From:	NoEmailAds AT execpc DOT com (Chris Giese)
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Mapping a data structure into DOS memory space
Date:	Tue, 05 Dec 2000 19:33:18 GMT
Organization:	PROPULSION GROUP
References:	<NEBBKABEDKFPFBKEMBFOCEEFCAAA DOT alex AT compuweigh DOT com>
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"Alex Oleynikov" <alex AT compuweigh DOT com> wrote:

>For that matter I will use a 128K NVRAM chip, which is mapped into a UMB
>memory space through a 32K frame. The data I have to store there is
>organized as a set of structures.
>Now I have a problem - how can I neatly access that data from my program? As
>far as I understand, there is no way to tell the linker to use the physical
>address for relocation of the data structures, since it is below the 1MB
>limit. I have investigated different possibilities of accessing that data
>( through _farX functions, or using "Fat DS" trick ), but they all
>effectively limit my ability to use trivial "dot-style" variable access
>within the structures.


How about this:


#include <stdio.h> /* printf() */
#include <sys/nearptr.h> /* __djgpp_conventional_base, __djgpp_nearptr_enable() */
#include <crt0.h> /* crtNNN */

typedef unsigned char	u8;
typedef unsigned short	u16;
typedef unsigned long	u32;

struct nvram_struct
{
	volatile u8 byte	__attribute__((packed));
	volatile u16 word	__attribute__((packed));
	volatile u32 dword	__attribute__((packed));
};
/*****************************************************************************
*****************************************************************************/
int main(void)
{
	struct nvram_struct *foo;

	if(!(_crt0_startup_flags & _CRT0_FLAG_NEARPTR))
	{
		if(!__djgpp_nearptr_enable())
		{
			printf("Could not enable nearptr access\n");
			return 1;
		}
	}
/* the NVRAM is at linear address 000D0000 (real-mode address D000:0000) */
	foo = (struct nvram_struct *)(0xD0000 + __djgpp_conventional_base);
/* write the 8-bit value */
	foo->byte = 0;
/* read values */
	printf("byte = 0x%X\n", foo->byte);
	printf("word = 0x%X\n", foo->word);
	printf("dword = 0x%lX\n", foo->dword);
	return 0;
}

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