Archive: Computer Collector Newsletter / Technology Rewind, Jan. 2004 - March 2006

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Bradley's invention of Ctrl-Alt-Del

by Evan Koblentz

It was either Ctrl-Alt-Del, or shorting two contacts with a screwdriver.

David Bradley chose the former method for doing a warm reboot of IBM's original PC, and he did a lot more.

"Early in '81, we were dealing with prototype software, prototype hardware, and as you would attempt to try things out it would hang up. We needed a faster way than turning the power off, waiting a moment, turning it back on. I stuffed a specific value in a location in memory and jumped to the reset vector," he explained, in an interview with Computer Collector today.

But why'd he pick those three keys? "It's not as if somebody said 'we need you to pick out three keys to reboot the machine'," he explained. "Two of them had to be shift keys," since most of the IBM PC's memory was already spoken for. "I'm already tracking whether these four shift keys are up or down. So I picked Ctrl and Alt as the two newest, least used keys. Ctrl-Alt-Delete has a better mnemonic feel than Ctrl-Alt-Plus," he said. "The systems we had in the lab, it was easy to reset them, you just struck a screwdriver across a couple of contacts."

For Bradley, now 55, it was hardly the highlight of his career. He graduated in 1971 from the University of Dayton (Ohio), the same year as Intel released the 4004 chip, and received his Ph.D. from Purdue in 1975, the year of MITS' Altair kit. "The first computer I ever used would have been a [IBM] 360," he said. He joined IBM in June 1975 and worked on the System/23 Datamaster in 1978 - IBM's first computer with a non-IBM processor. For the IBM PC, he wrote the entire BIOS. "I wrote virtually all of the code that's there with the exception of the cassette and the power-on self-test," he said.

Had they known it'd last 20 years, some things would have been done differently. For example, "The interrupts on what's now called the ISA bus are positive-edge triggered and we should have made them negative. The consequence of that decision is it was impossible to share interrupts," he said.

Bradley said he doesn't maintain a personal collection, but has owned various significant machines over the years. "For a while I had one of the original lab-built IBM PCs, but unfortunately in a move, I have no idea what happened to it. I may still have an old Timex Sinclair 1000 sitting around somewhere."