来源于QNX IDE
实质就是将物理内存physical映射到程序中，我们通过指针来取这个物理地址的内容；
这样做的好处是：大家可以共用这个物理地址的内容。
1、mmap_device_memory()函数
Map a device's physical memory into a process's address space
实质:将物理内存映射到程序中指针
#include <sys/mman.h>
void * mmap_device_memory( void * addr,
size_t len,
int prot,
int flags,
uint64_t physical );Arguments:
1）addr
NULL, or a pointer to where you want to map the object in the calling process's address space. //可以写为0
2）len
The number of bytes you want to map into the caller's address space. It can't be 0. //例如可以写为：0x1000，写一个很大的数2^12 ==4096
3）prot
The access capabilities that you want to use for the memory region being mapped. You can use a combination of at least the following protection bits, as defined in <sys/mman.h>:
访问内存的权限功能。
- PROT_EXEC — the region can be executed.
- PROT_NOCACHE — disable the caching of the region (e.g. to access dual-ported memory).//禁用区域缓存
- PROT_NONE — the region can't be accessed.
- PROT_READ — the region can be read.
- PROT_WRITE — the region can be written.
4）flags
Specifies further information about handling the mapped region. You can use the following flag://可以置为0
- MAP_FIXED — map the object to the address specified by addr. If this area is already mapped, the call changes the existing mapping of the area.
A memory area being mapped with MAP_FIXED is first unmapped by the system using the same memory area. See munmap() for details.
-
Use MAP_FIXED with caution. Not all memory models support it. In general, you should assume that you can MAP_FIXED only at an address (and size) that a call to mmap() without MAP_FIXED returned.
-
A memory area being mapped with MAP_FIXED is first unmapped by the system using the same memory area. See munmap() for details.
This function already uses MAP_SHARED ORed with MAP_PHYS (see mmap() for a description of these flags).
5）physical
The physical address of the memory to map into the caller's address space.//物理地址，例如：0x80000000
Returns:
The address of the mapped-in object, or MAP_FAILED if an error occurs (errno is set). // 返回值与参数1 addr 应该是一样的，所以，addr置0了。
例子：
/* Map in the physical memory; 0xb8000 is text mode VGA video memory */
ptr = mmap_device_memory( 0, len, PROT_READ|PROT_WRITE|PROT_NOCACHE, 0, 0xb8000 );
if ( ptr == MAP_FAILED ) {
perror( "mmap_device_memory for physical address 0xb8000 failed" );
exit( EXIT_FAILURE );
}