I’ve heard from multiple sources that the original goal of the “turbo” button was back-compat for a feature of the original 4.77Mhz PC that depended on clock-ra

I’ve heard from multiple sources that the original goal of the “turbo” button was back-compat for a feature of the original 4.77Mhz PC that depended on clock-rate: using an NTSC TV as a monitor. The original PC could have been 5Mhz, but 4.77 chosen instead so that the CGA graphics adapter could use the bus clock to sync up with the NTSC signal without needing an addition clock (saved a good 25 cents on the hardware that way!). When faster PCs came out, they lost the TV display feature. But, if you really needed it, you could clock down your fancy 6Mhz PC back to 4.77 to regain TV display capability. “TV back compat mode” is long and boring, so the button was named “turbo” in a brilliant marketing move. It became so popular that PCs added the ability to arbitrarily clock down at the touch of a button long after it made sense to do so.