So I'm working on 86Box's MPU-401 emulation again and I recently fixed it to work like the Windows NT 3.5 driver expects it to. This however, raised a question - what mode does the Sound Blaster 16's MPU-401 start in? Intelligent or UART? Based on the driver's source code in the Windows NT 3.5 DDK, it should be starting in intelligent mode, using the sound card's IRQ, with the MPU-401's IRQ state reflected in bit 2 of the SB16's (CT1745's) IRQ source register. But this contradicts both the DOSBox code which has an option to start in UART mode, and the common claim that the Sound Blaster 16's MPU-401 does not have intelligent mode. So I wonder what the Sound Blaster 16's MPU-401 actually does.