when turbo is turned on or off, and these numbers can be changed with two (or three) banks of jumpers on the back of the display, each bank changing each digit, as their purpose is to connect LED's in the display to (or disconnect from) power. The layout of the banks can vary. Use The feature was relatively common on systems running 286 to 486 CPUs, and less common on Pentium era computers. The frequency displays largely disappeared or were reprogrammed to display "HI"/"LO", "99", or were replaced with a three-digit display 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. 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, which offers an adjustable emulation rate. Modern PCs that support ACPI power management may provide software controls to switch ACPI performance states or other CPU throttling modes. This is used for power saving or to prevent CPU overheating rather than for compatibility, as modern applications use the real-time clock for timing instead of the CPU clock. Turbo = Slow? Some computers have wired the turbo button in a way that if the button is pressed in, the computer is running in the slower speed. While the turbo button can be configured this way, this is not the intended way of using the button, as the computer is intended to run at full speed when the button is pressed in, hence the name turbo. This caused a lot of confusion and misunderstanding about the purpose and naming of the button in retro computer forums and communities. This issue can be solved by reconnecting the wires in the correct way on either the display, or on the motherboard of the computer. If the turbo LED is on, that always means the CPU is running at full speed, regardless of how the switch is wired. References Turbo PC by Complete Business Systems, @1991 - 1994, Ira Kleiner Computing input devices Computer keys