On personal computers, the turbo button is a button which provides two run states for the computer, normal (full) speed, or a reduced speed.
The name is inspired by turbocharger, a turbine-driven forced induction device that increases an engine's power and efficiency.
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Effect
The turbo button was added to many computers using CPUs faster than the original 4.77 MHz Intel 8088 used in the IBM Personal Computer. Some software titles (games in particular) used the CPU's frequency for timing, so as faster chips came out, some of these games were unplayable. To provide a layer of compatibility for these titles, the "turbo" button was added.
Disengaging turbo mode slows the system down to a state compatible with original 8086/8088 chips. On most systems, turbo mode was with the button pushed in, but since the button could often be wired either way, on some systems it was the opposite. The turbo button was linked to a turbo LED or MHz display on the system case, although the indicated MHz was not always accurate.
The feature was relatively common on systems running 286 to 486 CPUs, and rarely on first generation Pentium CPU equipped computers. The MHz displays largely disappeared or were reprogrammed to display "HI"/"LO" when CPU speeds reached 100 MHz, since most systems only had a two-digit display. Soon afterwards, software creators used different methods for keeping time within games, making this feature obsolete.
Some systems also supported keyboard combinations Ctrl-Alt-+ and Ctrl-Alt-- for switching turbo mode on and off; ITT Xtra used Ctrl-Alt-\ to toggle.
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Software implementations
By 1988 software companies designed games that automatically compensated for varying speeds. While the implementation of the turbo button by manufacturers has disappeared, software developers have compensated with software replacements. One example is DOSBox, which offers full turbo button functionality with adjustable clock speed. Modern PCs that support ACPI power management may provide software controls to switch ACPI performance states or other CPU throttling modes.
Some keyboards had a turbo button too, located near right Shift. Unlike the turbo button that was common on computer cases, the turbo button on the keyboards didn't control the clock rate of the CPU; rather, it controlled the keyboard repeat rate.
Source of the article : Wikipedia
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