Advent of Computing - Reading - The Story of Mel

Episode Date: August 26, 2023

This episode is simply a reading of the Story of Mel. I opened last episode with an excerpt, but didn't feel right leaving it at that. So, I present, the Story of Mel as written by Ed Nather and preserved in the Jargon file.

Welcome to a supplemental thing. Don't worry, this isn't my episode for the bi-weekly feed. This is just something to go along with my previous episode on the LGP30. I started that episode with an excerpt of the story of Mel, which was kind of my jumping-off point for discussing the LGP30 and the larger history of weird old drum machines. This is a full reading of the story of Mel, as written by Ed Nather. This specific version comes from my copy of the New Hackers Dictionary, which is a physical version of the jargon file. So, without much more preamble, here's the full reading of the story of Mel. Once this is done, I'll catch you in a few days for my next full-length

episode. Hope you enjoy. Real programmers write in Fortran. Maybe they do now in this decadent era of light 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 RAT4. 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 LGP30, a small, cheap, by the standard of the day, drum memory computer, and it just started to manufacture the RPC4000, a much-improved, bigger, better, faster drum memory computer. 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 LG P30 and played blackjack with potential customers at computer shows. Its effect was always dramatic. The LG P30 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 rewrite the blackjack program for the RPC-4000. Port? What does that mean? The new computer had a 1-plus-1 addressing scheme, in which each machine instruction and the address of the needed operand had a second address that indicated where, on the revolving drum, the next instruction was located.

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 finishes its job, the next would be just arriving at the readhead and available for immediate execution. There was a program to do this 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 took me 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 part 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 bulky 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 readhead 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 initializers 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 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 customer 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 left the company for greener pastures, 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've 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, 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 came 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 the instruction finished, the next one was right under the drum's readhead, 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 location 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 0,

and the program went happily on its way. I hadn't kept in touch with Mel, so I don't know if he ever gave in to the flood of change that was 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.