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DOS u/blastprocessing ·

What did the Turbo button on old computer cases actually do?

I recently bought an old 486 PC to play classic DOS games, and I noticed a button on the front panel labeled Turbo next to a digital speed display. When I press it, the speed number changes, but the games actually seem to run slower. Why does a button called Turbo make the computer slower instead of faster?
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3 comments

  • u/grey_cart ·
    The Turbo button was actually designed to slow the computer down, rather than speed it up, to maintain compatibility with older software. Early PC games were coded to rely on the CPU clock speed for timing, meaning they ran at normal speed on a 4.77 MHz IBM PC but became unplayably fast on newer, faster processors like the 286 or 386.
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  • u/mode7mike ·
    By the time the Pentium processor era arrived in the mid-1990s, games had transitioned to using hardware timers instead of CPU clock cycles for game loops. This rendered the manual Turbo button obsolete, and computer manufacturers stopped including it on PC cases.
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  • u/joystickjen ·
    Pressing the Turbo button toggled the CPU between its maximum speed and a reduced clock rate, or disabled the CPU cache, to mimic a slower system. This allowed users to play early DOS games at their intended speed without them running too fast to control. The digital LED display on the front of the case would change to show the relative speed mode.
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