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====== Embarrassing lDebug bug in ensuredebuggeeloaded ======

While modifying lDebug's ''ensuredebuggeeloaded'',
which is responsible for creating an empty process as a debuggee,
I erred severely [[https://hg.pushbx.org/ecm/ldebug/rev/354311dec5b0|early in the morning of last Wednesday]].

===== =====

The bug was that I moved around a call to ''.zeroregs''
(later [[https://hg.pushbx.org/ecm/ldebug/rev/aa39f4276687|made into a non-local function]])
to after the attempt to allocate memory for the new process-to-be.
However, ''.zeroregs'' uses the ''ax'' register,
resetting it to zero to be precise.
But the allocation, if successful, left **the segment of the allocated memory block**
in ''ax'', to be used to continue the initialisation and eventually call
[[https://fd.lod.bz/rbil/interrup/dos_kernel/2155.html|interrupt 21h function 55h]] to create a child PSP.
Passing a zero to the subsequent code made this function call
destroy the (linear) first 256 bytes of memory, or the first 64 entries
of the Interrupt Vector Table that's located in the first 1024 bytes of memory in 86 Mode.
Obviously, this caused a crash.

The insidious thing is that this is a severe, unmistakeable error,
that occurs often and reliably,
but did not happen in the particular test cases I ran.
It crashed when:

  * Loading the debugger command line prompt without a debuggee file to load
  * Loading the debugger with a binary file to load (filename extension neither ''.COM'' nor ''.EXE'')
  * The debuggee process quitting by itself and returning control to the debugger
  * The debuggee process being terminated by the QA

The only case in which it did not crash,
basically,
was to load up an executable to run as a debuggee.
And even then, only until trying to terminate the debuggee.
(However, not when terminating the debuggee by entering a Q command to the debugger.)
Unfortunately, this slipped past my notice at first.
Next, I suspected it might be
[[:blog:pushbx:2022:0424_ldebugx_compatibility_tweaks_dosemu2_native_dpmi_bugfix|a new dosemu bug]]
because I first noticed the crash upon exiting the debugger
in my recent compilation of the current dosemu2
(located on our server in my home directory's ''/local.new/bin/'' subdirectory).
It didn't help that at first I was running lDebugX and lDDebugX
with [[https://hg.pushbx.org/ecm/dpmitest/file/fedbb3529c73/dpmimini.asm|the dpmimini example]],
so I guessed it might be DPMI-related.

Turns out it wasn't that!
It wasn't [[https://hg.pushbx.org/ecm/ldebug/rev/eb5aaca4a673|until the noon of Thursday]]
that I diagnosed and fixed this error.

{{tag>ldebug bugfix ensure}}


~~DISCUSSION~~