What are near far and huge pointers in C?

Answer 1

A near pointer is a 16 bit pointer to an object contained in the current segment, be it code segment, data segment, stack segment, or extra segment. The compiler can generate code with a near pointer and does not have to concern itself with segment addressing, so using near pointers is fastest, and generates smallest code. The limitation is that you can only access 64kb of data at a time, because that is the size of a segment - 64kb. A near pointer contains only the 16 bit offset of the object within the currently selected segment.

A far pointer is a 32 bit pointer to an object anywhere in memory. In order to use it, the compiler must allocate a segment register, load it with the segment portion of the pointer, and then reference memory using the offset portion of the pointer relative to the newly loaded segment register. This takes extra instructions and extra time, so it is the slowest and largest method of accessing memory, but it can access memory that is larger than 64kb, sometimes, such as when dealing with video memory, a needful thing. A far pointer contains a 16 bit segment part and a 16 bit offset part. Still, at any one instant of time, without "touching" segment registers, the program only has access to four 64kb chunks, or segments of memory. If there is a 100kb object involved, code will need to be written to consider its segmentation, even with far pointers.

Now, segments overlap. Each segment is 64kb in length, but each one overlaps the next and the prior by 65520 bytes. That means that every address in memory can be addressed by 64kb-1 different combinations of segment:offset pairs. The result is that the total addressible memory was only 1mb, and the total usable memory address space was 500kb to 600kb. That sounds odd, but Intel built it, Microsoft wrote it, and DOS/Windows 3.1 grew up around it. I still have that computer, and it still works just fine.

Now the huge pointer. The far pointer suffers because you can not just add one to it and have it point the the next item in memory - you have to consider segment:offset rules, because of the 16 byte offset issue. The huge pointer is a monolithic pointer to some item with a large chunk of memory, and there are no segment:offset boundaries.

Well, in order to get that, the pointer to segment:offset calculation has to be done every time you reference the pointer. It does allow you to create and manipulate a single monolithic object that is greater than 64kb, but it has its costs. Far pointers are the normalized pointers of four bytes which are used to access the main memory of the computer ...it can access both the data segment and code segment thus by modifing the offset you can modify refer two different addresses but refer to the same memory .

In a generic OS , memory is organised in a segment:offset fashion. Now say, it is of "X" MB and this "X" MB is made up of say "S" number of segments of each segment having "B" Bytes where S*B Bytes=X MB.

Near Pointer: A near pointer is that which will only point within the current segment say segment 3 (there are S number of segments numbered 0 to S-1) by containing only offset .

Far Pointer: A far pointer is that which will point anywhere in the X MB across segments by containing segment+offset .

The numbers X,S and B vary across diff operating system memory models under which you are programming .

Say for MS-DOS,X=1MB;B=16 Bytes and S=1Mega Bytes/16 Bytes . Here, near pointer will have 4 bits to express address from 0 to 16 Bytes in a segment . Here, far pointer will have "l" bits for segment part of memory where 2^l=S (Here S should be a multiple of 16 bytes i.e; B=offset memory) and 4 bits for offset part of memory, so far poinetr will have l+4 bits.

In DOS only 1 mb (10,48,580 bytes) of memory is accessible. Any of these memory locations are accessed using CPU registers. Under DOS the CPU registers are only 16 bits long. Therefore, the minimum value present in a CPU register could be 0, and maximum 65,535. Then how do we access memory locations beyond 65535th byte? By using two registers (segment and offset) in conjunction. For this the total memory (1 mb) is divided into a number of units each comprising 65,536 (64 kb) locations. Each such unit is called a segment. Each segment always begins at a location number which is exactly divisible by 16. The segment register contains the address where a segment begins, whereas the offset register contains the offset of the data/code from where the segment begins. For example, let us consider the first byte in B block of video memory. The segment address of video memory is B0000h (20-bit address), whereas the offset value of the first byte in the upper 32K block of this segment is 8000h. Since 8000h is a 16-bit address it can be easily placed in the offset register, but how do we store the 20-bit address B0000h in a 16-bit segment register? For this out of B0000h only first four hex digits (16 bits) are stored in segment register. We can afford to do this because a segment address is always a multiple of 16 and hence always contains a 0 as the last digit. Therefore, the first byte in the upper 32K chunk of B block of video memory is referred using segment:offset format as B000h:8000h. Thus, the offset register works relative to segment register. Using both these, we can point to a specific location anywhere in the 1 mb address space.

Suppose we want to write a character `A' at location B000:8000. We must convert this address into a form which C understands. This is done by simply writing the segment and offset addresses side by side to obtain a 32 bit address. In our example this address would be 0xB0008000. Now whether C would support this 32 bit address or not depends upon the memory model in use. For example, if we are using a large data model (compact, large, huge) the above address is acceptable. This is because in these models all pointers to data are 32 bits long. As against this, if we are using a small data model (tiny, small, medium) the above address won't work since in these models each pointer is 16 bits long.

