c++ - how - What are near, far and huge pointers?




what is void pointer in c (4)

Can anyone explain to me these pointers with a suitable example ... and when these pointers are used?


Difference between far and huge pointers:

As we know by default the pointers are near for example: int *p is a near pointer. Size of near pointer is 2 bytes in case of 16 bit compiler. And we already know very well size varies compiler to compiler; they only store the offset of the address the pointer it is referencing. An address consisting of only an offset has a range of 0 - 64K bytes.

Far and huge pointers:

Far and huge pointers have a size of 4 bytes. They store both the segment and the offset of the address the pointer is referencing. Then what is the difference between them?

Limitation of far pointer:

We cannot change or modify the segment address of given far address by applying any arithmetic operation on it. That is by using arithmetic operator we cannot jump from one segment to other segment.

If you will increment the far address beyond the maximum value of its offset address instead of incrementing segment address it will repeat its offset address in cyclic order. This is also called wrapping, i.e. if offset is 0xffff and we add 1 then it is 0x0000 and similarly if we decrease 0x0000 by 1 then it is 0xffff and remember there is no change in the segment.

Now I am going to compare huge and far pointers :

1.When a far pointer is incremented or decremented ONLY the offset of the pointer is actually incremented or decremented but in case of huge pointer both segment and offset value will change.

Consider the following Example, taken from HERE :

 int main()
    {
    char far* f=(char far*)0x0000ffff;
    printf("%Fp",f+0x1);
    return 0;
  }

then the output is:

0000:0000

There is no change in segment value.

And in case of huge Pointers :

int main()
{
char huge* h=(char huge*)0x0000000f;
printf("%Fp",h+0x1);
return 0;
}

The Output is:

0001:0000

This is because of increment operation not only offset value but segment value also change.That means segment will not change in case of far pointers but in case of huge pointer, it can move from one segment to another .

2.When relational operators are used on far pointers only the offsets are compared.In other words relational operators will only work on far pointers if the segment values of the pointers being compared are the same. And in case of huge this will not happen, actually comparison of absolute addresses takes place.Let us understand with the help of an example of far pointer :

int main()
{
char far * p=(char far*)0x12340001;
char far* p1=(char far*)0x12300041;
if(p==p1)
printf("same");
else
printf("different");
return 0;
}

Output:

different

In huge pointer :

int main()
{
char huge * p=(char huge*)0x12340001;
char huge* p1=(char huge*)0x12300041;
if(p==p1)
printf("same");
else
printf("different");
return 0;
}

Output:

same

Explanation: As we see the absolute address for both p and p1 is 12341 (1234*10+1 or 1230*10+41) but they are not considered equal in 1st case because in case of far pointers only offsets are compared i.e. it will check whether 0001==0041. Which is false.

And in case of huge pointers, the comparison operation is performed on absolute addresses that are equal.

  1. A far pointer is never normalized but a huge pointer is normalized . A normalized pointer is one that has as much of the address as possible in the segment, meaning that the offset is never larger than 15.

    suppose if we have 0x1234:1234 then the normalized form of it is 0x1357:0004(absolute address is 13574). A huge pointer is normalized only when some arithmetic operation is performed on it, and not normalized during assignment.

     int main()
     {
      char huge* h=(char huge*)0x12341234;
      char huge* h1=(char huge*)0x12341234;
      printf("h=%Fp\nh1=%Fp",h,h1+0x1);
      return 0;
     }
    

    Output:

    h=1234:1234
    
    h1=1357:0005
    

    Explanation:huge pointer is not normalized in case of assignment.But if an arithmetic operation is performed on it, it will be normalized.So, h is 1234:1234 and h1 is 1357:0005which is normalized.

    4.The offset of huge pointer is less than 16 because of normalization and not so in case of far pointers.

    lets take an example to understand what I want to say :

     int main()
      {
      char far* f=(char far*)0x0000000f;
      printf("%Fp",f+0x1);
      return 0;
      }
    

Output:

    0000:0010

In case of huge pointer :

      int main()
      {
      char huge* h=(char huge*)0x0000000f;
        printf("%Fp",h+0x1);
        return 0;
        }

        Output:
        0001:0000

Explanation:as we increment far pointer by 1 it will be 0000:0010.And as we increment huge pointer by 1 then it will be 0001:0000 because it's offset cant be greater than 15 in other words it will be normalized.


All of the stuff in this answer is relevant only to the old 8086 and 80286 segmented memory model.

near: a 16 bit pointer that can address any byte in a 64k segment

far: a 32 bit pointer that contains a segment and an offset. Note that because segments can overlap, two different far pointers can point to the same address.

huge: a 32 bit pointer in which the segment is "normalised" so that no two far pointers point to the same address unless they have the same value.

tee: a drink with jam and bread.

That will bring us back to doh oh oh oh

and when these pointers are used?

in the 1980's and 90' until 32 bit Windows became ubiquitous,


In the old days, according to the Turbo C manual, a near pointer was merely 16 bits when your entire code and data fit in the one segment. A far pointer was composed of a segment as well as an offset but no normalisation was performed. And a huge pointer was automatically normalised. Two far pointers could conceivably point to the same location in memory but be different whereas the normalised huge pointers pointing to the same memory location would always be equal.


The primary example is the Intel X86 architecture.

The Intel 8086 was, internally, a 16-bit processor: all of its registers were 16 bits wide. However, the address bus was 20 bits wide (1 MiB). This meant that you couldn't hold an entire address in a register, limiting you to the first 64 kiB.

Intel's solution was to create 16-bit "segment registers" whose contents would be shifted left four bits and added to the address. For example:

DS ("Data Segment") register:  1234 h
DX ("D eXtended") register:   + 5678h
                              ------
Actual address read:           179B8h

This created the concept of 64 kiB segment. Thus a "near" pointer would just be the contents of the DX register (5678h), and would be invalid unless the DS register was already set correctly, while a "far" pointer was 32 bits (12345678h, DS followed by DX) and would always work (but was slower since you had to load two registers and then restore the DS register when done).

(As supercat notes below, an offset to DX that overflowed would "roll over" before being added to DS to get the final address. This allowed 16-bit offsets to access any address in the 64 kiB segment, not just the part that was ± 32 kiB from where DX pointed, as is done in other architectures with 16-bit relative offset addressing in some instructions.)

However, note that you could have two "far" pointers that are different values but point to the same address. For example, the far pointer 100079B8h points to the same place as 12345678h. Thus, pointer comparison on far pointers was an invalid operation: the pointers could differ, but still point to the same place.

This was where I decided that Macs (with Motorola 68000 processors at the time) weren't so bad after all, so I missed out on huge pointers. IIRC, they were just far pointers that guaranteed that all the overlapping bits in the segment registers were 0's, as in the second example.

Motorola didn't have this problem with their 6800 series of processors, since they were limited to 64 kiB, When they created the 68000 architecture, they went straight to 32 bit registers, and thus never had need for near, far, or huge pointers. (Instead, their problem was that only the bottom 24 bits of the address actually mattered, so some programmers (notoriously Apple) would use the high 8 bits as "pointer flags", causing problems when address buses expanded to 32 bits (4 GiB).)

Linus Torvalds just held out until the 80386, which offered a "protected mode" where the addresses were 32 bits, and the segment registers were the high half of the address, and no addition was needed, and wrote Linux from the outset to use protected mode only, no weird segment stuff, and that's why you don't have near and far pointer support in Linux (and why no company designing a new architecture will ever go back to them if they want Linux support). And they ate Robin's minstrels, and there was much rejoicing. (Yay...)





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