Booting the Computer by Mark B. Rosenthal

Booting the Computer

by Mark B. Rosenthal

Nowadays, when people hear the phrase "boot the computer" many think of kicking it, as you would "kick-start" a motorcycle engine. But that's not what is meant by this use of "boot".

With a modern computer, when you first power it on, it already has some instructions in its memory. These instructions are in a special kind of memory called read-only-memory or ROM. The computer executes these instructions to read additional instructions from its disk, and those instructions then tell it how to load the operating system.

The first computers I ever wrote code for (DEC PDP-8's and PDP-11's) had no ROM. But they did have front panels with toggle switches that allowed you to manually enter values to be stored in memory. To start the computer you had to enter a short program, usually 6 or 7 machine instructions, in binary through the toggle switches. These instructions were a tight loop which was just barely enough to read the first block from some device - typically punched paper tape or DECtape. They contained no error checking. If something failed when reading in the first block, there was nothing to be done. But assuming all went well with the reading of the first block, the instructions would then jump into the code in the block they just read into memory. That code would read in the next block. Memory would now contain code capable of reading other blocks and capable of reporting any errors that might be