What if we are working in small data model and still want to access the first byte of the upper 32K chunk of B block of video memory? In such cases both Microsoft C and Turbo C provide a keyword called far.

normally Pointers are 32 bit length. which are divided as segment and offset.

which are represent as

seg : off 0000:0000 . . . 8000:FFFF. . . . FFFF:FFFF

first 4 hexa digits are segment, last 4 hexa digits are offset

C Program will allocate 64KB (only one segment) memory for data Part (dynamic memory allocation, Local variables).

by using 16 bit we can access that memory that's called near pointer(16 bit pointer).

suppose we need more than 64KB memory to a program / we want to access a particular memory location (in TSR Program)

at the time we neet 32 bit pointer. through 32 bit pointer we can access any segment and offset address.

there are 2 types of 32 bit pointers 1. far pointer. 2. Huge Pointer.

In TSR programming normally we use far Pointer.

The Main Difference between Far and Huge Pointer is

Far pointers are not Normalized.

Huge pointers are Normalized.

First let me state this is based on my current understanding. If someone can update or improve my version I will be grateful:

1.For those in hurry far simply means "its not here find it somewhere else in memory" 2. far, near and huge and not a part of the C standard( am I right?)

-So the answer to this is: "It depends" Its highly compiler specific and platform(processor) specific.

Different processors have different ways to handle memory, and also different amount of memory.

when you say "far" you are just telling the compiler find or put this data somewhere else.But the compiler does the hard work for you: writing the instructions to change the segment properly and accessing the correct memory location for you..and that's because it knows the processor..

I think previous answers are mostly related to Pentium family of processors..and perhaps turbo C or similar compilers.But remember that is only one of them!

Below are the examples which perhaps can have different meanings in different places:

1. far int* near a; 2. far int* a; 3. far int *far a;

So the best way is to try it out with your compiler and find out.

Pointers can either be near, far, or huge. Near pointers refer to the current segment, so neither DS nor CS must be modified to dereference the pointer. They are the fastest pointers, but are limited to point to 64 kilobytes of memory (the current segment).

Far pointers contain the new value of DS or CS within them. To use them the register must be changed, the memory dereferenced, and then the register restored. They may reference up to 1 megabyte of memory. Note that pointer arithmetic (addition and subtraction) does not modify the segment portion of the pointer, only its offset. Operations which exceed the bounds of zero or 65535 (0xFFFF) will undergo modulo 64K operation just as any normal 16 bit operation.

For example, the code below will wrap around and overwrite itself: char far* myfarptr = (char far*) 0x50000000L ; unsigned long counter ; for(counter=0; counter<128*1024; counter++) // access 128K memory *(myfarptr+counter) = 7 ; // write all 7s into it

The moment counter becomes (0x10000), the resulting absolute address will roll over to 0x5000:0000.

Huge pointers are essentially far pointers, but are normalized every time they are modified so that they have the highest possible segment for that address. This is very slow but allows the pointer to point to multiple segments, and allows for accurate pointer comparisons, as if the platform were a flat memory model: It forbids the aliasing of memory as described above, so two huge pointers that reference the same memory location are always equal.

This near, far & huge pointers comes only in DOS or other, which have only less memory accessible. In DOS only 1 Mb ( 10,48,580 Bytes) of memory is accessible. Any of these memory location are accessed using CPU registers. Under DOS the CPU registers are only 16 bits long. Therefore, the minimum value present in a CPU register is 0, and maximum is 65,535. If we want to access more than 65,535 ? how to ? We can do this by - using two registers ( Segment & Offset ). Here memory is divided into 64 Kb locations or chunks. C Program will allocate 64KB (only one segment) memory for data Part (dynamic memory allocation, Local variables). by using 16 bit we can access that memory that's called near pointer(16 bit pointer). suppose we need more than 64KB memory to a program / we want to access a particular memory location (in TSR Program) at the time we need 32 bit pointer. through 32 bit pointer we can access any segment and offset address. The Main Difference between Far and Huge Pointer is Far pointers are not Normalized. Ex: TSR Programs use Far Pointers Huge pointers are Normalized. I think by this u can have brief idea about near, far & huge pointers. According to my knowledge far pointers are mainly used to build I/O commands.Such as you can write "printf" function by using far pointer void main() { char far *vdu=0xB8000000; *(vdu+2)='a'; } If you run this program in C,you can see "a" in the DOS screen There are 2 blocks of memory above the 640KB RAM of 64KB each. The first 32KB are used by MA, rest by CGA/EGA/VGA. The "far" pointer is used to access the VDU memory directly. Hence the displaying functions or output functions are not required.

On 16-bit operating systems, a huge pointer allows the developer to access a single block of memory that's larger than 64 kilobytes in size.

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With the development of Intel's 80386 and later microprocessors that support the flat memory addressing model these variants of pointers became unnecessary and most modern C compilers no longer support them. While many modern compilers do accept this syntax for backward compatibility with old source code, they ignore them and treat all three just as ordinary pointers that can access any location in memory.