Switcher (2004)

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Switcher

Author: Andy Hertzfeld

Date: October 1984

Characters: Andy Hertzfeld, Neil Konzen, Bill Gates, Bud Tribble, Steve Jobs, Steve Capps, Charles Simonyi, Phil Goldman, Tom Petrie, Victor Bull, John Markoff, Burrell Smith, Guy Kawasaki

Topics: Software Design, Technical, Compensation

Summary: The Macintosh's first multi-tasking environment

The Switcher About Box

The first commercial product that I worked on after going on leave of absence from Apple in March 1984 (see Leave Of Absence) was a low cost, high resolution scanner for the Macintosh called Thunderscan, that I created in collaboration with a tiny company named Thunderware (see Thunderscan). I started working on it in June 1984, and by early October, it was almost complete.

Tom Petrie, one of the two principals at Thunderware (the other was Victor Bull, who I worked with on the Silentype thermal printer (see What Hath Woz Wrought), the first project that I did for Apple), arranged a few demos for various computer magazines, to curry favorable reviews to promote the product. On October 11th, 1984, I drove with Tom to an office in Hillsborough, to demonstrate Thunderscan for Byte magazine.

The Byte reviewer was John Markoff, a technology scribe for the San Francisco Chronicle who was one of the best reporters covering the personal computer industry. Tom described Thunderscan while I set up the demo and started scanning. John asked a few questions, taking notes with his IBM PC, which was running a character-based text editor that I viewed with the typical pious disdain of a Macintosh purist. As I was answering one of his questions, the phone rang.

"Excuse me," he told us, as he pressed a key combination on his keyboard, his monitor screen instantly changing to a different program. He talked on the phone for a minute or two, occasionally typing, before he finished the conversation and pressed a key combination to switch back to his Thunderscan notes.

"What did you just do?," I asked John, curious about the software that he was running. "How did you switch to another application so quickly?"

"Oh, I'm running Memory Shift. Haven't you seen it?" John responded. "It's a DOS utility program that keeps multiple applications resident in memory, and allows you to switch between them quickly. I've been using it a lot lately." John typed the switch command a few times in rapid succession, to show me how fast it could do its thing.

"You know, I think I could do that for the Macintosh", I suddenly blurted out, before I even thought about it consciously.

The 512K Macintosh, with four times the memory of the original, had just started shipping a few weeks ago. I had considered trying to run multiple applications simultaneously on the 512K Mac, but I was stymied by low memory conflicts and other potential gotchas. But now, as I observed Memory Shift in action on John's PC, I suddenly saw a simple way to do it, which didn't seem to be too hard to implement.

"Yeah, that would be cool," John agreed. I continued with the Thunderscan demo, but it was hard for me to concentrate on it, because I couldn't stop thinking about application switching. There were a few intricate problems to solve, but it seemed eminently doable, and I thought it would be incredibly useful if I got it to work.

Tom Petrie noticed how excited I was about the new idea and, as he drove me home from the demo, he reminded me of my prior commitments, and made me promise that I would finish the alpha release of Thunderscan as soon as possible, before daring to start something new. We agreed on a list of a dozen or so tasks, which I thought I could accomplish within two weeks. Once the alpha release was completed, I could take a short hiatus, in order to work on application switching, before diving back into Thunderscan to finish the 1.0 release, due by the end of November.

During the next two weeks, I focused on polishing Thunderscan to get it ready for the alpha release, but I also spent idle moments pondering the design of the application switcher. One fundamental decision was whether or not to load all of the applications into a single heap, which would make optimal use of memory by minimizing fragmentation, or to allocate separate "heap zones" for each application. I decided to opt for separate heap zones to better isolate the applications, but I wasn't sure that was right.

There were lots of little problems to solve. The most crucial one was that the system software kept lots of application-specific global variables in low memory (see Mea Culpa), which needed to be swapped during context switching, so each application could maintain its own set of them. The hard part was coming up with the precise list of exactly what needed to be swapped; many of the variables were obvious, but some were quite subtle, and dependent on how applications were using them. I knew that my first cut wouldn't be perfect, but I was confident that I could debug the inevitable problems once I saw how the applications were actually failing.

A few days after starting the push to finish Thunderscan, I received an intriguing phone call from Jeff Harbers, the manager of Microsoft's Macintosh applications team. Jeff told me that Microsoft had a very strategic project that they needed for the Macintosh, and that they thought that I was the ideal person to implement it. He wouldn't tell me anything else over the phone, but he offered to fly me up to Seattle to discuss it in person. Even though I was right in the middle of trying to complete Thunderscan, I was intrigued enough to accept his offer to fly up to Seattle and visit with him the following Tuesday.

Jeff picked me up at the airport, and we drove to Microsoft's main building where we were joined by Neil Konzen, a talented 23 year old who was Microsoft's main systems programmer on the Macintosh. I knew Neil from his days as an early Apple II hobbyist, when we collaborated on adding features to an assembly language development system when he was only 16.

Jeff asked me what I was working on, and I told him about Thunderscan, which he seemed to be interested in. But when I mentioned that I was about to start some experiments with an application switching utility, his jaw dropped, and he looked like he couldn't believe what I said.

"That's just what we wanted to talk with you about!", he exclaimed, "It's great that you're already working on it".

Jeff explained that Microsoft had put a lot of effort into getting their applications to run well in the tiny space available in the 128K Macintosh, which they considered to be a key competitive advantage. But as things stood, the 512K Mac would undermine their efforts, since it allowed applications to be much larger. Plus, Lotus had recently announced an integrated application suite for the 512K Macintosh called Jazz that made it easy to quickly switch between different functional areas. But if the Macintosh could run multiple applications simultaneously, the small memory footprint of the Microsoft apps would continue to be advantageous, since their lower memory requirements meant that more of them could run concurrently, and users could put together customized application suites on their own. The purpose of the visit was to convince me to write an applications switcher under contract to Microsoft.

Neil Konzen had contemplated a potential design, which he conveyed to me in front of a whiteboard. He decided to use the single heap approach that I had rejected, along with a few interesting twists to minimize memory fragmentation. I told him about the alternate approach of using separate heapzones, and how I thought that it was probably worth it to trade some memory fragmentation for greater robustness. I told him that I would give his approach some more thought.

Finally, my afternoon at Microsoft culminated in a p