How Computers BOOT: From Startup to Viruses

Dave explores how a computer starts up, or boots, from scratch. From the initial reset to loading the operating system, Dave shows you all the steps.

[oliverscratch]

Back in the 1980s I ran a chemistry laboratory instrument that was controlled by a PDP-8. To start that up you used the front panel switches to input a program that enabled the paper tape reader on a connected ASR-33 Teletype machine. The rest of the operating system and the instrument software was loaded through multiple paper tapes that had to be read in one-by-one. Our software library was a box full of 35mm film cans each holding a single paper tape. When the instrument was finally decommissioned the PDP-8 went to the university's technology museum. (PS: After reading what I just wrote I realized how very old I am.)

[harrkev]

Hardware engineer here. Usually, reset lines are active low, so to reset a chip, the reset line is pulled to ground. I believe that to be an artifact of when BJT transistors were still used, and NPN transistors were both faster and had a larger current capacity than PNP transistors. So pulling a line low was easier than pulling it high. You can check old 74 series logic data sheets and see that the current sink number was typically a lot larger than current source.

[WhyMustThisBeSoLong]

Just sat down and watched through this with my ~20 year old son who is just starting out in an IT sys-admin career. I think it took us a good hour of pause-restart to get through the whole video, but I think he now has a better understanding of some fundamentals that are not normally taught in modern computer courses.

Thanks Dave!

[glaubhafieber]

As an apprentice, I asked our top software engineer where does the H8 microcontroller know where to get my code from? He called hitachi and ordered a ton of documentation for me. I believe he mentioned that I’m the 16 year old kid in the R&D office. They were kind enough to send me a complete hardware emulator and i let nobody touch it until i graduated. I believe that thing was very expensive but the products that little boy designed are still in operation today. I saw one during a visit to a pharma company and said i wrote the software as a child haha. Luckily i added an easter egg that showed my name on the 7 segment display

[alphanarseven]

That was my first surprise interview question to the candidates while hiring to PSS in the late 90's. People would come prepped with all sort of things about SQL, Exchange etc but when initially confronted with this "basic" challenge, many would just freeze and needed few seconds to boot up and come back to life:) . At least for me it was a great way to understand how much deep the person could go for troubleshooting in the future if needed.

[Super_dada]

first time watching one of your vids, immediately got the feeling that this was a man, probably towards the end of his career, educating the younger generations in a very entertaining and engaging way! I don't even know why I'm commenting but I'm sure I'll like that video.

[muddyexport]

The mention of the "Stone Virus" reminds me of one of the earliest books about computer viruses and AI, "The Adolescence of P-1", by Thomas Ryan, 1977 AKA from way back.

[andrewduncan]

Thanks Dave, very informative. As a Software Engineer, I totally failed an interview question "What are the steps that occur when a computer boots up". Next time I will have a much better story to tell.

[jimnech]

This brings back memories. Back when the first IBM PCs first shipped I wrote a little TSR to make duplicationg diskettes much faster. I had to deal with low level issues like the 64k DMA transfer limit. Mucking around with the hardware at a very low level was a lot of fun!

[davetown]

Having been in IT since the days of the first PCs, I enjoy watching your videos. They're very informative and entertaining. Thanks for the time and effort you put in to bring them to us. Your mention of The Friendly Giant brings back memories of the occasional sick day when I was in my early elementary school years. I remember flipping around the TV dial (yep, I remember dials on TVs), and finding this program on the regional PBS station. Good memories from a bygone era. Also, love, love, LOVE the LEDs you have set up in your gadgets and gizmos. Thanks again!

[christopherg]

One reason BIOS Booting went: BIOS -> Boot Drive -> MBR -> Active Partition -> Partition Bootloader -> OS Bootloader, was simple size constraints.
You needed quite a bit if space to store a bootloader that is capable of properly reading and writing a full filesystem. There was just no way to fit even partial reading before the Partition Bootloader.
UEFI has much more storage space. So now we can write the OS bootloader directly into it. Sidestepping the whole MBR Relay Race.

[mikebarushok]

An additional interesting thing about the original IBM PC was that if no bootable disk was found it would load Cassette Basic from ROM. Thus, early compatibles, not having Cassette Basic could give the error 'Cassette Basic not found ", rather than the later error of "No Bootable Devices Found".

[JunYamog]

Thanks this brings me back memories. I remember during my high school days, the computer lab teacher would remove me from the class as I was one of only kid in school who understood about boot sector viruses. He just excepted me from the classes, as I also gave him grief whenever I ask questions or propose better code. So I sat in a corner and help fix the schools virus issues. I have a known disk that has no viruses and write protected. I then would get most of my classmates disks, go through them and using DEBUG to copy back the boot sector to its original address. I also can easily when there is boot sector virus, when you open the PC case, you can see drive A: head will move differently when an infected disk is booted, you can clearly see that will go the beginning cylinders initially but moves to end to load most of the real boot sector.

[icarvs_vivit]

You make such great, compendious videos that still somehow only take up 15 mins of my time.
I don't know how you do it but I'm glad you do.

[Richardincancale]

In the early days of PDP8s and 11s we would usually key in an actual program - typically 10-15 instructions long - that could read from a card reader or paper tape reader. So we’d load up cards or tape with a bootstrap program that could then read from a hard drive if you had one.

[sleepydragon]

I love the detail you've gone into on how a computer boots up. It was also a blast from the past, I remember getting the "Your PC is now Stoned!" message!

[expert-in-nothing]

Canadian nostalgia to my early 1980's childhood:, both the friendly giant, "Look up, waaay up, " and I have to assume a throwback to Stampede Wrestling with Ed Whalen's famous line. You better "batten the hatches and lock the doors, because it's stampede wrestlin' time once more!"

[bertruttan]

The good ole days. Never looked back. But i do remember fondly how I learned to even get these contraptions to work. The lights still mesmerize me though.

[chuck]

Always like the Friendly Giant outro. Brings back memories. Thanks Dave.

[sanfords]

For years I have thought that this subject would make a great reference book. So much happens at boot time and knowing that can really help you speed things up. thanks for taking this on!