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IPL_UBA already has the subtract built in: So it shouldn't be extracted again. The whole routine looks a little strange, but the way it works is that an interrupt from the UBA itself sets <bit 31> in the returned vector. Because the vector is read by code and not used by hardware, the flag bit is "harmless." UBA interrupts occur only under strange circumstances, like bad map pages and device NXMs. Under the simulator, with a debugged OS, they never happen.