The Story of Mel
This was posted to Usenet by its author, Ed Nather <nather@astro.as.utexas.edu>, on May 21, 1983:
#A recent article devoted to the macho side of programming# made the bold and unvarnished statement:
write in .
#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 and ,# wrote in .# Not . Not . Not, even, .# .# Raw, unadorned, inscrutable 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 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 ,# a small, cheap (by the standards of the day)# computer,# and had just started to manufacture# the , a much-improved,# bigger, better, faster — computer.# 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 # for this new marvel and Mel was my guide to its wonders.# Mel didn't approve of .
#"If a program can't rewrite its own code",# he asked, "what good is it?"
#Mel had written,# in ,# the most popular computer program the company owned.# It ran on the # and played blackjack with potential customers# at computer shows.# Its effect was always dramatic.# The 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 # (? What does that mean?)# The new computer had a one-plus-one# ,# in which each machine instruction,# in addition to the # and the address of the needed ,# 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 !# Put that in 's pipe and smoke it.
#Mel loved the # 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 "" 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 first,# so they would get first choice# of the address locations on the drum.# The optimizing assembler wasn't smart enough to do it that way.
#Mel never wrote , either,# even when the balky # 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 "" is an absolute term,# like "unique", it became common verbal practice# to make it relative:# "not quite " or "less "# or "not very ".# Mel called the maximum time-delay locations# the "most ".
#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 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 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 .# Mel was, I think, an unsung genius.
#Perhaps my greatest shock came# when I found an innocent that had no in it.# No . None.# Common sense said it had to be a closed ,# 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 computer had a really modern facility# called an index .# It allowed the programmer to write a program # that used an indexed instruction inside;# each time through,# the number in the index # 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 # each time through.# Mel never used it.
#Instead, he would pull the instruction into a machine ,# add one to its address,# and store it back.# He would then execute the modified instruction# right from the .# the 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 had no test in it.
#The vital clue came when I noticed# the index ,# the that lay between the address# and the in the instruction word,# was turned on —# yet Mel never used the index ,# 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 .# 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 .