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