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From: pwb@newt.phys.unsw.OZ.AU (Paul W. Brooks)
Newsgroups: alt.msdos.programmer,comp.sys.ibm.pc,aus.computers.ibm-pc
Subject: Re: Setting the Turbo mode by software
Summary: On my PC...
Message-ID: <1375@usage.csd.unsw.oz.au>
Date: 19 Apr 91 04:27:39 GMT
References: <1991Apr10.104737.9962@cc.newcastle.edu.au>
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In article <1991Apr10.104737.9962@cc.newcastle.edu.au>, eepjm@cc.newcastle.edu.au writes:
> 
> Background: on many IBM-AT clones, one has a "turbo" mode or something similar
> to select the processor clock rate.  On some such machines the switching is
> obviously hard-wired since the only way to change the speed is with a
> front-panel pushbutton.  On many others, however, the speed can be changed
> via the keyboard (using Ctrl/Alt/+ or something similar), which implies that
> the speed can be changed by software.
> 
> Question: how does one do this?  Presumably by writing something to a port,
> but which port and what is the code?
> 
> (Yes, I know that the answer might be different for different clones, but
>  if I can find out the answer for at least one clone, that gives me a
>  starting point.  And you never know, a de facto standard might have arisen.)
> 

Well, according to the Hardware Manual for my NEC Powermate IV (a '286
AT clone) the speed selection is done with a CTRL-ALT-'+' combination as
you describe. But internally, it sets a special bit in the Keyboard
controller chip output, and is not accessable to the rest of the system
The chip is an 8742 / 8042. What that does electrically to the
motherboard is anyones guess.
Best of Luck!

through a 'normal' I/O port.
Paul Brooks        |Internet: pwb@newt.phys.unsw.edu.au
Uni. of N.S.W.     |If you have trouble sleeping, try lying on the end of
Kensington NSW 2033|   your bed. With a little luck you'll drop off. 
AUSTRALIA          |                              - Mark Twain.