Halt and Catch Fire

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It will be interesting to see how many "old wives tales" they use in this series.

When I worked for MITS we used Motorola 6800 CPUs in some of our products and  never then or at any other time in those days was there an instance of a computer literally catching fire because of  executing code. The HCF was always a used as a joke.

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In early CPUs

The HCF instruction was originally a fictitious instruction, said to be under development at IBM for use in their System/360 computers, along with many other amusing instructions like "Execute Programmer".

One apocryphal story about the HCF instruction goes back to the late 1960s, when computers used magnetic core memory. The story goes that in order to speed up the core memory on their next model the engineers increased the read/write currents in the very fine wires that were threaded through the cores. This worked fine when the computer was executing normal programs, since memory accesses were spread throughout memory. However, the HALT instruction was implemented as a "Jump to self". This meant that the same core memory location was repeatedly accessed, and the very fine wires became so hot that they started to smoke — hence the instruction was labeled "Halt and Catch Fire".[1]

Totally bogus story. The only way to "speed up" the core memory was to increase the master clock cycle time of the CPU.  That was done when practical but there was never a way to heat the write current enough to melt the wires.

When I worked for Ampex Computer Products at the Univ. of Cincinnati computer center, the Ampex engineers in Calif. had found a way to double the access speed of the Ampex core memories, as compared to the IBM memories, which they had sold to the Univ. to replace the IBM 360 memories on the Univ.'s 360 system. However this required going into the backplane of the IBM CPU and rewiring a few wires. Of course the IBM field engineers on site weren't about to let anyone from a rival company to fool with their computer.

So....Ampex 'devised' a way to rewire the backplane without IBM knowing about it. They sent out an eng. who would arrive late at night and the Ampex field service engineer on site (me) would take him into the computer room and then with no IBM or Univ. computer operators on duty he would quickly rewire the backplane, we would leave and no one would be the wiser. The computer would execute jobs a little faster, the Univ. was happy and IBM didn't have a clue.

This went on for awhile at various computer sites where Ampex had replaced IBM memories. Eventually they were caught in the act and then the Ampex CEO had to talk to the IBM execs. How that conversation went I have no idea.....  

Anyway....another tale from the computer trenches.....those were the days......


This series takes place about 12 year after the above.