What is the difference between far pointers and near pointers?

by

On a 16-bit x86 segmented memory architecture, four registers are used to refer to the respective segments:

  • DS → data segment
  • CS → code segment
  • SS → stack segment
  • ES → extra segment

A logical address on this architecture is written segment:offset. Now to answer the question:

  • Near pointers refer (as an offset) to the current segment.

  • Far pointers use segment info and an offset to point across segments. So, to use them, DS or CS must be changed to the specified value, the memory will be dereferenced and then the original value of DS/CS restored. Note that pointer arithmetic on them doesn’t modify the segment portion of the pointer, so overflowing the offset will just wrap it around.

  • And then there are huge pointers, which are normalized to have the highest possible segment for a given address (contrary to far pointers).

On 32-bit and 64-bit architectures, memory models are using segments differently, or not at all.

More Related Contents:

  1. Warning in array of pointers "initialization from incompatible pointer type"
  2. Constant Value assignment to Integer Pointer in C works for this case
  3. Adjacency List for Graphs in C
  4. Assignment makes pointer from integer without a cast and vice versa in c
  5. C prog. Pointers and strings [closed]
  6. Clarifications about pointers use
  7. What is the difference between NULL, ‘\0’ and 0?
  8. Passing by reference in C
  9. returning a local variable from function in C [duplicate]
  10. How do you pass a function as a parameter in C?
  11. How can I get the size of an array from a pointer in C?
  12. Should I explicitly cast malloc()’s return value? [duplicate]
  13. Function pointers and address of a function
  14. Using pointers to remove item from singly-linked list
  15. Passing a 2D array of structs
  16. In C, can a const variable be modified via a pointer?
  17. Why can I change a local const variable through pointer casts but not a global one in C?
  18. Pointer Arithmetic: ++*ptr or *ptr++?
  19. Segmentation Fault when writing to a string [duplicate]
  20. Why is a point-to-volatile pointer, like “volatile int * p”, useful?
  21. In C, why can’t an integer value be assigned to an int* the same way a string value can be assigned to a char*?
  22. Swap nodes in a singly-linked list
  23. Segmentation fault occurring when modifying a string using pointers?
  24. Pointer to 2D arrays in C
  25. Pointer initialisation gives segmentation fault
  26. Printing pointers in C
  27. lvalue required as increment operand
  28. pointer default value .?
  29. Is it possible to store the address of a label in a variable and use goto to jump to it?
  30. C pointer notation compared to array notation: When passing to function

Leave a Comment