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[MUSIC] So what is this class about? 
Well, this class is about the hardware, 

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software interface. 
But to understand that, first let's see 

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what's hardware. 
Well, hardware is the part of the 

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computer made of atoms, it's the physical 
part of your, computing device. 

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Okay. 
In software, it's this thing made of bits 

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which is the program that runs on top of 
your hardware. 

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Okay. 
But for that to happen effectively there 

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are many many layers of abstractions 
between the hardware and the software, 

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okay? 
And this will be called the hardware 

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software interface. 
And the reason we want you to understand 

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what the hardware software interface is 
is that it's going to help you understand 

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better how computers work. 
It's going to make you a more effective 

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programmer, it's going to make you better 
at writing efficient codes. 

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And it's just going to make you a better 
computer science, scientist. 

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So, let's look at these three pieces of 
code here. 

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So, here we have a simple statement that 
does a conditional and performs an 

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assignment. 
And this is written in, you know, a 

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language like C or Java, okay? 
Now, this piece of code here is also a, 

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well it's also a program. 
And this is written in what we call the 

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assembly language, okay? 
The assembly language is just a much 

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lower level language that's pretty to 
close to hardware. 

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And it just happens to be an abstraction, 
a simple way of looking at machine code. 

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Okay, so the machine code is just a 
sequence of 0's and 1's. 

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Processors only understand 0's and 1's it 
interprets them in different ways. 

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Now, so what's in common between these 
three pieces of codes? 

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Well all of them we're exactly the same 
thing their equivalent but you'd rather 

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write in this language here because it's 
much easier. 

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it's much more natural for humans to 
write, okay, so it's much more human 

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friendly. 
But as I said before the hardware likes 

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bit strings. 
So 0s and 1s. 

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So a digital computer likes 0s and 1s 
because it's very easy to represent with 

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electronic sequence because 0s is a low 
voltage and 1 can, can be represented as 

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a high voltage, okay? 
So and the, the, the machine machine 

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structure is actually much shorter than 
the number of bits we would need to 

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represent the characters in the assembly 
language. 

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It's a, much, it's a much more dense 
encoding of the assembly language, the 

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assembly instructions that we see here. 
Okay? 

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There's a very direct correspondence 
between these, simple assembly 

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instructions, and some of the bits in 
machine code. 

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Now let's look at a hard or soft 
interface from a historical perspective. 

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So at first the hardware, the hard or 
soft interface was very, very simple 

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because in because hardware was simple. 
Hardware was very, very primitive when 

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computers started. 
Okay. 

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So humans was were able to write code 
directly to hardware without any 

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abstraction at all. 
But then people got [INAUDIBLE] and that 

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means that software was very primitive. 
Because the the soft improvements 

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reflected the hardware pretty closely. 
But then people got really excited about 

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computers. 
It's like hey,computers can do all these 

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things. 
So they decided, they end up writing more 

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and more complex pieces of code. 
That meant that humans could no longer, 

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you know keep up with the complexity of 
writing machine code directly. 

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So, uh,the assembly language was 
invented. 

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The assembly language, as I said before 
is a very very simple computer 

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programming language. 
That's pretty close what a hardware can 

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do in terms of it's primitives and one 
assembly instruction translates to one 

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machine instruction. 
Okay? 

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So but the big difference is that the 
assembly language is much easier for 

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humans to read because they have, first 
of all, they have characters strings as 

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opposed to just bits. 
Okay so humans can understand much, much 

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easier so it's much easier to read and 
write. 

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And you can also use symbolic names for 
values. 

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And you can use thing like writes their 
names, a, b and so on, okay. 

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So now there's a piece of code here 
called Assembler. 

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That takes a program written in Assembly 
and generates zeros and ones Machine 

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code, which is what the hardware likes. 
But even then that, that wasn't enough 

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because you know, programs were getting 
more and more complex. 

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We needed yet another level of 
abstraction that's much higher level. 

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Okay? 
So this is thing so these languages such 

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as languages like C and java they're 
much, much higher level. 

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You can write statements like you know, a 
equals b plus c for example. 

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Okay? 
And one statement in, this high level 

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language can translate to many, many, 
assembly language instructions. 

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Okay? 
So, but now, we have yet another, another 

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component in our path from the user to 
the hardware here, which is a C compiler. 

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Okay. 
The C Compiler takes C code and generates 

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assembly code. 
[UNKNOWN] this is assembly. 

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Okay. 
So and that's much high lever that means 

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you must have much more productive at 
writing code for that. 

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So now, let's look at the entire lifetime 
of a computer program. 

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And computer program has three basic 
steps in it's life time. 

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There's coding time which is the time 
that humans spent writing the program 

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high level language like C. 
Compile time is the time it takes to get 

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your C program, and generates machine 
code executed by the hardware. 

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Okay? 
And when the hardware's executing the 

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code, we call that run time. 
Okay? 

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So, now compare time is something that 
happens just once, and if you execute 

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Your program many, many, many times. 
That means that it can advertise a cost 

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of compilation a lot, because, you know, 
if you spend some time compiling the 

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program you're going to execute so many 
times. 

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It doesn't matter if you spend a little 
bit more time doing compilation, okay? 

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And in fact, during this process here. 
During the compilation process, the 

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compiler can spend a little bit more time 
generating the code. 

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In order to do optimizations that are 
going to make. 

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You program faster without having the 
programmer improve the code itself. 

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Although if you do want to generate 
really fast codes, you do want to 

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regenerate, you do want to start with 
good algorithms first. 

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Okay? 
So a big thing in this class then is how 

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to hardware software interface, okay? 
So just, so that includes how to 

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hardware, you know, with 0s and 1s and 
processor executing instruction. 

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Relates to software In other words your, 
your for example your Java program. 

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And the other big, the other part of this 
thing is that computer's about 

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abstractions. 
Computer systems are complex. 

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In order for humans to understand and 
design effective computer systems, we 

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need abstractions. 
Okay. 

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So, computer's a lot about abstractions 
but, we can't really forget reality. 

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Well, the goal of an abstraction is to 
abstract reality and make it simpler to 

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think about. 
But we don't to completely ignore reality 

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because that leads to inefficiencies and 
leads to other problems. 

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Okay. 
So, what, we also going to talk about 

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what are the abstractions that, that we 
use and what do you need to know about 

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them, okay. 
So well, you know when they break down 

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and you have to peek under the hood. 
You also want to know because it will 

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help [INAUDIBLE] bacause it helps with 
what kind of bugs can they cause and how 

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you find them and so on. 
But knowing the hardware and software 

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interface ultimately is going to make you 
better programmers. 

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It's going to help you begin to 
understand important concepts that have 

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evolved er, in building ever more complex 
computer systems.