
The blog of the original author of DOS: Tim Paterson - fogus
http://dosmandrivel.blogspot.com/
======
Andys
"The original IBM PC had a clock speed of 4.77 MHz. The 8088 processor inside
was actually specified to run at 5 MHz, so how did IBM end up at 4.77?"

" At some point an IBM hardware design engineer made a leap: The Color
Graphics Adapter would need a 3.579545 MHz signal to create a color
subcarrier; one way to get that signal was to divide 14.31818 MHz by four;
14.31818 MHz is only about 5% less than the maximum speed of the Intel 8284
clock generator (which would divide it by three to get 4.77273 MHz for the
8088 processor). Thus by sacrificing 5% in performance, around $0.50 could be
saved in parts"

Thus setting the tone of the future of the PC.

~~~
to3m
One man's cheap'n'nasty is another man's elegance. And $0.50 was a lot of
money back in those days ;)

Anyway for computers of the era that generated TV output, it seems like it was
common to start with whatever clock speed was necessary for the video output,
and work back from there. Hence the odd CPU speeds of the C64, Amiga, Atari
8-bit, etc.

~~~
__david__
For some real excitement, look at how the Apple 2's graphics worked. If you
ever worked with hi-res mode you know how screwey the colors were. Turns out
they basically serialized the bits right out of ram into a crude NTSC signal
and the bits ended up making different frequencies--hence different colors
(the high bit of each byte specified whether to delay the rest of the 7 bits
by half a clock).

Looking at an Apple 2 schematic is eye-opening. There's hardly anything there!

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michael_dorfman
Wow, that brings back some memories. I haven't thought about CP/M or the S-100
bus in quite some time.

~~~
yuhong
Yea, what is now MS-DOS and associated with IBM PC and compatibles was
actually originally developed before the IBM PC even existed!

------
arocks
> I proposed to start with a “quick and dirty” OS that would eventually be
> thrown away.

DOS is a great example of Worse is Better

------
n3m6
Nearly 25 years later and he still can't get over it.

~~~
clawrencewenham
As his first post reveals, he started writing the blog a week after his libel
suit was dismissed. It's never too late to correct what you believe are
inaccuracies that hurt your reputation.

Everything he's posted since then has been fascinating technolore, worth
reading.

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jaysonelliot
I was always under the impression that DOS (or QDOS, if we want to pick nits)
was essentially a clone of CP/M, even to the point of using CP/M code.

I know the historical record is muddy, and there are competing versions of the
story, but I'm uncomfortable calling Tim Paterson the "original" author, when
CP/M creator Gary Kildall was the one who really did so much from scratch.

EDIT: A very strong piece of evidence is the recent libel suit Paterson
brought and lost against an author who called DOS a copy of CP/M:

"The Judge also agreed that Paterson copied CP/M's API, including the first 36
functions and the parameter passing mechanism, although Paterson renamed
several of these. Kildall's "Read Sequential" function became "Sequential
Read", for example, while "Read Random" became "Random Read"."

[http://www.theregister.co.uk/2007/07/30/msdos_paternity_suit...](http://www.theregister.co.uk/2007/07/30/msdos_paternity_suit_resolved/)

~~~
michael_dorfman
Did you read his blog? Because he addresses this very topic at some length.

Short version: he's never seen the CP/M code, and the similarity between the
products isn't surprising, since they both are implementations of the same
API.

~~~
jaysonelliot
He claims to have never seen the CP/M code, and I certainly don't have any
inside knowledge as to whether that's true or not.

I've read claims that actual lines of CP/M could be found in QDOS. I've also
read that Paterson "bought a CP/M manual and used it as the basis to write his
operating system in six weeks."

Whatever the truth of the matter, it seems clear that QDOS got a real head
start thanks to the existence of CP/M.

I'm not saying Paterson didn't do a lot of hard work, I'm just saying that the
word "original" might be a stretch.

~~~
yid
Then the Linux kernel is totally unoriginal.

~~~
bronson
No, too far. The Linux kernel is MOSTLY unoriginal. I think most people on
LKML would agree with this.

