PC speed was Re: 8' DSDD disk

[Randy McLaughlin]

From: "Jules Richardson" <julesrichardsonuk at yahoo.co.uk>
Sent: Monday, June 06, 2005 6:54 AM


> On Sun, 2005-06-05 at 17:45 -0700, Eric Smith wrote:
>> Patrick wrote:
>>> Ok, I don't understand that... Colorburst is (on NTSC), about 3.58 MHz,
>>> which isn't easy to derive from 4.77MHz.  I highly doubt IBM's reason
>>> for using that speed had anything to do with (at least NTSC) video.
>>
>> Yes, that was the reason.  They had a 4x colorburst crystal (14.31818 
>> MHz).
>> They divided by three for the CPU clock, and by four for the colorburst.
>
> Does that mean that IBM machines running PAL display rates had a
> slightly different CPU clock frequency as the main crystal would be
> different?
>
> [btw, where did we suddenly get an 8-foot floppy from!?]
>
> If the 8088 has an 8 bit bus and the 8086 is 16 bits, is there a good
> reason why it didn't get named the 80816? I'm assuming that's what the
> last digit in the number signifies...
>
> cheers
>
> Jules

Part of my original point about using 4.77Mhz is that all of the CGA cards I 
ever saw had their own crystals and ignored the MB clock.

The fact that the PC used 5Mhz parts but only ran at 4.77Mhz was just a loss 
of speed.  One of my real peeves about IBM's design.

As far as the 8' disks I always had to get a couple of buddies to help 
change disks, the real bitch was the lock down bar or was that holding down 
the shift key when typing ;-}


Randy
www.s100-manuals.com