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<v Speaker 1>Welcome back to the deep dive. You know, I was

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<v Speaker 1>looking at the stack of notes for today we are

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<v Speaker 1>tackling William Stallings's massive text Operating Systems, internals and design principles,

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<v Speaker 1>and it really hit me. We spend all this money

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<v Speaker 1>on faster processors, better ram but the thing that actually

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<v Speaker 1>decides if we have a good experience is this invisible

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<v Speaker 1>layer we almost never talk about.

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<v Speaker 2>It's the ghost in the machine, literally.

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<v Speaker 1>And Stallings makes this comparison right out of the gate

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<v Speaker 1>that to stop me. He doesn't compare the operating system

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<v Speaker 1>to a brain or you know, an engine. He compares

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<v Speaker 1>it to a.

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<v Speaker 2>Government, which, depending on your political views, might make you

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<v Speaker 2>a little nervous about turning on your laptop.

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<v Speaker 1>It definitely gives you pause. But when you dig into

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<v Speaker 1>the book the analogy it fits surprisingly well. The OS

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<v Speaker 1>doesn't actually produce anything, does it.

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<v Speaker 2>No, it doesn't write your email or edit your photo.

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<v Speaker 2>It just it creates the environment where those things can happen.

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<v Speaker 2>That's the core function. It paves the roads, polices the intersectctions,

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<v Speaker 2>and it collects taxes in the form of memory and

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<v Speaker 2>processing power.

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<v Speaker 1>And like a government, it's constantly fighting this sort of

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<v Speaker 1>three way war with itself. Stallings lays these out really clearly. Convenience, efficiency,

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<v Speaker 1>and the ability to evolve, and.

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<v Speaker 2>Those three things do not naturally go together.

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<v Speaker 1>Not at all. In fact, they seem like they would

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<v Speaker 1>actively sabotage each other.

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<v Speaker 2>They absolutely do. I mean, think about convenience. That's you,

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<v Speaker 2>the user. You want a beautiful desktop, you want transparent windows,

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<v Speaker 2>plug and plate devices. You do not want to see

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<v Speaker 2>the wiring.

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<v Speaker 1>I definitely do not want to see the wiring. I

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<v Speaker 1>just want to click the button and have it work.

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<v Speaker 2>But hiding the wiring is expensive. Yeah, that eats up

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<v Speaker 2>system resources, which fights against objective number two efficiency. The

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<v Speaker 2>hardware just wants to run raw code as fast as possible,

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<v Speaker 2>not paint pretty pictures for you. Every cycle spent drawing

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<v Speaker 2>a drop shadow on a window is a cycle not

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<v Speaker 2>spent calculating a spreadsheet.

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<v Speaker 1>So the OS is basically stuck in the middle trying

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<v Speaker 1>to make me happy without making the CPU quit in frustration, while.

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<v Speaker 2>Also handling objective three evolution. It has to be built

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<v Speaker 2>in a way that allows it to change completely next

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<v Speaker 2>year without breaking all the software you bought five years ago.

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<v Speaker 2>It's a juggling act.

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<v Speaker 1>An unbelievable juggling.

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<v Speaker 2>App billions of times a second.

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<v Speaker 1>So let's get into the mechanics of this. Stallings breaks

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<v Speaker 1>it down to the instruction cycle. Fetch, execute, fetch execute.

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<v Speaker 1>It sounds so simple.

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<v Speaker 2>It is. It's the heartbeat of the machine. But there's

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<v Speaker 2>a massive physiological problem with this computer body. Okay, the brain,

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<v Speaker 2>the CPU is moving at the speed of light, but

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<v Speaker 2>the hands and feet, the input output devices like your

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<v Speaker 2>hard drive or your Wi Fi card are moving through molasses.

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<v Speaker 1>Stalling's had a stat on this that just blew my mind.

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<v Speaker 1>The speed difference isn't a little bit, it's orders of magnitude.

