mem_map初始化

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#define LOW_MEM 0x100000                       //1 MB
#define PAGING_MEMORY (15*1024*1024)           //只管理1MB以上的空间，只有15MB
#define PAGING_PAGES (PAGING_MEMORY>>12)       //15 MB的页数
#define MAP_NR(addr) (((addr)-LOW_MEM)>>12)
#define USED 100
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static long HIGH_MEMORY= 0;
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static unsigned char mem_map [PAGING_PAGES]= {0,};  //注意每个元素一个字节
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void mem_init(long start_mem, long end_mem)  //start_mem传进来的值为buffer_memory_end
{
    int i;
    HIGH_MEMORY= end_mem;
    for (i=0;i<PAGING_PAGES;i++)
        mem_map[i]= USED;
    i= MAP_NR(start_mem);                        //start_mem为6 MB(虚拟盘之后)
    end_mem -= start_mem;
    end_mem >>= 12;                              //16 MB的页数
    while (end_mem-->0)
        mem_map[i++]=0;
}