Booting the Computer

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 encountered. This code was able to read in and load the rest of the operating system.

There's an old expression which describes someone, usually a poor immigrant to the U.S., who started with nothing and became a success -- "he pulled himself up by his own bootstraps". The sequence necessary to start a computer had much the same feel of starting with nothing and each step building on the previous step until you had successfully loaded the operating system. So the code in the first blocks read in came to be called the "bootstrap loader", and toggling in the instructions to start the process came to be called "bootstrapping" the computer - or "booting" for short.

The next time someone jokes about booting the computer by giving it a good