[Phys-L] hardware, software, and counting

[John Denker <jsd@av8n.com>]

On 09/01/2014 03:15 PM, Bernard Cleyet wrote:

> **More correctly, I suspect the “machine commands” were in the
> "wiring”.

Wiring is a lower level, lower even than machine code.

I've built computers from scratch, doing the wiring by
hand.  I've also hacked the wiring on factory-built
computers ... even fairly recently.

> I’m from an older time.  Machine was the lowest level.
> 
> bc  used machine w/ a PDP-8 **

I've programmed PDP-8s and PDP-10s in machine code.  I 
wrote in assembly language but compiled it by hand and 
toggled the bits into the machine.

I've also disassembled many tens of thousands of lines
of machine code.

I did my share of that.  Got paid for it, too.  You
wanna know what I did with the money?  I hired the
smartest guy in town and paid him to write a high-
level compiler, so I wouldn't have to write assembly 
code any more (except for special occasions).

This supports the point I was making:  I do *not*
recommend students start by building their own 
hardware, or by reading or writing machine code,
or even writing low-level javascript.  Folks should
start by learning the /concepts/ of a high-level 
language.  After they understand the concepts, they 
can (maybe) discipline themselves to implement a 
sound design in terms of lower-level resources.
If they started at the bottom, they would have to
re-invent 50 years of computer engineering, which
is unlikely to be a good use of their time.

Modern machines are designed to be compiler-friendly.
They are not meant to be hand-coded.  The chance that
you can write machine code by hand that works better
than compiler-generated code is reeeally small.
At the very least, this requires extraordinarily 
specialized skills.  It is far removed from physics, 
especially introductory-level physics.

===========================

Separate topic:  Since people seem to be somewhat
interested in the countdown program:  There is actually
one slightly tricky/interesting thing in the otherwise
straightforward code.

Taking the negative of a number does /not/ commute with 
truncating insignificant trailing digits.  This creates
a conceptual problem if you wanted to have a countdown 
timer with fractional-second resolution.  I've never 
seen such a thing, and I reckon people would be very
likely to misinterpret it.

I quote from the comments in the current version of
  https://www.av8n.com/computer/countdown.html

   Post-launch, the display "00:07"
   indicates 7 /or more/ seconds elapsed.  This is the result you
   would naturally get by truncating a more-detailed representation
   such as 7.777.  Pre-launch, the same display ("00:07") indicates 7
   /or less/ seconds remaining.  Alas, this is *not* the result you
   would get by truncating a more-detailed representation such as
   6.543.  The post-launch case is easy, but we need to look closely
   at the pre-launch case.

   Pre-launch, during most of the interval in question, the time is
   "minus 6 point something".  That is, "7" is displayed during the
   interval {-6.5 ± .5}.  That gets you into trouble if you try to
   display any resolution beyond the decimal point, e.g. -6.543 or
   something like that.  You cannot format the time as a decimal
   fraction and then look only at the part in front of the decimal
   point.  Instead you have to start by taking the floor() of the
   number, so that -6.543 becomes -7, and then display that.  Any
   sub-second resolution is lost.

For additional details, read the source.

==========================

Note that I have been using the word "concept" to refer to deep, 
important, useful concepts.  A discussion about the meaning of
numerals and numbers seems kinda deep IMHO.  I mention this 
because it offends me to see that the term "conceptual physics" 
has become a synonym for "bonehead physics" -- more-or-less 
devoid of anything I would recognize as physics concepts.