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Anyone remember Periscope? It was a DOS debugger with the hardware button attached to the NMI pin on the chip to break in even if interrupts are disabled! It was awesome.

Still unequalled if you ask me, although gdb-dashboard comes pretty close.

EDIT: A quick bit of searching yielded a scanned copy of the manual courtesy of the os2museum.com:

https://www.os2museum.com/files/docs/periscope/periscope-man...

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Being able to enter a debugger on NMI is great because now you can trigger the NMI from any set of hardware conditions too - you can connect logic gates to the address bus to decode a range of addresses, "and" it with the memory write assert pin or IORQ pin and now you have hardware breakpoints for a machine that didn't originally support it.

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