This is cool. In the old days... computers came with these things built into the ROM (BIOS, Firmware, etc.). They were mostly called a 'ROM Monitor' or just the 'Monitor'. I never knew why they were called that, but I would guess that it was probably a term DEC or IBM invented.

On a Sun, NeXT, or Cisco box, you could hit a magic keyboard sequence (or send an RS-232 BREAK over the console) to get to the monitor. It was a very powerful thing to be able to interrupt the running OS, modify it, and then resume it. On the NeXT, you could even work with OS structures or sync the disks. The monitor typically had a disassembler and a simple single step debugger.

Earlier microcomputers also had these, and were helpful, but less powerful. For example, the Apple ][ had a monitor, but later ROMs did not allow you to invoke it with a key sequence.

On the later Suns and Power Macs, the monitor was implemented as a standard called Open Firmware. This was basically a standardized Forth acting as a Monitor. Device drivers could be embedded as a bytecode like sequence of Forth words for ethernet or display cards. That way, you could use newer devices even with old firmware and with any CPU. Very cool.

I wish this functionality would come back.