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B1 Intermediate US 86 Folder Collection
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Hi, I'm Carrie Anne, and welcome to Crash
Course Computer Science!
Computers in the 1940s and early 50s ran one
program at a time.
A programmer would write one at their desk,
for example, on punch cards.
Then, they'd carry it to a room containing
a room-sized computer, and hand it to a dedicated
computer operator.
That person would then feed the program into
the computer when it was next available.
The computer would run it, spit out some output,
and halt.
This very manual process worked OK back when
computers were slow, and running a program
often took hours, days or even weeks.
But, as we discussed last episode, computers
became faster... and faster... and faster
– exponentially so!
Pretty soon, having humans run around and
inserting programs into readers was taking
longer than running the actual programs themselves.
We needed a way for computers to operate themselves,
and so, operating systems were born.
INTRO
Operating systems, or OS'es for short, are
just programs.
But, special privileges on the hardware let
them run and manage other programs.
They're typically the first one to start
when a computer is turned on, and all subsequent
programs are launched by the OS.
They got their start in the 1950s, as computers
became more widespread and more powerful.
The very first OSes augmented the mundane,
manual task of loading programs by hand.
Instead of being given one program at a time,
computers could be given batches.
When the computer was done with one, it would automatically and near-instantly start the next.
There was no downtime while someone scurried
around an office to find the next program
to run.
This was called batch processing.
While computers got faster, they also got
cheaper.
So, they were popping up all over the world,
especially in universities and government
offices.
Soon, people started sharing software.
But there was a problem…
In the era of one-off computers, like the
Harvard Mark 1 or ENIAC, programmers only
had to write code for that one single machine.
The processor, punch card readers, and printers
were known and unchanging.
But as computers became more widespread, their
configurations were not always identical,
like computers might have the same CPU, but
not the same printer.
This was a huge pain for programmers.
Not only did they have to worry about writing
their program, but also how to interface with
each and every model of printer, and all devices
connected to a computer, what are called peripherals.
Interfacing with early peripherals was very
low level, requiring programmers to know intimate
hardware details about each device.
On top of that, programmers rarely had access to every model of a peripheral to test their code on.
So, they had to write code as best they could,
often just by reading manuals, and hope it
worked when shared.
Things weren't exactly plug-and-play back
then… more plug-and-pray.
This was clearly terrible, so to make it easier
for programmers, Operating Systems stepped
in as intermediaries between software programs
and hardware peripherals.
More specifically, they provided a software
abstraction, through APIs, called device drivers.
These allow programmers to talk to common
input and output hardware, or I/O for short,
using standardized mechanisms.
For example, programmers could call a function
like “print highscore”, and the OS would
do the heavy lifting to get it onto paper.
By the end of the 1950s, computers had gotten
so fast, they were often idle waiting for
slow mechanical things, like printers and
punch card readers.
While programs were blocked on I/O, the expensive
processor was just chillin'... not like
a villain… you know, just relaxing.
In the late 50's, the University of Manchester,
in the UK, started work on a supercomputer
called Atlas, one of the first in the world.
They knew it was going to be wicked fast,
so they needed a way to make maximal use of
the expensive machine.
Their solution was a program called the Atlas
Supervisor, finished in 1962.
This operating system not only loaded programs
automatically, like earlier batch systems,
but could also run several at the same time
on its single CPU.
It did this through clever scheduling.
Let's say we have a game program running
on Atlas, and we call the function “print
highscore” which instructs Atlas to print
the value of a variable named “highscore”
onto paper to show our friends that we're
the ultimate champion of virtual tiddlywinks.
That function call is going to take a while,
the equivalent of thousands of clock cycles,
because mechanical printers are slow in comparison
to electronic CPUs.
So instead of waiting for the I/O to finish,
Atlas instead puts our program to sleep, then
selects and runs another program that's
waiting and ready to run.
Eventually, the printer will report back to
Atlas that it finished printing the value
of “highscore”.
Atlas then marks our program as ready to go,
and at some point, it will be scheduled to
run again on the CPU, and continue onto the
next line of code following the print statement.
In this way, Atlas could have one program
running calculations on the CPU, while another
was printing out data, and yet another reading
in data from a punch tape.
Atlas' engineers doubled down on this idea,
and outfitted their computer with 4 paper
tape readers, 4 paper tape punches, and up
to 8 magnetic tape drives.
This allowed many programs to be in progress
all at once, sharing time on a single CPU.
This ability, enabled by the Operating System,
is called multitasking.
There's one big catch to having many programs running simultaneously on a single computer, though.
Each one is going to need some memory, and
we can't lose that program's data when
we switch to another program.
The solution is to allocate each program its
own block of memory.
So, for example, let's say a computer has
10,000 memory locations in total.
Program A might get allocated memory addresses
0 through 999, and Program B might get 1000
through 1999, and so on.
If a program asks for more memory, the operating
system decides if it can grant that request,
and if so, what memory block to allocate next.
This flexibility is great, but introduces
a quirk.
It means that Program A could end up being
allocated non-sequential blocks of memory,
in say addresses 0 through 999, and 2000 through 2999.
And this is just a simple example - a real
program might be allocated dozens of blocks
scattered all over memory.
As you might imagine, this would get really
confusing for programmers to keep track of.
Maybe there's a long list of sales data
in memory that a program has to total up at
the end of the day, but this list is stored
across a bunch of different blocks of memory.
To hide this complexity, Operating Systems
virtualize memory locations.
With Virtual Memory, programs can assume their
memory always starts at address 0, keeping
things simple and consistent.
However, the actual, physical location in
computer memory is hidden and abstracted by
the operating system.
Just a new level of abstraction.
Let's take our example Program B, which
