ftp.barnyard.co.uk/2015.02.ftp.barnyard.co.uk.tar/ftp.barnyard.co.uk/cpm/walnut-creek-CDROM/ZSYS/ZNODE-12/I/REALPROG.TXT

Text File | 2000-06-30 | 9KB | 172 lines

                Real Programmers Write in Fortran


Maybe they do now, in this decadent era of Lite beer, hand
calculators and "user-friendly" software but back in the Good
Old Days, when the term "software" sounded funny and Real
Computers were made out of drums and vacuum tubes, Real
Programmers wrote in machine code.  Not Fortran, not RATFOR.
Not, even, assembly language.  Machine Code.  Raw, unadorned,
inscrutable hexadecimal numbers.  Directly.

Lest a whole new generation of programmers grow up in ignorance
of this glorious past, I feel duty-bound to describe, as best I
can through the generation gap, how a Real Programmer wrote
code.  I'll call him Mel, because that was his name.

I first met Mel when I went to work for Royal McBee Computer
Corp., a now-defunct subsidiary of the typewriter company.  The
firm manufactured the LGP-30, a small, cheap (by the standards
of the day) drum-memory computer, and had just started to
manufacture the RPC-4000, a much-improved, bigger, better,
faster -- drum-memory computer.  Cores cost too much, and
weren't here to stay, anyway.  (That's why you haven't heard of
the computer, or the computer.)

I had been hired to write a Fortran compiler for this new marvel
and Mel was my guide to its wonders.  Mel didn't approve of
compilers.

"If a program can't rewrite its own code," he asked, "what good
is it?"

Mel had written, in hexadecimal, the most popular computer
program the company owned.  It ran on the LGP-30 and played
blackjack with potential customers at computer shows.  Its
effect was always dramatic.  The LGP-30 booth was packed at
every show, and the IBM salesmen stood around talking to each
other.  Whether or not this actually sold computers was a
question we never discussed.

Mel's job was to re-write the blackjack program for the
RPC-4000.  (Port?  What does that mean?)  The new computer had a
one-plus-one addressing scheme, in which each machine
instruction, in addition to the operation code and the address
of the needed operand, had a second address that indicated
where, on the revolving drum, the next instruction was located.
In modern parlance, every single instruction was followed by a
GO TO!  Put *that* in Pascal's pipe and smoke it.

Mel loved the RPC-4000 because he could optimize his code:  that
is, locate instructions on the drum so that just as one finished
its job, the next would be just arriving at the "read head" and
available for immediate execution.  There was a program to do
that job, an "optimizing assembler," but Mel refused to use it.

"You never know where it's going to put things," he explained,
"so you'd have to use separate constants".

It was a long time before I understood that remark.  Since Mel
knew the numerical value of every operation code, and assigned
his own drum addresses, every instruction he wrote could also be
considered a numerical constant.  He could pick up an earlier
"add" instruction, say, and multiply by it, if it had the right
numeric value.  His code was not easy for someone else to modify.

I compared Mel's hand-optimized programs with the same code
massaged by the optimizing assembler program, and Mel's always
ran faster.  That was because the "top-down" method of program
design hadn't been invented yet, and Mel wouldn't have used it
anyway.  He wrote the innermost parts of his program loops
first, so that they would get first choice of the optimum
address locations on the drum.  The optimizing assembler wasn't
smart enough to do it that way.

Mel never wrote time-delay loops, either, even when the balky
Flexowriter required a delay between output characters to work
right.  He just located instructions on the drum so each
successive one was just *past* the read head when it was needed;
the drum had to execute another complete revolution to find the
next instruction.  He coined an unforgettable term for this
procedure.  Although "optimum" is an absolute term, like
"unique," it became common verbal practice to make it relative:
"not quite optimum" or "less optimum" or "not very optimum".
Mel called the maximum time-delay locations the "most pessimum".

After he finished the blackjack program and got it to run,
("Even the initializer is optimized," he said proudly) he got a
Change Request from the sales department.  The program used an
elegant (optimized) random number generator to shuffle the
"cards" and deal from the "deck," and some of the salesmen felt
it was too fair, since sometimes the customers lost.  They
wanted Mel to modify the program so, at the setting of a sense
switch on the console, they could change the odds and let the
customer win.

Mel balked.  He felt this was patently dishonest, which it was,
and that it impinged on his personal integrity as a programmer,
which it did, so he refused to do it.  The Head Salesman talked
to Mel, as did the Big Boss and, at the boss's urging, a few
Fellow Programmers.  Mel finally gave in and wrote the code, but
he got the test backwards, and, when the sense switch was turned
on, the program would cheat, winning every time.  Mel was
delighted with this, claiming his subconscious was
uncontrollably ethical, and adamantly refused to fix it.

After Mel had left the company for greener pa$ture$, the Big
Boss asked me to look at the code and see if I could find the
test and reverse it.  Somewhat reluctantly, I agreed to look.
Tracking Mel's code was a real adventure.

I have often felt that programming is an art form, whose real
value can only be appreciated by another versed in the same
arcane art; there are lovely gems and brilliant coups hidden
from human view and admiration, sometimes forever, by the very
nature of the process.  You can learn a lot about an individual
just by reading through his code, even in hexadecimal.  Mel was,
I think, an unsung genius.

Perhaps my greatest shock came when I found an innocent loop
that had no test in it.  No test.  *None*.  Common sense said it
had to be a closed loop, where the program would circle,
forever, endlessly.  Program control passed right through it,
however, and safely out the other side.  It took me two weeks to
figure it out.

The RPC-4000 computer had a really modern facility called an
index register.  It allowed the programmer to write a program
loop that used an indexed instruction inside; each time through,
the number in the index register was added to the address of
that instruction, so it would refer to the next datum in a
series.  He had only to increment the index register each time
through.  Mel never used it.

Instead, he would pull the instruction into a machine register,
add one to its address, and store it back.  He would then
execute the modified instruction right from the register.  The
loop was written so this additional execution time was taken
into account -- just as this instruction finished, the next one
was right under the drum's read head, ready to go.  But the loop
had no test in it.

The vital clue came when I noticed the index register bit, the
bit that lay between the address and the operation code in the
instruction word, was turned on -- yet Mel never used the index
register, leaving it zero all the time.  When the light went on
it nearly blinded me.

He had located the data he was working on near the top of memory
-- the largest locations the instructions could address -- so,
after the last datum was handled, incrementing the instruction
address would make it overflow.  The carry would add one to the
operation code changing it to the next one in the instruction
set: a jump instruction.  Sure enough the next program
instruction was in address location zero, and the program went
happily on its way.

I haven't kept in touch with Mel, so I don't know if he ever
gave in to the flood of change that has washed over programming
techniques since those long-gone days.  I like to think he
didn't.  In any event, I was impressed enough that I quit
looking for the offending test, telling the Big Boss I couldn't
find it.  He didn't seem suprised.

When I left the company, the blackjack program would still cheat
if you turned on the right sense switch, and I think that's how
it should be.  I didn't feel comfortable hacking up the code of a
Real Programmer.


                     --Anonymous [seen on USENET; author unknown]