[POST cards] . . . I don't find it that useful, actually. I have used it on this PC when I had a little processor problem, but for most faults a logic probe is a lot more use.

The PERQ (how do I always manage to get that machine mentioned) had a thing called the DDS - Digital Diagnostic System. It was a 3 digit decimal counter that was cleared by the system reset button, and could be incremented by a CPU instruction. The official use for it was that it would be incremented as the diagnostics tested out bits of the machine during booting. If the DDS stopped somewhere, you looked up the code in the manual and it (hopefully) told you what was wrong. In fact, the manual wasn't that useful if you were really trying to fix the machine. Some of the error codes were explained very badly. Don't ask how I found that out.

However, it could also be used as you described. There was a system routine to set the DDS to a given value (bascially, the OS kept the current count and sent the appropriate number of pulses to the DDS to set it to the value you wanted). It was useful for indicating where a program had got to