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Concept
Killer poke
Summary
In computer jargon, a killer poke is a method of inducing physical hardware damage on a machine or its peripherals by the insertion of invalid values, via, for example, BASIC's 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 and Commodore PET.)
The Z1 (1938) and Z3 (1941) computers built by Konrad Zuse contained illegal sequences of instructions which damaged the hardware if executed by accident.
The PET-specific killer poke is connected to the architecture of that machine's video rasterizer circuits. In early PETs, writing a certain value to the memory address of a certain I/O register (POKE 59458,62) made the machine able to display text and graphics on the screen 106% faster. This was accomplished by disabling a “wait to print to screen” safeguard designed to reduce static/noise by preventing the shared VRAM from being read by the display at the same time as it was being written to by the CPU. With this safeguard disabled, graphics could appear on the screen twice as fast, but small bits of static would also appear. Despite the static, some games designed for early PETs included this POKE in their source code in order to benefit from the faster graphics.
When the PET range was revamped with updated hardware, the video rasterizer circuits were redesigned to run at a faster speed and without the need for a “wait to print” safeguard. Thus, the old POKE trick no longer resulted in faster graphics. Instead, performing the old trick on the new hardware led to strange behavior by the new video chip, which could cause signal contention and possibly damage the PET's integrated CRT monitor. This is because the exact pin targeted by the POKE command used to control display timing, but in the upgraded video chip, that pin controlled the vertical sync.
Official source
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