Near Pointer In C

Question

Hi Friends....... Can you explain me the concept of near pointer? Thanks.............

Sam Hobbs · Answer

You can ignore the terms near and far for addresses. They do not apply to operating systems that support multiple processes or that support virtual storage. The segmented memory model is usually called real-address mode or real mode by Intel. Near and far apply only to real mode. Addresses in the real mode are 20 bits which means that real mode is limited to exactly 1 MB (MB not GB) including operating system and real mode does not support paging (virtual storage). DOS had a way to use an additional 64KB but that is getting more technical than you need to be concerned about. In the Windows SDK header files NEAR and FAR are ignored. In the Windows SDK strings are usually called LPSTRING and the L means long which means far but the long part is archaich. Note that even the latest version (Windows 3.1) of 16-bit Windows does not use real mode and therefore near and far does not apply to it either. I know that when I was developing a Windows driver for Windows 3.1 I could not figure out how to address the memory in the 640KB to 1MB range which is generally reserved for device drivers. I don't remember for sure; it might have been that I needed to do that for a configuration program that was separate from the actual device driver. Windows protected that area of memory and it could not have protected it using real mode.

Vulpes · Answer

In x86 systems which use a segmented memory model (such as 16 bit Windows) a near pointer is a pointer to an address in the current segment (64KB of memory). If you don't know what a segmented memory model is I'd read this article: http://en.wikipedia.org/wiki/Intel_Memory_Model