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<v Speaker 2>It's astronomical. If the CPU's a Ferrari driving it at

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<v Speaker 2>two hundred miles an hour, the hard drive is a tractor. No,

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<v Speaker 2>that gives a hard drive too much credit.

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<v Speaker 1>It's a snail.

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<v Speaker 2>It's a snail exactly.

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<v Speaker 1>So you didn't have a smart operating system and you

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<v Speaker 1>told the computer to save file that Ferrari effectively has

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<v Speaker 1>to slam on the brakes and just idle for what

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<v Speaker 1>feels like one hundred years while the snail writes the data.

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<v Speaker 2>That's the polling method. CPU just sits there, asking the

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<v Speaker 2>drive Are you done yet? Are you done yet? Are

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<v Speaker 2>you done yet?

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<v Speaker 1>What a waste.

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<v Speaker 2>It's a massive waste, a billion dollar hardware. You have

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<v Speaker 2>the world's smartest mathematician sitting there waiting for a printer

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<v Speaker 2>to warm up.

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<v Speaker 1>So they invented interrupts. And I love this term because

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<v Speaker 1>it sounds rude, but it's actually the most polite thing

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<v Speaker 1>the hardware can do.

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<v Speaker 2>It's save computing with interrupts. The Ferrari sends the save

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<v Speaker 2>command to the snail and then immediately peels out and

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<v Speaker 2>goes to do something else.

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<v Speaker 1>Okay, so it goes off and runs a spreadsheet, plays music, whatever.

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<v Speaker 1>But wait, how does it know when the snail is done?

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<v Speaker 2>The snail has a direct line to the CPU. When

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<v Speaker 2>it's finished, it sends an electrical signal and interrupt. It

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<v Speaker 2>forces the CPU to pause whatever it's doing to acknowledge

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<v Speaker 2>that the task is complete.

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<v Speaker 1>See that's the part that stresses me out. If I'm

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<v Speaker 1>in the middle of a complex math problem and you

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<v Speaker 1>interrupt me, I lose my place. I have to start over.

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<v Speaker 2>And that is the magic trick. Yeah, the OS performs

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<v Speaker 2>a context switch. Okay, imagine you're reading a really dense

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<v Speaker 2>novel and the doorbell rings, You don't just throw the

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<v Speaker 2>book on the floor, right, you put a bookmark in.

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<v Speaker 2>You write down exactly which line you were on, maybe

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<v Speaker 2>even the thought you were having in that split second.

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<v Speaker 1>So that's the state of the process.

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<v Speaker 2>Correct, the program counter, the register values. Everything gets pushed

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<v Speaker 2>onto a control stack. It's a snapshot of reality frozen

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<v Speaker 2>in time. Wow, you go answer the door, handle the

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<v Speaker 2>big job, and when you come back, you pop that

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<v Speaker 2>snapshot back off the stack. The software doesn't even know

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<v Speaker 2>it was passed, so.

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<v Speaker 1>To me, it looks like everything happened at once exactly.

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<v Speaker 2>It completely decouples the user's perception of time from the

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<v Speaker 2>computer's reality.

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<v Speaker 1>But what happens if the doorbell rings while I'm already

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<v Speaker 1>on the phone?

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<v Speaker 2>Ah, the nightmare scenario multiple.

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<v Speaker 1>Interrupts, is the system just crash?

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<v Speaker 2>Depends on how it's designed. You can disable interrupts basically

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<v Speaker 2>take the phone off the hook while you answer the door,

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<v Speaker 2>or you can have nested interrupts.

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<v Speaker 1>Which implies a hierarchy. Some things are more important.

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<v Speaker 2>Precisely, if your network card says incoming data that's urgent.

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<v Speaker 2>If your printer says I'm out of paper, that can

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<v Speaker 2>wait The OS prioritizes the urgent interrupt handles it then

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<v Speaker 2>goes back to the less urgent one, and finally back

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<v Speaker 2>to your program.

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<v Speaker 1>And all of this happens so fast that I just

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<v Speaker 1>see a smooth mouse cursor.

