Killer poke

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In computer jargon, a killer poke is a method of inducing physical hardware damage on a machine and/or its peripherals by the insertion of invalid values, via e.g. BASICs POKE command, into a memory-mapped control register. The term is typically used to describe a family of fairly well-known tricks that can overload the analog electronics in the CRT monitors of computers lacking hardware sanity checking (notable examples being the IBM Portable[1] and Commodore PET; a similar trick is reported having been done to Atari ST displays[citation needed]).

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Specific examples

The Commodore PET

The PET-specific killer poke is connected to the architecture of that machine's video rasterizer circuits. In early model PETs, writing a certain value to the memory address of a certain I/O register made the machine able to display text on the screen much faster. When the PET range was revamped with updated hardware, it was quickly discovered that performing the old trick on the new hardware led to disastrous behavior by the new video chip, causing it to destroy the PET's integrated CRT monitor.[2]

TRS-80 Model III

The TRS-80 Model III had the ability to switch between a 32-character-wide display and a 64-character display. Doing so actuated a relay in the video hardware, and was accomplished by writing to a specific memory-mapped control register. Programs that repeatedly switched between 32 and 64 character modes at high speed (either on purpose or accidentally) could permanently damage the video hardware. While this is not a single "killer poke", it demonstrates a software failure mode that could permanently damage the hardware.[citation needed]

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