Originally posted by TJSwoboda:
It even had a "turbo" button on it to run it at 50 MhZ; they thought you might want to run it at 25 MhZ most of the time...

  The original IBM PC, IBM PC-XT, and all the earliest clones all had 8088 processors running at 4.77 MHz.  In those days, if you were writing a program that was timing-sensitive, you wrote it to waste as many CPU cycles as needed, by executing spurious instructions, in order to run at the desired speed.  This, of course, depended on being able to make solid assumptions about what processor you were running on, and at what speed.  (I have some memory of using a similar technique when I wrote a program for the Apple ][ to drive a homemade 300 bps modem through the joystick port.  I needed, in a few places, to have loops that took exactly 1/300 of a second to run through one iteration; so I inserted the necessary numbers of NOP instructions and other spurious instructions in order to eat up exactly the right number of CPU clock cycles.)

  When the PC clones started pushing the 8088 to speeds beyond 4.77 MHz, timing-sensitive programs written on the assumption that they would run on a 4.77 MHz 8088, started breaking.  That's where the ??Turbo? switch originated.  Clones started utilizing a switch that would allow you to choose to run either at 4.77 MHz, or at whatever higher speed the machine was capable of.  You'd use the 4.77 MHz speed for running timing-sensitive programs that broke at the higher speed, and the higher ??Turbo? speed for runing everything else.

  Once PC compatibles started moving to other processors beyond the 8088, the whole ??Turbo? concept really became rather meaningless.  You couldn't step an 8086, an 80186, an 80286, an 80386, an 80486, or a Pentium back to any speed that would exactly duplicate the performance of an 8088 at 4.77 Mhz.  By this time, any software that still depended on being able to make that assumption about processor speed was screwed.  But as a matter of convention, the ??Turbo? switch remained a feature of most of these machines at least into the early Pentium models; allowing you to switch the machine into a ??non-Turbo? mode that would, in some way, reduce the machine to somewhat slower performance.  I think this only really ended with the newer ATX-style cases and motherboards.

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