What is a "boot", anyway?

A boot is a bootstrap.  When you turn on a computer, it knows nothing.  The process by which it starts has been likened to "pulling oneself up by one's own bootstraps".

Computers in ages gone by were far less fancy than they are today. Data was stored in tiny magnetic cores, speeds of 1 Megahertz were attainable only through state-of-the-art engineering, and programs were kept on punch cards, paper tape, or if you had the big bucks, magnetic tape.

Say you wanted to load and run an analysis program that you had on a "mag tape". The computer knew nothing of how to operate a tape drive. To do so, you would have to load a program to operate the tape drive. That program might be on a paper tape.  But how do you load the paper tape?

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The computer would have a paper tape reader, but it would need a program to instruct it how to read the paper tape.  See where this is going?  It all has to start somewhere.

Generally, a device like a paper tape reader was simple enough to operate that it could be programmed with just a few instructions. Now the ideal tape reading program would be sophisticated enough to understand where to load the program, maybe load the data in another part of memory, check the tapes for read errors (checksum), and even stop at the end. So you create that program, put it on a paper tape, and then you write a really small, really simple program to load that program, and this one you have to put into the computer manually.

Manual program entry was done using a row of switches and indicator lights on the "front panel" of the machine. Each machine instruction was entered by setting the switches according to the binary code and then pressing a button to store it. Naturally the goal was to create as small a program as possible that would still do the job. Fortunately, the core memory in use at the time was non-volatile, meaning the program would stay in there even after the power was turned off.  As an added protection, a write-protect switch kept the program safe from being overwritten when it was not in use. Even here, there were two steps, because the actual program that "lived" in this protected space was fairly fancy, so you had a dirt simple one that you ran to load the fancy one.

So, to start the machine from scratch, you did the following:

• Enabled the protected area of memory and "toggled in" the instructions for a basic binary loader (about 12 instructions or so). Under normal circumstances, this program would already be there, but if it didn't work you would at least have to step through the program and verify that each instruction was still intact. The mark of a true hacker (original meaning) was someone who knew that program by heart.
• Placed the first tape in the reader and started the program. The 12-instruction program was so simple it did not even know when to stop.  The tape would just fly out of the reader and land on the floor. You pushed the halt switch to stop the program.
• Placed the second tape in the reader and started the program you just loaded off the first tape. If this tape read to the end and stopped, congratulations, you had a good loader now.  
• The next tape loaded would either be an application program, or some kind of utility to enable you to use mag tape or even some kind of disc to read programs into memory.  The sophisticated versions of these were called operating systems as they would make the system console come alive and you could actually type requests right into the computer and have it load programs from tape or disk.

So, in this way, you would load smarter and more capable programs until you had the computer running the desired task(s). From nothing, to operating software, each step in turn, "pulling itself up by its own bootstraps".

Advancing technology eventually created programs that were permanent. If your research budget allowed, you could purchase a card covered with several hundred diodes, which when read by the computer looked like a loader program. Then PROM's and EPROM's evenually got large enough to hold a small program. The days of the front panel were numbered. Before they disappeared entirely, they were recreated using microprocessors so that a keypad and numeric displays could replace the "bits" represented by switches and lights. The final step was when the programs got smart enough to use the computer's standard peripherals (console TTY or CRT) so that you were "talking" to the computer right from the start.

The peripherals were getting smarter, too.  The disk drives now could be commanded to perform an initial program load (IPL). The computer simply said "let me have it" and the disk could go to track zero/sector zero and serve up bytes one after another which the computer would store into memory.  This was enough to get started.

So, you booted from a paper tape; you booted from a mag tape, and if your budget committee was generous enough, you could boot you operating system right from a disk drive. This concept of booting from a device was plainly obvious to the people who were operating these machines.

Nowadays, things are so automatic, that you just press the reset switch--no, just turn on the power; the reset is done automatically, too--the computer starts, first with the "BIOS ROM" bootstrap, then the master boot record, then the partition boot sector, then the operating system kernel...and when the smoke clears, you are looking at your shell prompt ('#' or 'C:>' ), or your full color GUI screen. No wonder you hear people say their computer boots to C:, or boots in Windows. They have no idea what just happened. This is why the boot is so misunderstood.

This booting process has another feature: The entire software load can be altered or changed without making any modifications to the hardware. Most of us are aware of boot managers or "multi-boot" programs that let you choose a different operating system to run.  But at a lower level, you can choose the boot loader itself. DOS has a loader that boots DOS. OS/2 and NT come with a loader that lets you select one of your disk partitions to load from. And LILO, our favorite bootloader, lets you choose a boot device, a disk partition, and even an operating system image for maximum flexibility.