far pointer
{{short description |Address to memory outside immediate vicinity}}
{{Use American English |date=February 2024}}
{{Use mdy dates |date=February 2024}}
{{More citations needed |date=February 2024}}
In a segmented architecture computer, a far pointer is a pointer to memory in a specific context, such as a segment selector making it possible to point to addresses outside of the default segment.
Comparison and arithmetic on far pointers is problematic: there can be several different segment-offset address pairs pointing to one physical address.
In 16-bit x86
{{further|x86 memory models}}
For example, in an Intel 8086, as well as in later processors running 16-bit code, a far pointer has two parts: a 16-bit segment value, and a 16-bit offset value. A linear address is obtained by shifting the binary segment value four times to the left, and then adding the offset value. Hence the effective address is 21 bits. There can be up to 4096 different segment-offset address pairs pointing to one physical address. To compare two far pointers, they must first be normalized to a form with only one representation address. Such a normalized form may be one that minimizes the segment (maximizing the offset), minimizes the offset (maximizing the segment), or converts the 2-part segmented address to a (20-bit) linear representation. One commonly used normalized form minimizes the offset part of the address to a value less than 16 (10 hex): such a normalization can be computed simply by taking the low-order 4 bits of the unnormalized offset as the new offset, and adding to the unnormalized segment the unnormalized offset shifted right by 4 bits.
On C compilers targeting the 8086 processor family, far pointers were declared using a non-standard {{code |lang=C |far}} qualifier; e.g., {{code |lang=C |char far *p;}} defined a far pointer to a char. The difficulty of normalizing far pointers could be avoided with the non-standard {{code |lang=C |huge}} qualifier. On other compilers it was done using an equally non-standard {{code |lang=C |__far}} qualifier.
Example of far pointer:
- include
int main() {
char far *p =(char far *)0x55550005;
char far *q =(char far *)0x53332225;
*p = 80;
(*p)++;
printf("%d",*q);
return 0;
}
:Output of the following program: 81; Because both addresses point to same location.
:Physical Address = (value of segment register) * 0x10 + (value of offset).
:Location pointed to by pointer {{code |lang=C |p}} is : 0x5555 * 0x10 + 0x0005 = 0x55555
:Location pointed to by pointer {{code |lang=C |q}} is : 0x5333 * 0x10 + 0x2225 = 0x55555
:So, {{code |lang=C |p}} and {{code |lang=C |q}} both point to the same location 0x55555.
Notes
{{reflist|group=Note|refs=
Early x86 processors only had a 20-bit address bus so results above 1MiB wrapped around to zero, discarding the carry bit. PC's using the 80286 or newer, which had the necessary address lines, implemented the A20 gate to toggle this behavior for backwards compatibility with older software.
}}
References
{{reflist |refs=
{{cite Q |Q1120519 |title=Pointers in Far Memory |first1=Ethan L. |last1=Miller |first2=George |last2=Neville-Neil |first3=Achilles |last3=Benetopoulos |first4=Pankaj |last4=Mehra |first5=Daniel |last5=Bittman |date=December 2023 |journal=Communications of the ACM |volume=66 |number=12 |url=
https://m-cacm.acm.org/magazines/2023/12/278153-pointers-in-far-memory/fulltext |access-date=2024-02-11 }}
}}
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