In previous articles in the History of the Computer series, we discussed machine codes, programs, and memories. We also mentioned that many types of memory are volatile, that is, if we cut the power, the memory loses its data. So how did the data on your computer get there at this point? Is this like the Big Bang theory of the universe? Did it just show up? If you’ve ever had to reload an operating system, along with all the other programs you use, you’ll know that putting it on disk is no easy task. After that, you have to save it to memory, every time you ‘boot’!

What do we mean by boot? My old instructor in my first computer engineering course called it a “binary boot loader”. That’s a clue! The term is said to have originated from fictional tales of a German character you may have heard of, Baron von Munchhausen.

His exploits were serialized in comic strips and the like, along with Dick Tracey etc. One famous tale had him riding a cannonball and, deciding it wasn’t a good idea, switch to an enemy cannonball going in the opposite direction, to come back to his side.

The scandalous story that concerns us here refers to the moment when he fell into a swamp, but was able to save himself by pulling his hair. Later versions of this story had him stand up for himself. This expression ‘getting up on your own’ was common at one time. It meant getting out of his own problems.

All of this doesn’t have much to do with computers, but it does describe the way they start up!

Early digital computers were loaded with paper tape or punch cards, then magnetic tape, and then disk. You may know that before Windows there was DOS, which refers to a disk-based operating system.

In a mainframe computer there is an operating system that acts as an interface between the machine and the user. This operating system is known as ‘Exec’ (Executive) or ‘MCP’ (Master Control Program), etc. it depends on the proprietary computer system in use. (New hires were shocked to learn they had to ‘fire the executive every morning’ – high CEO turnover?). This is roughly equivalent to the Windows or Linux operating system on your PC.

The operating system handles all user input, allocates time slots and prioritizes jobs, controls print jobs, allocates mass storage, and so on. When you talk to the computer, it only listens to what the operating system tells it! The operating system can talk to the computer in its own language. Therefore, the operating system must be able to load from disk or tape into memory.

To do this, we have a system whereby a block of data, known as a “boot block”, and located in the first block of a paper tape, magnetic tape, or address 00 on a disk, is read into memory at zero location onwards. This ‘boot command’ is usually a read instruction from address 00 on device 00.

The data, contained in the boot block, loads the buffer areas needed to handle a data I/O transfer into the ‘reserved’ area of ​​memory, etc., ready for communication with the boot device, e.g. example, the disk containing the operating system Having set up all the necessary buffers, the computer reads and executes instructions from 00 onwards. These instructions, freshly loaded from the disk’s boot block, complete the setup process.

The final instruction of the boot block is a read command to disk to load the rest of the data for the operating system. This data overwrites the boot block data in memory with the normal data needed to run the system. Once the data is loaded, the operating system can start operating.

You must initialize all subsystems and check what resources are available. You need to know how much memory there is, what data is available on mass storage, what network resources are available, etc. To do this, it can look at a previous configuration, which it then compares to what it can see now. Alternatively, for a ‘clean’ boot, check what resources you have and work from there.