[corysama]

The original IBM PC could have been 5Mhz. But, it was set to 4.77Mhz instead because that way the NTSC out feature of the video card could piggyback it's signal generator off of the CPU clock rather than paying for an additional clock. That way you didn't necessarily have to buy a computer monitor. You could cheap out and use your existing TV instead.

Later models obviously ran faster, but that broke the TV out feature. It seems bad to sell people a newer, more expensive model that loses a popular feature. So, a TV back-compat button was put in so you could still use your TV in cases where you really need to. Technically, you didn't lose the feature when you upgraded.

Also, some games were written with the assumption of a fixed 4.77Mhz clock and would run unplayably fast on newer machines. The back-compat button would slow the machine down so you could still play last-year's games.

But "NTSC Backwards Compatibility Button" is not a sexy name for a feature. So, some genius labeled it the "Turbo Button" even though it actually slowed down the machine.

Once the early machines had it, all the later machines had to have it too. For a long time it simply cut the clock rate in half. Eventually, it did nothing and was abandoned soon after.

[phicoh]

The 286 could still run at a low enough clock speed that changing the clock frequency made sense.

On the 386 that didn't work anymore, but there was an external cache. So typically, the turbo button would disable external cache.

Then the 486 came with built-in L1 cache. And it was always way too fast. At that point the turbo button didn't make sense any more. I guess there were software solutions, but I don't recall any name.

[mlyle]

I turn off 486 cache (in software -- bits in CR0) to run 386 games. Works rather well to hit mid-386 levels of performance.

[TheCycoONE]

As you brought it up, one of the software solutions I used was Mo'Slo. It appears to still have a web presence. http://www.hpaa.com/moslo/

[ant6n]

My family's 486 had a turbo button that switched between 33 and 66 MHz, with the frequency being shiwn on the case like a digital clock.

[caf]

That screen was just telling you whether the switch was on or off, though - the logic to decide what to display was just a series of jumpers in the case front. That's why some were configured to read

  |_| |
  | | |

and

  |   _
  |_ |_|

instead. It's not like it was actually measuring the bus frequency.

[hakfoo]

There was definitely a "cache enable" pin on the 486 socket.

I had a 5x86/133 that had that pin broken. It worked but unimaginably slowly. Wedging a snapped off pin from another CPU into the socket and trying to keep the whole mess bodged/glued/soldered together made it usable again.

[Beldin]

<ShifL>-<Alt>-<ShiftR>!!

(To switch between 4.77 MHz and 8 MHz)