Heisenbug

The year was 1974. Computers looked like computers, just as you'd expect from the movies. Below is the maintenance panel of a Univac 1110 computer, like the one I worked on at the time. Operators interacted with the computer through a separate console, with a CRT terminal and page printer, but the maintenance panel provided access to the low-level circuitry of the computer, allowing hardware maintenance engineers and systems programmers (like me, who worked on the operating system at a low level) to examine its operation in binary, stepping the multi-million dollar machine instruction-by-instruction through programs late at night while paying customers were asleep.That wouldn't do. I was a systems programmer! This machine was what we now call a symmetric multiprocessor: it had two central processing units (CPUs) which shared access to common memory. In single processor systems, it was usual that when the system was idle it would simply while away the time in an infinite loop until interrupted when work arrived, but this was a poor choice for a multiprocessor: retrieving the infinite loop instruction over and over from memory would impede other processors and input/output devices from accessing it. (Today's computers would keep such an instruction in a CPU cache local to the processor, but this was the 1970s, where such extravagances were like flying cars.)

The indicators at the right showed the most basic registers of the computer such as the program counter and processor state register, while the knobs at the left allowed selecting a variety of internal registers to be displayed or modified, with a legend above the binary display mechanically rotated to label the bits in the selection. This was a 36-bit word machine, so the registers were of that length.

When the computer was running its normal work load, the maintenance console, while in view from the operator's console, displayed what was essentially a random pattern of lights: the registers were changing so rapidly they were averaged out into a blur.