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 );
}