In-circuit emulator (ICE)
Debug hardware that replaces the processor on the target board with its own transparent stand-in, giving the developer sight of and control over every bus cycle.
You pulled the CPU out of its socket and plugged in a connector cabled to a box, the "pod". Inside sat a special version of the chip — a bond-out part with extra pins exposing internal signals — plus trace memory and breakpoint logic. Intel was selling ICE modules for its development systems in the 1970s, and through the 80s and 90s the tool was standard kit for anyone writing firmware.
The appeal was complete real-time visibility: every instruction traced without the program noticing. The drawbacks were price, often tens of thousands of dollars, and the need for a separate pod for every processor variant. Once debug logic moved onto the die via JTAG and later SWD, a cheap probe was enough and the ICE became a niche.