It has taken me a whole week to catch up with Classiccmp. So this is on oldish
threads, I'm afraid...
Tim Shoppa wrote (starting with a quotation from I don't know whom):
>>C= PET VisiCalc EPROM
>>---------------------
>>The Commodore PET version of the VisiCalc spreadsheet came with a chip that
>>plugged into a socket on the main board. This was probably an EPROM, used for
>>copy-protection. I have an original VisiCalc package, minus this EPROM. Does
>>anyone know where I can download an image of the EPROM from?
>
> I'd be interested to know exactly what this chip did. It was never
> perfectly clear to me that it was used for copy-protection.
I was always under the impression that the ROM that came with Visicalc contained
most of the code. I never used VZCalc on a PET (I had an old ROM PET, not
compatible and no expansion sockets, and in those days no knowledge to change
matters), but could the ROM have contained the code common to all modules?
The argument on microcode and that which it spawned on ABS are a bit old, but
since the subject line says "Several things" I may as well put in my bit.
MICROCODE, Compilers, Assemblers.
=================================
Microcode compilation is a good example of differences between humans and
computers. Humans have (one hopes, if they are working at it at all) a BETTER
UNDERSTANDING of the job (in one sense, computers have no understanding at all).
Computers are FASTER and LESS PRONE TO MISTAKES [1]. In something like
microcode compilation/optimisation, it is often a requirement to get the best
possible result. If your processor knocks one clock cycle off an addition,
because you spotted a shortcut in the microcode that your competitors'
compiler/assembler didn't, you will have a faster processor. Probably
significantly faster.
But you can't claim that humans will always exhibit better PRODUCTIVITY (sorry
William B, otherwise I agree with you) because doing it by hand is usually _far_
slower.
The distinction between a compiler and an assembler is irrelevant here. As
usual, there is a continuous spectrum of techniques and things like Hex
monitors, assemblers, macro assemblers, compilers are names given to parts of
the spectrum. No matter how you define them there will always be examples that
are hard to place in one category or another. I think the point is: the more
the computer does for you, the easier and faster it is to get a good result, but
the more hassle it is to get the best result...
Spc said "There's no such thing as compiled code - everything is interpreted"
Definitely everything is interpreted. But unless the code fed to the
interpreter, be it software, microcode or hardware, is what was written, it's
compiled as well.
Tony said, on processor design, you can either have one flip-flop to each
machine state (like a P850) or microcode. Again there are intermediate points.
I claim you can do quite well by numbering the machine states in a suitably
chosen binary code and having one flip-flop to each bit. Logic for changing
flip-flops is often _easier_ than when you have one flip-flop per state. (I
have done both designs for the same circuit BTW). If you put this logic into a
ROM, this becomes in a sense a microcode ROM, but you can do it combinatorially
as well...
[1] In principle anyway, computers will do what they've been told, rather than
forget things. Microsoft have managed to write a suite of programs that do make
random silly mistakes just like humans, but even Microsoft software is more
predictable in its mistakes than humans are.
ABS.
====
To try and keep this on topic, how did Ferguson do ABS in the early 1970s? I
don't believe they would have used a microprocessor. I'd guess at an analogue
computer, probably not even electronic. Would this be less frightening to Tony?
Theory of ABS.
--------------
The consensus (and I agree) seems to be that a sliding tyre has less good grip
than a rolling one. If when braking, you start to skid, take the foot off and
re-apply. This will shorten your stoping distance. I once saw (via television)
a demo carried out at a skid pan. Car 1 jammed on brakes and skidded a long
way. Car 2, same speed, pumped the brakes quite slowly (less than 1 Hz) and
stopped in 1/2 the distance. ABS, by pumping brakes, will stop you in a shorter
distance than straight skidding.
Now ABS pumps the brakes very fast. A further improvement can be achieved by
slow pumping. Why? The car will bounce up and down like anything. If you pump
at the resonant frequency of the suspension, you can arrange always to brake
when it is down. This gives you extra pressure and extra grip, so you can stop
sooner. This is what I understand by the phrase "cadence braking".
But as several people have pointed out, the BIG advantage of ABS is extra
control.
Practice of ABS.
----------------
I will admit that I used to think as Tony does - ABS is undesirable because it
fails in a nasty way.
However, having driven cars with and without ABS, my opinion has changed.
Please note that this is only an opinion. You are welcome to think differently.
If you have skid training (I don't, but would like to one day), you perhaps
ought to think differently. I don't know.
1. Emergency stop doesn't require pumping as a matter of course. In my 1972
Marcos it is very seldom that I skid and have to reapply brakes.
2. Therefore the technique in most situations is the same whether you have ABS
or not: If you start skidding, pump. But not until.
3. As I said in the argument on earth leakage protection, a safety device is to
get you out of trouble not into it. ABS is no excuse for driving too fast or
too close.
4. There are situations in which ABS if it works can save you when no amount of
braking technique from the driver without it can. Emergency stop on a corner
springs to mind (see below). It is amazing how far a car goes between
applications of the brake pedal. At 60mph, in one second you travel 88 feet.
Pumping brakes quite fast, your skids will be 20 feet long. Pumping at a more
humanly achievable rate, probably nearer 40 feet. This is quite far enough to
leave the road and hit a tree.
(Last month emergency stop on a corner sprang rather forcibly to more than just
my mind. Car: dead. Diver: minor whiplash and bruising to joints of right leg.
Without ABS I am still confident that, had I been able to go in a straight line,
I would have stopped safely. With it, I wouldn't have hit the tree. With
failed ABS I would be no worse off than I am now.)
5. I now believe that on balance, ABS does more good than harm. To me, that
is.
6. But I refuse to have an airbag in my car! I don't like to carry explosives
at the best of times, and an accident is hardly the best of times. (Seriously,
airbags are generally inflated by an explosive that generates a lot of gas.
Sometimes it goes off when you are not crashing. If the airbag has a pinhole
leak, escaping gas can cause serious injuries (fortunately I wear glasses when
driving)). Seat belts and crumple zones are quite sufficient when used
properly.
Well, I've got it off my chest too. But boy, has the traffic on the list been
high this last week!
Philip.
Received on Thu Apr 08 1999 - 12:40:26 BST
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