5 Jun
2005
5 Jun
'05
5:25 p.m.
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.
Also, using an 8-bit bus let them keep the cost lower, as they required
1/2 as many latches for data, and it was easier to get 8-bit interface
parts (like 8255s and other 82xx series chips) than 16-bit parts cheaply
in 1981..
You'll note that when the AT came out, they did have a full 16-bit data
bus (and 24-bit address bus) available on the expansion bus.
<pet-peeve>BTW, m means milli, M means mega, b means bit, and B means
byte.</pet-peeve>
The excuse of running
4.77mhz was for color burst but I never saw a video card that didn't
use its own crystal negating the need for the 4.77mhz. In any case
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.
By the way, the difference in speed is less than 5% between what IBM used
and what the processor was rated at. IMO, 5% is a meaningless
difference.
Pat
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