From: spam@merlyn.demon.co.uk   
      
   JRS:  In article , dated Thu,   
   12 May 2005 03:34:57, seen in news:comp.lang.pascal.borland, Jason   
   Burgon  posted :   
   >"Dr John Stockton"  wrote in message   
   >news:wePQrJAUCngCFwFE@merlyn.demon.co.uk...   
   >> JRS:  In article , dated   
   >> Wed, 11 May 2005 00:18:47, seen in news:comp.lang.pascal.borland,   
   >> Jason Burgon  posted :   
   >   
   >> >That wasn't the reason for the choice of crystal frequency at all. That   
   >> >frequency in fact produces quite poor standard UART bit rates. The   
   >> > reason was to do with the video display and IIRC, was chosen because   
   >> >of some NTSC-specific timing parameter.   
   >>   
   >> Get a copy of The Timing FAQ !   
   >>   
   >>  358631 Feb 1 1996 ftp://garbo.uwasa.fi/pc/programming/pctim003.zip   
   >>  pctim003.zip Timing on the PC under MS-DOS, K.Heidenstrom   
   >>   
   >> Then you won't need any IIRCs.   
   >   
   >I already have it, and from it I extract the following paragraph:   
   >   
   >>>   
   >Historical note:  If you were wondering "Wouldn't 1 MHz have been easier?",   
   >yes it would, but that would have required an extra crystal.  IBM were...   
   >er, 'clever' - they used a master clock of 14.31818 MHz, and used logic   
   >chips to derive the 4.77 MHz CPU clock, the timer clock, and the NTSC colour   
   >subcarrier frequency for the CGA card, so they could save a few dollars.   
   ><<   
      
   However, 10 MHz would have been more sensible than 1 MHz, from what I   
   remember of commercial offerings at around the time.  Less quartz; and   
   division by 10 was cheap even then.  Better performance too, if they   
   were using a 5 MHz CPU (I have a 1979 data sheet set including the 5 MHz   
   8086).   
      
   >So before you get on you high horse and repremand a poster for not checking   
   >some FAQ or other   
      
   That was not a reprimand : it was advice for anybody that needs that   
   sort of information, as you appeared to.  By using PCTIM, one can avoid   
   the need for such an IIRC.   
      
      
   >, I suggest you read (and understand) them yourself.   
   >   
   >All you had to say in your post that my IIRC was indeed correct. Why the   
   >sactimonius repremand?   
      
   Please repair your spelling checker.   
      
      
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    © John Stockton, Surrey, UK.  ?@merlyn.demon.co.uk   Turnpike v4.00   MIME. ©   
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