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In-Circuit Emulators

July 16, 2020

Does anyone remember in-circuit emulators (ICEs)?

Around 1975 Intel came out with the 8080 microprocessor. This was a big step up from the 8008, for the 8080 had a 64k address space, a reasonable ISA, and an honest stack pointer (the 8008 had a hardware stack a mere 7 levels deep). They soon released the MDS 800, a complete computer based on the 8080, with twin 8" floppy drives. An optional ICE was available; this was, as I recall, a two-board set that was inserted in the MDS. A ribbon cable from those boards went to a small pod that could be plugged into the 8080 CPU socket of a system an engineer was developing.

The idea was that the MDS could act as the device's under test (DUT) CPU. It was rather like today's JTAG debuggers in that one could run code on the DUT, set breakpoints, collect trace data, and generally debug the hardware and software. For there was no JTAG then.

We had been developing microprocessor-based products using the 8008, but quickly transitioned to the 8080 for the increased computational power and address space. I begged my boss for the money for an MDS, which was $20k (about $100k in today's dollars), and to my surprise he let us order one. Despite slow floppies that stored only 80 KB each this tool greatly accelerated our work.

Before long ICEs were the standard platform for embedded work. Remember: this was before PCs so there were no standard desktop computers. The ICE was the computer, the IDE (such as it was) and the debugger.

In the mid-80s I was consulting and designed a, uh, "data gathering" system for our friends in Langley, VA, using multiple NSC-800 CPUs. There were few tools available for this part so I created a custom ICE that let me debug the code. Then a light bulb went on: why not sell the thing? There was practically no market for NSC-800 tools so I came up with versions for the Z80 and 8085 and slapped a $695 label on it. Most ICEs at the time cost many thousands so sales spiked.

Back then we still drew schematics on large D-size (17" x 22") vellum with a pencil. I laid out the PCBs on mylar with black tape for the tracks, as was the norm at the time.

This ICE is perhaps the design I'm most proud of in my career. It was only 17 ICs but was the epitome of an embedded system. Software replaced the usual gobs of hardware. On a breakpoint, for instance, the hardware switched from using the DUT stack to a stack on the emulator, but since the user's stack pointer could point anywhere, and the RAM in the ICE was only a few KB, the hardware masked off the upper address bits and lots of convoluted code reconstructed the user environment.

At the time ICEs advertised their breakpoints; most supported no more than a few as comparators watched the address bus for the breakpoint. My ICE used a 64k by one bit memory that mirrored the user bus. Need a breakpoint at, say, address 0x1234? The emulator set that bit in the memory true. Thus, the thing had 65K breakpoints. One of my dumbest mistakes was to not patent that, as all ICE vendors eventually copied the approach.

The trouble with tools is support. An ICE replaces the DUT CPU, and interfaces with all sorts of unknown target hardware. Though the low clock rates of the Z80 meant we initially had few problems, as we expanded the product line support consumed more and more time. Eventually I learned it was equally easy to sell a six-thousand-dollar product as a six-hundred-dollar version, so those simple first emulators were replaced by much more complex many-hundred chip versions with vast numbers of features.

But the market was changing. By the mid-90s SMT CPUs were common. These were challenging to connect to. Clock rate soared making every connection a Maxwell Law nightmare. I sold the business in 1997 and went on to other endeavors. Eventually the ICE market disappeared.

One regret from all those years is that I didn't save any of the emulator's firmware or schematics. In this business everything is ephemeral. We should make an effort to preserve some of that history.

Feel free to email me with comments.

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