8" DSDD disk

Tony Duell ard at p850ug1.demon.co.uk
Mon Jun 6 16:39:54 CDT 2005


> 
> Randy McLaughlin declared on Sunday 05 June 2005 06:12 pm:
> > They took a 5mhz brain dead version of a 16 bit chip and ran it on an 8 
> > bit bus @ 4.77mhz and put a RAM limit of 448mb of RAM.
> 
> Err, I think you have the memory limit wrong (I'm assuming you meant 
> 448kB), 640kB (or 64kB on the original motherboard if you don't use a 
> memory expansion card) is more accurate.

I thought there was an issue with the very first version of the PC DIOS 
where it would only use somewhat less than 640K RAM.

> 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.

It most certainly does!. Read the Intel 8284 clock generator data sheet 
(which explains the divide-by-3), look at the PC motherboard and CGA card 
schematics. Everyting is derrived from a 14.3... MHz crystal.

-tony



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