https://pushbx.org/ecm/web/#projects-ldebug

lDebug is a 86-DOS debugger based on the MS-DOS Debug clone FreeDOS
Debug. It features DPMI client support for 32-bit and 16-bit segments,
a 686-level assembler and disassembler, an expression evaluator, an InDOS
and a bootloaded mode, script file reading, serial port I/O, permanent
breakpoints, conditional tracing, buffered tracing, auto-repetition
of some commands, and a number of extensions. There is also a symbolic
debugging branch being developed.

The manual (text, PDF) is available online. The repo is hosted
at https://hg.pushbx.org/ecm/ldebug/ and proper releases are
available at https://pushbx.org/ecm/download/ldebug/ A current
release is available at https://pushbx.org/ecm/download/ldebug.zip
and a current release of the debuggable debugger is available at
https://pushbx.org/ecm/download/lddebug.zip (You only need this
one if you want to debug the debugger itself.) Further, a current
release of the conditionally debuggable debugger is available at
https://pushbx.org/ecm/download/lcdebug.zip (This one can switch
debuggable mode off and on.)