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This sounds a lot like Walmart's codebase for their POS registers. Except, the kicker is that there are zero unit tests, zero test frameworks, etc. You just have to run it through the shitty IBM debugger and hope that you don't step on anyone else's work. Up until 2016, they didn't even have a place to store documentation. Each register has ~1000 flags, some of which can be bit switched further into testing hell.