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<v Speaker 2>Well underneath, it's just controlled chaos.

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<v Speaker 1>So we have the CPU maximizing its time with interrupts.

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<v Speaker 1>But there's another bottleneck, right memory. We all want terabytes

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<v Speaker 1>of storage that's instantly accessible. But Stallings points out the

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<v Speaker 1>iron triangle of memory capacity, speed, and cost.

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<v Speaker 2>And you can pick two because you cannot have all three.

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<v Speaker 2>Fast memory like the registers right inside the CPU, is

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<v Speaker 2>incredibly expensive and tiny. Big memory like your hard drive

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<v Speaker 2>is cheap but slow.

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<v Speaker 1>So the OS has to lie to us again. It

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<v Speaker 1>has to create the illusion that we have unlimited, super

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<v Speaker 1>fast memory.

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<v Speaker 2>And it does that through the memory hierarchy. It's a pyramid.

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<v Speaker 2>Data is constantly moving up and down. But the secret sauce,

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<v Speaker 2>the thing that makes the lie convincing is this concept

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<v Speaker 2>of locality of reference.

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<v Speaker 1>This is essentially the OS gambling right. It's betting on

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<v Speaker 1>what I'm going to do next.

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<v Speaker 2>It's a rigorous statistical gamble. They are two types. First,

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<v Speaker 2>temporal locality Okay, if you looked at a photo five

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<v Speaker 2>seconds ago, the odds are really high you'll look at

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<v Speaker 2>it again in the next five seconds. Think about a

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<v Speaker 2>loop and a piece of code.

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<v Speaker 1>It's the same instructions over and over, so you keep

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<v Speaker 1>that data close exactly.

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<v Speaker 2>And then there's spatial locality. If you're reading a document

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<v Speaker 2>at line ten, there is a ninety nine percent chance

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<v Speaker 2>you're going to need line eleven next.

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<v Speaker 1>You almost never jump from line ten to line five thousand.

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<v Speaker 2>Right, So the OSCs you're reading line ten and quietly

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<v Speaker 2>in the background, it runs down to the slow warehouse,

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<v Speaker 2>grabs lines eleven through twenty and puts them on the

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<v Speaker 2>fast on the top, right next to the CPU that's cashing.

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<v Speaker 2>That's cashing, and it works surprisingly well. A well toned

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<v Speaker 2>o S can get a hit on that cash maybe

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<v Speaker 2>ninety percent of the.

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<v Speaker 1>Time, which explains why when I open a huge app

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<v Speaker 1>for the very first time, it sometimes chugs for a second.

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<v Speaker 2>That's a miss, right. The chef reached for the salt,

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<v Speaker 2>but it wasn't on the counter. He had to run

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<v Speaker 2>to the pantry. But once he brings it out, it

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<v Speaker 2>stays on the counter. It stays on the counter. The

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<v Speaker 2>next time you click. It's instant.

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<v Speaker 1>It's fascinating to think about this because it means my

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<v Speaker 1>computer is constantly trying to predict the future based on

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<v Speaker 1>the immediate past.

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<v Speaker 2>And when a guess is right, you feel like you

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<v Speaker 2>have a supercomputer. When a guess is wrong, you get

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<v Speaker 2>the spinning wheel of death.

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<v Speaker 1>It's so easy to take this for granted. Now we

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<v Speaker 1>have casing, we have interrupts. Everything feels instant. But Stallings

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<v Speaker 1>takes us back to the nineteen forties and fifties to

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<v Speaker 1>show us the dark Ages.

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<v Speaker 2>It really was the dark Ages. No operating system, no mouse,

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<v Speaker 2>no screen. It's a clipboard, a literal clipboard outside the

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<v Speaker 2>computer room. You signed up for a thirty minute block.

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<v Speaker 2>You walked in with your deck of punch cards. You

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<v Speaker 2>loaded them into the machine, and you prayed.

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<v Speaker 1>And if you had a typo, a light.

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<v Speaker 2>Flashed, the machine stopped, and your thirty minutes were up.

