8" DSDD disk

Patrick Finnegan pat at computer-refuge.org
Sun Jun 5 19:25:36 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.

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
-- 
Purdue University ITAP/RCAC       --- http://www.rcac.purdue.edu/
The Computer Refuge               --- http://computer-refuge.org



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