To all:

I read with interest the posts about the MI emulator.  Not that I would ever 
remotely attempt such a thing (nervous laugh) or want to, but because of the 
interesting background information.  I had, for some time, assumed that older 
versions of OS/400 were essentially written in MI, and newer versions in C++.  
I guess that's approximately correct?

Anyway, I am working on an MI source debugger program, just as an interesting 
project.  Not  w r i t t e n  in MI, but for the purposed of  d e b u g g i n g 
 MI.  Sadly, programming is really now more a hobby for me than a vocation, 
since I am going into a different field.  But I digress.

Most of the usual debugger functions ("Find", "Set Breakpoint", etc.) didn't 
present too much of a problem.  But, the "Single Step" exececution function is 
a big hurdle.  It  s h o u l d  of course, cause the program to run exactly one 
MI instruction and then stop at the next instruction.  If a branch instruction 
is encountered, such as "CmpBla(b)  FieldA, FieldB / Eq (Equal-Branch-Point);", 
the program should stop at Equal-Branch-Point if the branch is in fact taken.  
This is what I cannot figure out how to do!

My stab at "emulating" this function was to set a breakpoint at the NEXT 
sequential MI instruction from the instruction at which the program is 
currently stopped, then let the program continue to run (and stop at the next 
instruction).  This of course works fine when the instruction stream is in fact 
executed perfectly in sequence, but of course that is often not the case, 
program branching being the "MI way" to support an IF/ELSE construct.

SO:  I hope I have explained the little problem clearly.  Does anyone know how 
IBM does this, for example in the (OPM) "Start Source Debugger" (STRISDB) 
product.  It obviously is possible.  I thought about setting breakpoints for 
ALL instructions in the program.  That would be overkill worthy of a politician 
(not saying which flavor)!


Rich Hart

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