBug offers many commands for disassembling, searching, and viewing data as well as control over processor registers. MacsBug is not installed by default with Mac OS, although every Macintosh since the Macintosh Plus includes a debugger in ROM known as MicroBug.

Users that stumble into MacsBug by accident need only to enter G and press return to escape from MacsBug; however, MacsBug is not installed by default, requiring a system extension, so a typical user environment does not include it. However, it was occasionally installed by end users to provide very basic error recovery. As the classic Mac OS lacked memory protection "hard crashes" where an application crash simply froze the entire system weren't uncommon. With MacsBug installed, instead of an unresponsive system, the user would be dumped into MacsBug, where they could type ES to Exit to Shell (force quit the crashed application and return to the Finder) or RB for ReBoot, which restarted the system. Such recovery efforts were often not successful, with the only alternative a hard reset.