Fun_People Archive
6 Oct
The Story of Mel - A classic

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From: Peter Langston <psl>
Date: Fri,  6 Oct 100 16:15:56 -0700
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Subject: The Story of Mel - A classic

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From: Nev Dull <nev@sleepycat.com>
Source: usenet: utastro!nather, May 21, 1983.

A recent article devoted to the *macho* side of programming made the bald
and unvarnished statement:

                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 company,
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 its 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 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 surprised.

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."

© 2000 Peter Langston