8' DSDD disk

[Eric Smith]

Patrick wrote:
> 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.

On the original PC, the limit was less than 640K.  It was determined
by the DIP switch settings, and there weren't settings that went as
high as 640K.  To convince the BIOS and DOS to use more memory, you
had to run a program that fiddled with the pointers in low memory.

> 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.

Eric