Why a moth?

by John Feagans
The moth sculpture outside Coover Hall on the Iowa State Ames campus.

Why a moth? It represents the first computer bug found when Admiral Grace Hopper removed a moth stuck between relay contacts. There are pure bugs and then there are design flaws that are also bugs.

In the 8k PET we started noticing a period of time where the PET would seem to hang then wakeup again.  I traced it down to using all of string space and the garbage collect would kick in. When we had prototypes of the dynamic RAM 32k PET in early 1979  this seeming hang became more pronounced.

10 A$=TI$
20 ? A$
30 GOTO 10

Each time a string variable instanced, the previous data became garbage unusable memory. To free and reuse this memory, garbage collection was necessary.  Unfortunately, Microsoft programmed the descriptors and data as a singularly linked list meaning that moving through string space, every location required a linear search of the descriptors to see if anyone was pointing at the data based on the address and length of the string. If a location was found unused, the entire space was moved down and the descriptors for other string variables updated for the new location of their data. The execution time could literally be minutes when garbage collection became necessary and it was a foreground task.

I called Bill Gates.  Microsoft had just moved from Albuquerque to new offices in Bellevue, Washington, a city across Lake Washington from Seattle. Gates agreed to consult for a day on the problem for $500.  I got a purchase order for consulting services signed and flew to Seattle.

As I recall, Microsoft was located on the entire second floor of a bank high rise building. We met and moved to a conference room where Bill and I went through various scenarios for a solution that I was to program. 

Bill took me to lunch at Shakeys Pizza! He drove his used purple Porsche 911 and we talked about things other than work. I had been an undergraduate at the University of Washington. We had both met Gary Kildall, founder of Digital Research, and I was surprised to learn Bill had been frequenting the same machine room where I punched my program cards though we never knew each other.

Presented with several solutions, I chose to implement a doubly linked list so the string space could be traversed without a linear search of the variable descriptors. Back in Palo Alto, I wrote the code and tested on the MDT, then sent a listing to Microsoft to include with a number of other bug fixes for Commodore BASIC 2.0.

Popular posts from this blog

If Looks Could Kill

by Leonard Tramiel At one of the many computer trade shows over the years I had a really funny interaction with Bill Gates. I was doing demos on the PET and answering the questions of those that walked by. I noticed a large group of people approaching, but at first I didn’t recognize anyone. As they got closer I saw that it was Bill Gates leading the others around. There was a translator repeating what Bill said into Japanese. When they reached me Bill walked up to a PET and typed WAIT 6502,10. This caused the machine to hang and he froze. I said, quietly enough in the loud hall that only he could hear, “Bill, there’s nothing at 6502 so that’s just going to freeze”. He turned and gave me a look that was a perfect example of a look that could kill. I knew what he was doing so I turned to the group and said something along the lines of, “Microsoft BASIC is a valuable part of this machine”. Bill relaxed and the group left the suite. The WAIT command in Microsoft BASIC was an unusual addit
Read more

Calculators, codes, and hidden messages

Image
by Leonard Tramiel Looking at a list of the chips used in Commodore calculators we see some letter and number combinations that are quite meaningful, if you know the stories. Some of the most sophisticated processors that Commodore made were based on a custom chip set that used a common processor and different ROMs (or two) that contained the code for that particular model. I wrote the firmware for a few of those. Like this one: Photo credit: Dave  McMurtrie One of my favorite memories is the way those ROMs were programmed, as described here . This processor was a fully custom chip and it required a LOT of engineering work, time, and money. When my father, Jack Tramiel, was persuaded to approve the project he told the person in charge "God help you" if this doesn't work. The chips all have product names the contain GHU for God Help Us. The first set of calculator chips that Commodore engineers developed included as much of the circuitry as thought possible into a single
Read more

Creating PETSCII

Image
by Leonard Tramiel When Chuck Peddle was given the go ahead to bring his ideas of what would become the PET computer into reality many of the ingredients were only loosely constrained. One example is the character set. It would clearly need to have alpha-numeric characters and the basic punctuation. But that was far  too limiting for many recreational uses of the machine. It should be possible to create a set of graphics characters that would allow a wide range of images to be created yet still have the hardware simplicity of a character based system. He knew that one of the likely uses of the computer was going to be card games. The game of Blackjack was a favorite of both Chuck and my dad. That meant the only thing that was well defined about the set was that it needed to have the four suits; hearts, clubs, spades and diamonds. He gave me the task of designing that character set. Each screen location would store an 8 bit number, 256 possibilities. The top bit would swap on and off pi
Read more