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<v Speaker 2>You had to go back to your desk and debug

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<v Speaker 2>by just staring at holes in paper.

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<v Speaker 1>The inefficiency is painful to even think about. While you

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<v Speaker 1>were walking around loading cards, checking the lights, this million

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<v Speaker 1>dollar machine was just sitting.

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<v Speaker 2>There burning electricity, doing nothing. That idle time is what

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<v Speaker 2>drove the invention of the OS. It wasn't about user comfort,

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<v Speaker 2>it was about money. They needed batch processing to keep

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<v Speaker 2>the machine.

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<v Speaker 1>Fed, and this is where we see the first monitor programs.

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<v Speaker 2>Yes, you'd hand your cards to an operator. The operator

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<v Speaker 2>fed them to the machine one after another. The monitor

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<v Speaker 2>was a simple program that just said Okay, job one

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<v Speaker 2>is done, load job two.

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<v Speaker 1>But even then, if a job had to wait for

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<v Speaker 1>a tape to rewind, the CPU starped again.

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<v Speaker 2>Which led to the biggest leap inefficiency multi programming, the

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<v Speaker 2>idea that you could hold multiple job and memory at once.

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<v Speaker 1>So if job A waits for a.

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<v Speaker 2>Tape, the OS switches to Joe B.

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<v Speaker 1>Stallings mentions that uniprogramming had something like what three percent

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<v Speaker 1>CPU utilization? Multiprogramming just blew that away.

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<v Speaker 2>It changed the economics of computing, but it was still impersonal.

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<v Speaker 2>You submitted a job and came back hours later for

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<v Speaker 2>the print.

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<v Speaker 1>Out, which brings us to the moment the computer became

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<v Speaker 1>personal time sharing.

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<v Speaker 2>This is the MIT system CTSS. Before this, the goal

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<v Speaker 2>was always maximized the CPU. With time sharing, the goal

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<v Speaker 2>shifted to maximize the user.

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<v Speaker 1>And this is where we get the concept of time

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<v Speaker 1>slicing right.

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<v Speaker 2>Yes. Imagine a chess master playing fifty games at once.

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<v Speaker 2>He walks to board one, makes a move, walks to

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<v Speaker 2>board two, makes a.

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<v Speaker 1>Move, and if he moves fast enough.

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<v Speaker 2>Every player feels like they have his full attention.

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<v Speaker 1>So the OS is the chess master. It runs my

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<v Speaker 1>browser for a fraction of a second, then switches to

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<v Speaker 1>your music player, then the system clock and back to

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<v Speaker 1>my browser.

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<v Speaker 2>Every point two seconds in those early systems. Now it's

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<v Speaker 2>much much faster. It creates the illusion of concurrency.

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<v Speaker 1>You think you're multitasking, but the computer is actually just

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<v Speaker 1>panic switching.

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<v Speaker 2>Panic switching between tasks fast enough to fool your eyes.

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<v Speaker 1>This required a huge leap in software architecture, though Stallings

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<v Speaker 1>really emphasizes the concept of the process.

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<v Speaker 2>It sounds abstract, but it's so vital. A program is

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<v Speaker 2>just a static set of instructions on a disc. It's dead.

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<v Speaker 2>A process is that program in execution. It's alive.

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<v Speaker 1>It has baggage, it has.

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<v Speaker 2>Context, it has ownership of memory, it has security clearance,

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<v Speaker 2>it has a priority level. By treating running programs as processes,

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<v Speaker 2>the OS could finally manage them safely.

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<v Speaker 1>It could isolate them.

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<v Speaker 2>Isolate them, so if one process crashes, it doesn't necessarily

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<v Speaker 2>kill the others.

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<v Speaker 1>And this ties into virtual memory too, because if you

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<v Speaker 1>have fifty processes running, they can't all fit in the

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<v Speaker 1>physical RAM.

