5 Jun
2005
5 Jun
'05
6:01 p.m.
On Sun, 5 Jun 2005 17:45:31 -0700 (PDT)
"Eric Smith" <eric at brouhaha.com> wrote:
Patrick wrote:
On early PC and PC-XT motherboards, there was a trimmer capacitor in the
crystal clock circuit specifically for tweaking the color burst
frequency. It was labeled as such. When the IBM-PC first came out,
it's important to remember how expensive all that stuff still was.
There were two, and only two 'processors' in an original IBM machine.
The CPU on the motherboard, and the 8048 in the keyboard.
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.