has been allocated a block of memory from
address 1000 to 1999.
As far as Program B can tell, this appears
to be a block from 0 to 999.
The OS and CPU handle the virtual-to-physical
memory remapping automatically.
So, if Program B requests memory location
42, it really ends up reading address 1042.
This virtualization of memory addresses is
even more useful for Program A, which in our
example, has been allocated two blocks of
memory that are separated from one another.
This too is invisible to Program A.
As far as it can tell, it's been allocated
a continuous block of 2000 addresses.
When Program A reads memory address 999, that
does coincidentally map to physical memory
address 999.
But if Program A reads the very next value
in memory, at address 1000, that gets mapped
behind the scenes to physical memory address
2000.
This mechanism allows programs to have flexible
memory sizes, called dynamic memory allocation,
that appear to be continuous to them.
It simplifies everything and offers tremendous
flexibility to the Operating System in running
multiple programs simultaneously.
Another upside of allocating each program
its own memory, is that they're better isolated
from one another.
So, if a buggy program goes awry, and starts
writing gobbledygook, it can only trash its
own memory, not that of other programs.
This feature is called Memory Protection.
This is also really useful in protecting against
malicious software, like viruses.
For example, we generally don't want other
programs to have the ability to read or modify
the memory of, let say, our email, with that
kind of access, malware could send emails
on your behalf and maybe steal personal information.
Not good!
Atlas had both virtual and protected memory.
It was the first computer and OS to support
these features!
By the 1970s, computers were sufficiently
fast and cheap.
Institutions like a university could buy a
computer and let students use it.
It was not only fast enough to run several
programs at once, but also give several users
simultaneous, interactive access.
This was done through a terminal, which is
a keyboard and screen that connects to a big
computer, but doesn't contain any processing
power itself.
A refrigerator-sized computer might have 50
terminals connected to it, allowing up to
50 users.
Now operating systems had to handle not just
multiple programs, but also multiple users.
So that no one person could gobble up all
of a computer's resources, operating systems
were developed that offered time-sharing.
With time-sharing each individual user was
only allowed to utilize a small fraction of
the computer's processor, memory, and so
on.
Because computers are so fast, even getting
just 1/50th of its resources was enough for
individuals to complete many tasks.
The most influential of early time-sharing
Operating Systems was Multics, or Multiplexed
Information and Computing Service, released
in 1969.
Multics was the first major operating
system designed to be secure from the outset.
Developers didn't want mischievous users
accessing data they shouldn't, like students
attempting to access the final exam on their
professor's account.
Features like this meant Multics was really
complicated for its time, using around 1 Megabit
of memory, which was a lot back then!
That might be half of a computer's memory,
just to run the OS!
Dennis Ritchie, one of the researchers working
on Multics, once said:
“One of the obvious things that went wrong
with Multics as a commercial success was just
that it was sort of over-engineered in a sense.
There was just too much in it.”
T
his lead Dennis, and another Multics researcher,
Ken Thompson, to strike out on their own and build a new, lean operating system… called Unix.
They wanted to separate the OS into two parts:
First was the core functionality of the OS,
things like memory management, multitasking,
and dealing with I/O, which is called the
kernel.
The second part was a wide array of useful
tools that came bundled with, but not part
of the kernel, things like programs and libraries.
Building a compact, lean kernel meant intentionally
leaving some functionality out.
Tom Van Vleck, another Multics developer,
recalled:
“I remarked to Dennis that easily half the
code I was writing in Multics was error recovery
code."
He said, "We left all that stuff out of Unix.
If there's an error, we have this routine
called panic, and when it is called, the machine
crashes, and you holler down the hall, 'Hey,
reboot it.'"”
You might have heard of kernel panics, This
is where the term came from.
It's literally when the kernel crashes,
has no recourse to recover, and so calls a
function called “panic”.
Originally, all it did was print the word
“panic” and then enter
an infinite loop.
This simplicity meant that Unix could be run
on cheaper and more diverse hardware, making
it popular inside Bell Labs, where Dennis
and Ken worked.
As more developers started using Unix to build
and run their own programs, the number of
contributed tools grew.
Soon after its release in 1971, it gained
compilers for different programming languages
and even a word processor, quickly making
it one of the most popular OSes of the 1970s
and 80s.
At the same time, by the early 1980s, the
cost of a basic computer had fallen to the
point where individual people could afford
one, called a personal or home computer.
These were much simpler than the big mainframes
found at universities, corporations, and governments.
So, their operating systems had to be equally
simple.
For example, Microsoft's Disk Operating
System, or MS-DOS, was just 160 kilobytes,
allowing it to fit, as the name suggests,
onto a single disk.
First released in 1981, it became the most
popular OS for early home computers, even
though it lacked multitasking and protected
memory.
This meant that programs could, and would,
regularly crash the system.
While annoying, it was an acceptable tradeoff,
as users could just turn their own computers
off and on again!
Even early versions of Windows, first released
by Microsoft in 1985 and which dominated the
OS scene throughout the 1990s, lacked strong
memory protection.
When programs misbehaved, you could get the
blue screen of death, a sign that a program
had crashed so badly that it took down the
whole operating system.
Luckily, newer versions of Windows have better protections and usually don't crash that often.
Today, computers run modern operating systems,
like Mac OS X, Windows 10, Linux, iOS and
Android.
Even though the computers we own are most
often used by just a single person, you! their
OSes all have multitasking and virtual and
protected memory.
So, they can run many programs at once: you
can watch YouTube in your web browser, edit
a photo in Photoshop, play music in Spotify
and sync Dropbox all at the same time.
This wouldn't be possible without those
decades of research and development on Operating
Systems, and of course the proper memory to
store those programs.
Which we'll get to next week.
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B1 Intermediate US

Operating Systems: Crash Course Computer Science #18

86 Folder Collection
黃柏鈞 published on October 6, 2018

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