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On IBM PC compatible computers, the turbo button selects one of two run states: the default "turbo" speed or a reduced speed closer to the

Intel 8086 The 8086 (also called iAPX 86) is a 16-bit microprocessor chip designed by Intel between early 1976 and June 8, 1978, when it was released. The Intel 8088, released July 1, 1979, is a slightly modified chip with an external 8-bit data bus (allowi ...

CPU. It was relatively common on computers using the

Intel 80286 The Intel 80286 (also marketed as the iAPX 286 and often called Intel 286) is a 16-bit microprocessor that was introduced on February 1, 1982. It was the first 8086-based CPU with separate, non-multiplexed address and data buses and also the fi ...

Intel 80386 The Intel 386, originally released as 80386 and later renamed i386, is a 32-bit microprocessor introduced in 1985. The first versions had 275,000 transistors80486 processors, from the mid 1980s to mid 1990s. The name is inspired by

turbocharger In an internal combustion engine, a turbocharger (often called a turbo) is a forced induction device that is powered by the flow of exhaust gases. It uses this energy to compress the intake gas, forcing more air into the engine in order to pro ...

, a device which increases an engine's power and efficiency. When pressed, the "turbo" button is intended to let a computer run at the highest speed for which it had been designed.

Purpose

With the introduction of CPUs which ran faster than the original 4.77 MHz

Intel 8088 The Intel 8088 ("''eighty-eighty-eight''", also called iAPX 88) microprocessor is a variant of the Intel 8086. Introduced on June 1, 1979, the 8088 has an eight-bit external data bus instead of the 16-bit bus of the 8086. The 16-bit registers an ...

used in the

IBM Personal Computer The IBM Personal Computer (model 5150, commonly known as the IBM PC) is the first microcomputer released in the IBM PC model line and the basis for the IBM PC compatible de facto standard. Released on August 12, 1981, it was created by a team ...

, programs which relied on the CPU's frequency for timing were executing faster than intended. Games in particular were often rendered unplayable. To provide some compatibility, the "turbo" button was added. Disengaging turbo mode slows the system down to a state compatible with original

8086 The 8086 (also called iAPX 86) is a 16-bit microprocessor chip designed by Intel between early 1976 and June 8, 1978, when it was released. The Intel 8088, released July 1, 1979, is a slightly modified chip with an external 8-bit data bus (allowi ...

8088 The Intel 8088 ("''eighty-eighty-eight''", also called iAPX 88) microprocessor is a variant of the Intel 8086. Introduced on June 1, 1979, the 8088 has an eight-bit external data bus instead of the 16-bit bus of the 8086. The 16-bit registers and ...

chips.

Switching

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 could be linked to a turbo LED or two-digit segmented display on the system case, although in some cases, the indicated frequency (in MHz) was not a measure of the actual processor clocks, but the two "fast" and "slow" display options set by

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on the

motherboard A motherboard (also called mainboard, main circuit board, mb, mboard, backplane board, base board, system board, logic board (only in Apple computers) or mobo) is the main printed circuit board (PCB) in general-purpose computers and other expand ...

. Some systems also supported keyboard combinations -- and -- for switching turbo mode on and off, such as the Packard Bell 486ES 3x3 (the 4x4 and MT models had a dedicated Turbo button). With all the 486ES models—those with and without a turbo button—the power light would show green in normal (“Turbo”) mode, and orange in Slow mode. ITT Xtra used -- to toggle. Some keyboards had a turbo button as well, located near right Shift. Unlike the turbo button that was common on computer cases, the turbo button on the keyboards did not control the clock rate of the CPU; rather, it controlled the keyboard repeat rate.

Use

The feature was relatively common on systems running 286 to 486 CPUs, and rarely on first generation

Pentium Pentium is a brand used for a series of x86 architecture-compatible microprocessors produced by Intel. The original Pentium processor from which the brand took its name was first released on March 22, 1993. After that, the Pentium II and Pe ...

CPU equipped computers. The frequency 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. As new computers continued to get faster and had a wide variety of available speeds, it became impossible for software to rely on specific CPU speed. As software began to rely on other timing methods, the turbo feature became mostly irrelevant to new programs. 1988 game advertising automatic compensation for CPU speed.

Software implementations

While the implementation of an actual hardware turbo button has disappeared on modern machines, software developers have compensated with software replacements. One example is

DOSBox DOSBox is a free and open-source emulator which runs software for MS-DOS compatible disk operating systems—primarily video games. It was first released in 2002, when DOS technology was becoming obsolete. Its adoption for running DOS games i ...

, which offers an adjustable emulation rate. Modern PCs that support

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power management may provide software controls to switch ACPI performance states or other

CPU throttling A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, a ...

modes. This is used for

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or to prevent CPU overheating rather than for compatibility, as modern applications use the real time clock for timing instead of the

CPU clock In computing, the clock rate or clock speed typically refers to the frequency at which the clock generator of a processor can generate pulses, which are used to synchronize the operations of its components, and is used as an indicator of the ...

References

*Turbo PC by Complete Business Systems, @1991 - 1994, Ira Kleiner {{keyboard keys Computing input devices Computer keys