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<v Speaker 2>Virtual memory is the ultimate illusion. The OS breaks a

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<v Speaker 2>process into these fixed sized blocks called pages. It can

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<v Speaker 2>scatter them those pages anywhere, some in RAM, some on

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<v Speaker 2>the desk.

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<v Speaker 1>With the program thinks it's all in one continuous piece.

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<v Speaker 2>The program sees a continuous, beautiful block of memory. It

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<v Speaker 2>uses virtual addresses. The hardware, specifically, the memory management unit

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<v Speaker 2>translates those virtual desks into physical addresses.

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<v Speaker 1>On the fly, so a developer can write a program

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<v Speaker 1>that uses four gigabytes of RAM even if the machine

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<v Speaker 1>only has two gigabytes free. It's basically gas lighting the software.

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<v Speaker 1>Oh yeah, you totally have all the space while frantically

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<v Speaker 1>shuffling data in and out of the back door.

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<v Speaker 2>Gas Lighting is a harsh word for it, but technically accurate.

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<v Speaker 2>It's abstraction. You abstract the harsh reality of the hardware

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<v Speaker 2>to create a better reality for the software.

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<v Speaker 1>Speaking of harsh reality, let's talk about the hardware we

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<v Speaker 1>use today. We aren't just using single processors anymore.

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<v Speaker 2>We have SMP and multi core Symmetric multiprocessing. SMP was

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<v Speaker 2>the first step. You plug two or four separate CPUs

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<v Speaker 2>into one motherboard.

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<v Speaker 1>They share everything, they.

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<v Speaker 2>Share memory IO. The US has to schedule tasks across

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<v Speaker 2>all of them.

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<v Speaker 1>Which is great for real life liability. If one burns out,

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<v Speaker 1>the other keeps going.

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<v Speaker 2>But multi core is where we are now putting multiple

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<v Speaker 2>brains on a single chip. It's faster, more efficient, but

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<v Speaker 2>it creates a huge headache for the OS designer called

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<v Speaker 2>cash coherence.

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<v Speaker 1>Because each core has its own little cash right, its

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<v Speaker 1>own little countertop of ingredients exactly.

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<v Speaker 2>So Core A changes the value, you say, updates your

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<v Speaker 2>bank balance in its private cache, but COREB tries to

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<v Speaker 2>read that balance from its own cash.

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<v Speaker 1>COREB has old information. You have a synchronization nightmare.

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<v Speaker 2>So the OS and the hardware have to constantly gossip

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<v Speaker 2>with each other. Hey, I changed this value, invalidate your copy.

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<v Speaker 2>It has a lot of complexity.

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<v Speaker 1>It's fascinating that all these tricks interrupts caching, time slicing.

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<v Speaker 1>They're universal, but how they're packaged differs. Stallings compares the

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<v Speaker 1>big three Windows, Linux, and Android, and they have very

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<v Speaker 1>different personalities.

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<v Speaker 2>They do take Windows. Windows is the ultimate corporate manager.

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<v Speaker 2>It's highly modular. It has this thing called the HL,

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<v Speaker 2>the Hardware abstraction.

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<v Speaker 1>Layer, which sounds like the villain from two thousand and

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<v Speaker 1>one A Space Odyssey.

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<v Speaker 2>Fortunately it's helpful. The HAL is a translator. It means

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<v Speaker 2>the core of Windows does need to know if you're

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<v Speaker 2>running an Intel chip or an AMD chip, or what

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<v Speaker 2>motherboard you have. AHL handles the specifics.

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<v Speaker 1>It isolates the kernel.

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<v Speaker 2>From the messiness of the real world. This is why

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<v Speaker 2>you can build a PC out of random parts and

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<v Speaker 2>Windows generally just boots up.

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<v Speaker 1>It prioritizes compatibility, compatibility, and portability.

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<v Speaker 2>It uses a client server model. Even internally, different parts

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<v Speaker 2>of Windows talk to each other by sending messages, almost

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<v Speaker 2>like they're on a network.

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<v Speaker 1>Now contrast that with Linux. Stallings calls it monolithic. That

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<v Speaker 1>sounds heavy.

