6 Jun
2005
6 Jun
'05
2:39 p.m.
Randy McLaughlin declared on Sunday 05 June 2005 06:12 pm:
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.
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. 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