The modular debugger (mdb) is an extensible, low-level debugger developed by Sun Microsystems for the Solaris 7 operating system.
It is now open sourced, under the Common Development and Distribution License (CDDL).
[1] The mdb project was started in 1997 by Mike Shapiro and others when the Solaris operating system was adding support for 64-bit architectures.
Up until that point, Solaris was using the aging adb debugger developed by Steve Bourne (initially for Seventh Edition UNIX[2]).
It was very difficult to simply port adb from a 32-bit architecture to a 64-bit architecture, so Sun engineers decided to make a new debugger that would feature enhanced debugging capabilities, while being backward compatible with adb.