...NEAR FAR POINTER AND FAQS ON POINTERS

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Answer by Vadiraj Purohit
Submitted on 11/11/2004
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near Pointers: Pointers to near objects are stored using two bytes (16 bits). The bytes are stored little endian or low to high order. In the tiny memory model, near pointers contain a 16-bit memory address. In all other memory models, near pointers contain a 14-bit offset in bits 0-13 and a 2-bit DPP selector in bits 14 and 15. far Pointers: Pointers to far objects are stored using four bytes (32 bits). The bytes are stored little endian or low to high order. The first word contains the 14-bit memory offset (bits 14 and 15 are always 0). The second word contains the page number (or segment number for function pointers). The memory address is calculated as follows: Variable Address = (Page * 0x4000L) + Offset Function Address = (Segment * 0x10000L) + Offset

Answer by Monu
Submitted on 8/28/2005
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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

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Question by SIRISHA
Submitted on 9/2/2003
Related FAQ: comp.compilers monthly message and Frequently Asked Questions
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WHAT IS NEAR FAR POINTER AND FAQS ON POINTERS

Answer by hemanthdas
Submitted on 8/31/2004
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i like to have the details about pointers

Answer by c_razy
Submitted on 1/29/2005
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far near int a; pointer to a object beyond the current segment. the pointer itself resides in the current segment. normally far int a; etc. normally are always "near far" pointers . njoy!

Answer by anju
Submitted on 6/20/2005
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czxcxz

Answer by Shri
Submitted on 9/22/2005
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near pointer are the one which points to the memory locations in the code segment only.Far pointers are the one which point to memory locations in different segments.

Answer by rahul
Submitted on 1/25/2004
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Answer by wind
Submitted on 1/4/2006
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footballer of England....

Answer by ganesh
Submitted on 4/3/2006
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what is near & far pointer?

Answer by SunnyPalSingh
Submitted on 4/25/2006
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In a segmented architecture computer, a far pointer is a pointer which includes a segment number, making it possible to point to addresses outside of the current segment.

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