near vs. far vs. huge pointers

Question

3.00/5 (2 votes)

See more:

C

Hi,
What is the difference between "near", "far" and "huge" pointers?

Posted 19-Aug-10 6:24am

Yuvaraj Gogoi

Add a Solution

2 solutions

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Aescleal · Accepted Answer · 2010-08-19T06:38:00

The differences are only relevant on 16 bit [1] intel architectures and stopped being significant about 15 years ago.

In a nutshell virtual addresses on an Intel x86 chip have two components - a selector and an offset. The selector is an index into a table of base addresses [2] and the offset is added onto that base address. This was designed to let the processor access 20 bit (on a 8086/8, 186), 30 bit (286) or 46 bit (386 and later) virtual address spaces without needing registers that big.

So...

'near' pointers don't have a selector - they have an implied selector. They can access 64k off the virtual address space on a 286 and earlier and 4Gb of the addresss space on a 386 or later.

'far' pointers have an explicit selector. However when you do pointer arithmetic on them the selector isn't modified.

'huge' pointers have an explicit selector. When you do pointer arithmetic on them though the selector can change.

'based' pointers... oh, you didn't ask about them.

Basically ignore this. It's not worth the hassle to learn about unless you're an archaeologist.

Cheers,

Ash

[1] Okay, modern intel processors have all the old segmented guff still hanging about but the OS tends to set the selectors once and then forget about them.

[2] In real mode there is no table of base addresses, the address is just the selector multipled by 16.

Nyarost · Accepted Answer · 2010-08-19T06:35:00

Solution 1

Google it

Posted 19-Aug-10 6:35am

Nyarost

Comments

Yuvaraj Gogoi 19-Aug-10 12:43pm

I have google it! But I didn't get much of the difference between far and huge pointers,except that far is not normalized and huge is normalized!

Aescleal 19-Aug-10 18:40pm

Normalisation is the only difference between far and huge pointers. If a pointer is normalised it means that the pointer contains the minimum possible offset from the selector it refers to. Exactly what that means depends on the OS, the C compiler, phase of the moon...

As a quick example real mode programs have selectors with a base address 16 bytes apart so a huge pointer in a DOS program will never have an offset greater than 15. For contiguous blocks of memory in 16 bit windows programs huge pointers have offsets 64k apart so the offset can be up 64k - 1.

Yuvaraj Gogoi 15-Oct-10 6:15am

Thanks for the answar!