Path: utzoo!utgpu!attcan!uunet!littlei!intelisc!omepd!jhunt
From: jhunt@omews3.intel.com (Jim Hunt)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Increasing clock speed of IBM PC/AT
Keywords: 6mhz 8mhz
Message-ID: <3723@omepd>
Date: 4 Aug 88 18:45:42 GMT
References: <24213@teknowledge-vaxc.ARPA>
Sender: news@omepd
Reply-To: jhunt@omews3.UUCP (Jim Hunt)
Organization: Intel Corp., Hillsboro
Lines: 28

In article <24213@teknowledge-vaxc.ARPA> 1dharvey@teknowledge-vaxc.ARPA (Doug Harvey) writes:
>I have read that the older IBM PC/AT's can be altered to run
>at higher than 6mhz.  
>
>Can anyone provide me with *specifics*, or pointers to such
>information on whether this is possible and the approximate 
>cost.  
>
>Reply via e-mail or this newsgroup.  I will summarize any re-
>sponses I get.
>
>Doug Harvey
>email: 1dharvey@teknowledge-vaxc.arpa


It depends on the revision of the BIOS ROM in the PC/AT.  Early versions
would not check the clock frequency.  In those machines, you could replace
the 12 MHz crystal with a 16 MHz crystal and your machine would (probably)
run at 8 MHz.  I say "probably", because when you increase the clock speed
beyond IBM's basic spec, you increase the odds of the machine not working
reliably.  Most machines would, in fact, work OK at 8 MHz.  I believe BYTE
magazine at one time published a chart showing clock frequency versus
the probability that a random AT would run at that frequency.

After IBM figured out that people were running their 6 MHz machines at 
8 MHz (remember, IBM was planning on coming out with its own 8 MHz AT
eventually), it put code in the BIOS to check clock speed.  If the clock
was faster than 6 MHz, it shut the machine down.