What is far pointer and where should we use it?

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sanjay ujjainkar

  • Sep 12th, 2007
 

Far pointer can address memory location which is not pointed by normal pointer. Normal pointer being 2 byte can address up to 65536.

VDU is devided into a matrix of memory locations which can be accessed by Far pointer,using far pointer we can display some char without using printf() function.

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jintojos

  • Jun 18th, 2008
 

Use Of Far Pointer..........
Each program has its own data and code segments which is allocated by the compiler.
The normal pointer or near pointer can address locations which are in its data segment. far pointer is used to address locations which are outside the data segment. for example finding the size of RAM we want to check the location 0x417.
To find the RAM size we use the code 
          char far *siz=(char far*)0x417;
          printf("The Ram Size IS :%d",*siz);

Also for using far pointer we want to specify the segment and offset address........
ex: char far *scr=(char far*)0xb0008000; is the address of VDU memory.
Here b000 is the segment address and 8000 is the offset address........
    

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kbjarnason

  • Jul 1st, 2010
 

C has no "far" pointers, it only has pointers.

Some (generally ancient) compilers, notably for DOS, did offer near, far, and huge pointers.  This was due to how DOS managed memory.

Basically, in DOS, memory is broken up into "segments", which are a maximum of 64K in size.  By default, most programs were limited to one segment for code, one for data - meaning you could have a maximum of 64K of code, a maximum of 64K for data.  Not all that great, ya know?

Well, there's a couple ways around this.  One of them is to use "far" pointers when allocating memory (and, in turn, use a special allocation function).  By doing this, any given allocated block can be in a different data segment.  Like this:

void far *ptr1;
void far *ptr2;

ptr1 = farmalloc(somesize);
ptr2 = farmalloc(somesize);

Here, ptr1 and ptr2 can go into entirely different segments.  Each can still only cope with 64K of data, max, but between them you now have up to 128K of data - 64K each.  This is a distinct improvement upon a strict 64K total limit.

So why not make all pointers far by default?  Efficiency.

In the X86 world of DOS, when accessing memory, you use both a segment and an offset to specify the memory you want to access.  In assembler, it looks something like this:

   mov ax, [DS:1037]

This means "read memory at the segment specified by DS, at offset 1037, and store the results in the ax register."

In general, this is simplified a little, as (by default), all memory access is done in the segment specified by DS - so wriiting it out is a waste.  What you'd normally see in actual code would be more like:

  mov ax, [1037]

It means exactly the same thing - in the segment indicated by DS, go to offset 1037, read the value there, store it in the ax register.

And what about far?

If you recall, when we allocated our two far pointers, the whole point to it was that each could live in its own segment, thus giving us more effective usable memory - but at a cost.  Assume ptr1 uses segment 204 and ptr2 uses segment 319.  Assume we want to load whatever value is in memory at offset 100 in each segment.  Our code - in assembly - might look something like this:

   mov ax, ds ; save the current segment value
   push ax
   mov ax, 204
   mov ds, ax  ; load segment for ptr1
   mov bx, 100 ; load offset
   mov cx, [bx] ; get value at offset 100
   mov ax, 319
   mov ds,ax ; load segment for ptr1
   mov bx, 100
   mov dx,[bx] ; get value af offset 100
   pop ax
   mov ds, ax ; restore the segment value

Now cx has the value from ptr1+100, dx has the value from ptr2+100

Contrast that to the default "near" pointers:

char *ptr1 = malloc(somesize);
char *ptr2 = malloc(somesize);

val1 = ptr1[100];
val2 = ptr2[100];

In assembly:

   mov bx, offset val1
   add bx, 100
   mov cx,[bx]
   mov bx, offset val2
   add bx,100
   mov cx,[bx]

Because "near" pointers all share the same segment, there's no need to diddle about trying to load and save the segment registers; you just get the address of the buffer, add the offset and you're done.  Less code, faster operation.


Fortunately, unless you're dealing with DOS, or some other segmented architecture with ridiculously small segments that forces you into this sort of thing, you simply need not worry about it.


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suchi kumari

  • Jul 19th, 2016
 

What is the meaning of char far in c is not clear? Why it is used for a pointer declaration

Code
  1. main()

  2. {

  3.  char far *s1,*s2;

  4.  printf("%d%d",sizeof(s1),sizeof(s2));

  5. }

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