Definition
Halt and catch fire is a computer machine code instruction that, when executed, can cause the computer’s central processing unit (CPU) to stop working or behave erratically.
It was adopted as a debugging tool for CPU engineers and designers to test and ascertain the CPU’s operation. A CPU receiving an HCF command should end all processes and enter an infinite loop.
However, because of the way instructions were implemented, it could potentially destroy the hardware in some CPUs.
Examples of the Halt and Catch Fire Instruction
Initially, HCF’s risks were misunderstood, and its dangers only became evident after extensive use and testing over time. So, the HCF instruction was replaced with safer debugging tools:
- In 1997, the CIH or Chernobyl virus utilized the HCF instruction as its payload, causing the CPU to overheat and stop working. The virus caused significant damage, affecting a substantial number of computers.
- In 1985, Mark Probst was disqualified from the University of Illinois for designing software that employed this virus to make a particular IBM PC model overheat and shut down. At that time, the HCF instruction was still evident in some CPUs and was a potentially risky debugging tool. Probst’s program, designed as a prank, exploited this feature and caused many computers to fail. The university perceived Probst’s actions as a significant violation of computer usage policies and ethical standards, leading to his expulsion.