A2 Basic 5 Folder Collection
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So without pointers
we would find it really difficult to build a general purpose CPU
that can do anything
rather me drawing it
let me get out my book of the original IBM PC's 8088
and effectively when the mac boot up
when the pc boot up
they boot up like this
now what we have inside the CPU
well is a bit of it witch will do the arithmetic, do the logic, add numbers together
do various Boolean algebra, will forget about them for now
will talk about them some other time probably already talk about them
the other thing you got are registers, so registers are literally
a small where you can store a bit of information
well some of them do store a bit but actually store few more bits
and then you will get a Byte or maybe two Bytes worth
so let's go in what the 8088 had
and it can store generally in the register eater a Byte or two Bytes
if you have a look on this book "THE 8088 Project Book"
It's got a list in here of the various bits inside it
so we have got the arithmetic logic unit
some registers here, some more temporary registers
and then these general one's here
and a few more that we have got around
so we have got some 8 bit and 16 bit ? combined
to use eater way and then we have got the 16 bit
pointers will come back to that
so what all these registers do is allowing to store a 16 bit value
or an 8 bit value
so they are built into the cpu and the instruction in the cpu
will let you set a value in them
add a value to what is inside or read a value from one of them
that's fine
you have got one,two,three,four
16 bit or 8
8 bit values
that is not much data
and you have got the problem
where do you get the program from
how to you get more data out of your memory
to a? it, well the way you do that
is if you look you have other registers
witch are called pointer and index registers
and this why you need pointers or you cant build a computer
because what these are used for
is these say, well ok I dont have this value here but I am gonna store here where it is in memory
so I am gonna store for example
a stack pointer, not the stack but I am gonna store a value that points where the stack is in memory
now , really easy way to understand how pointers work
think it like the index of the book
so if I want to know about stack pointer
I will go to the index, if I can find the index
and somewhere it says stack pointer
here we are, stack pointer, see page 9-10
So if i want to find out something I do not go to page 9-10 where it is
I go to the index and it tell's me
where it is and then go page 9-10
pointers work exactly like that
they tell you where to find the information
just like and index in a book, simple
Right, now why do we need them to make the computer work
there is two reasons why
firstly how do we execute code
the code, the program, the instruction that it is going to execute
stored in memory and you need to know where that is
how the CPU does that
go back to are thing
and if we look carefully, we find among these registers we talked about
16 bit points to registers
look over here "16 bit segment registers and the instruction pointer"
an other pointer, a special register
and all this is doing is storing the value of where in memory to find the instruction you can execute
so when the CPU wants to execute an instruction
witch is what it does all the time
it goes to the instruction pointer
and it has got the address of where that instruction is
so it will look in the instruction pointer
and it will contain say : FFF ,0 in HEX
That is the initial point where a PC will start executing code when you witch it on
So it goes to that address
looks it up in memory
and get the value that is in memory and reads it into
the CPU where is can start processing it
and it will get that instruction, execute it whatever it is
and then it will increment the instruction pointer by how over big that instruction was
8088 instruction get very in size every one bye, two bytes
these days it can go up to 16 bytes on x86 chip
it will increase the instruction pointer to point to the next instruction and does the same things
it looks up where the instruction is in memory, fetch is it, and so one, and so one
so if we do not have pointers
we can't even execute code
we need that instruction pointer or what it is called on other processors program counter, same thing
just clear where on the x86
this instruction pointer we need that to know where to go to get the instruction
so without pointers you can't make a CPU work
or get values from memory at least
that is fine for executing code
but this thing is execute on data
how do we get the data in, the same thing
we have pointers that contains the address or registers that contain the address of where data is in memory
So every time you access a variable in C
the assembly that is generated is almost certainly getting to re getting
that ? from memory putting it into a register in the CPU
and that will be done with having a pointer value where this is in memory
going and fetching it and then using that
in the instruction that is being executed where ever that might be
to add say : on to an other register or where the CPU needs doing
so, again without pointers you can not get data from memory you can not store data back into memory
so you can not build a computer !
without computers you can forget your
functional program you can forget your hash tables
your link list anything, you cannot build a computer without pointers
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Essentials: Hidden Pointers - Computerphile

5 Folder Collection
林宜悉 published on March 28, 2020
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