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<v Speaker 2>It is, but in a good way. In Linux, the

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<v Speaker 2>kernel is one giant, highly optimized brain. All the drivers,

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<v Speaker 2>the file systems, the memory management, it's all living in

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<v Speaker 2>the same address space.

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<v Speaker 1>Isn't that dangerous If the audio driver crashes, doesn't it

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<v Speaker 1>take down the whole brain?

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<v Speaker 2>Historically, yes, that was the big criticism, but Linux got clever.

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<v Speaker 2>They added loadable.

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<v Speaker 1>Modules like Lego bricks sort of.

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<v Speaker 2>It means you can plug in a new device driver

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<v Speaker 2>or a file system while the kernel is running. You

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<v Speaker 2>don't have to shut down and rebuild the whole LOS

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<v Speaker 2>just to add support for a new webcam.

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<v Speaker 1>So you get the raw speed of a monolithic kernel with.

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<v Speaker 2>The flexibility of a modular one. It's the Borg approach,

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<v Speaker 2>adapt and assimilate without stopping.

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<v Speaker 1>And then Android is basically Linux putting on a hiking backpack.

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<v Speaker 2>It is Linux optimizing for survival power management on a server.

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<v Speaker 2>Linux assumes it has a wall socket. It runs tasks

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<v Speaker 2>whenever it wants. On a phone, Android has to be ruthless.

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<v Speaker 2>It uses wayclocks to prevent apps from keeping the CPU awake.

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<v Speaker 1>It kills background processes.

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<v Speaker 2>Aggressively because the dead battery makes the smartest OS completely useless.

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<v Speaker 1>True.

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<v Speaker 2>Android also uses a unique software stack for apps. It

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<v Speaker 2>doesn't run standard Linux programs. It runs apps inside a virtual.

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<v Speaker 1>Machine, which creates a sandbox.

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<v Speaker 2>Right, so if an angry bird crashes, it doesn't crash

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<v Speaker 2>your phone dialer.

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<v Speaker 1>It really highlights that the OS is the unsung hero.

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<v Speaker 1>We've talked about efficiency, convenience, evolution, but stallings leaves us

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<v Speaker 1>with one final, slightly terrifying concept, fault tolerance.

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<v Speaker 2>It's the realization that hardware will fail, not might will.

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<v Speaker 1>It's the law of large numbers. If you have billions

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<v Speaker 1>of transistors, one of them is going to flake out.

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<v Speaker 2>Eventually, a cosmic ray is going to hit your hard drive.

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<v Speaker 2>So the OS has to be paranoid. It relies on redundancy.

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<v Speaker 1>Redundancy in what sense.

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<v Speaker 2>Three senses actually spatial redundancy, having backup hardware like raid discs.

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<v Speaker 2>If one fails, the other takes over. Temporal redundancy. If

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<v Speaker 2>a calculation looks weird or fails, run it.

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<v Speaker 1>Again, just turn it off and on again at the

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<v Speaker 1>microchip level basically.

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<v Speaker 2>And information redundancy using air correcting codes to check if

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<v Speaker 2>a one accidentally flipped to a zero.

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<v Speaker 1>So the OS isn't just a manager. It is a

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<v Speaker 1>disaster prevention team.

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<v Speaker 2>Ideally, yes, it's managing a constant state of low level crisis.

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<v Speaker 2>It's masking the chaos of the physical world so that

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<v Speaker 2>you can exist in the pristine world of software. If

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<v Speaker 2>the OS does its job right, you never know how

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<v Speaker 2>close he came to a crash.

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<v Speaker 1>Well, on that comforting note. Next time your phone buzzes

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<v Speaker 1>with a notification, just remember that's an interrupt signal, saving

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<v Speaker 1>you from staring at a frozen screen. And that smooth scrolling,

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<v Speaker 1>that's a gamble that paid off. Thanks for listening to

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<v Speaker 1>the deep dive.

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<v Speaker 2>We'll catch on the next one.