text: // Memory layout
  
  #define EXTMEM  0x100000            // Start of extended memory
  #define PHYSTOP 0xE000000           // Top physical memory
  #define DEVSPACE 0xFE000000         // Other devices are at high addresses
  
  // Key addresses for address space layout (see kmap in vm.c for layout)
  #define KERNBASE 0x80000000         // First kernel virtual address
  #define KERNLINK (KERNBASE+EXTMEM)  // Address where kernel is linked
  
  /*
    虚拟地址 = 0x80100000 + 偏移量
    物理地址 = 0x100000 + 偏移量
           = 0x100000 + 虚拟地址 - 0x80100000
           = 虚拟地址 - 0x80000000
   */
  #define V2P(a) (((uint) (a)) - KERNBASE)
  #define P2V(a) (((void *) (a)) + KERNBASE)
  
  #define V2P_WO(x) ((x) - KERNBASE)    // same as V2P, but without casts
  #define P2V_WO(x) ((x) + KERNBASE)    // same as P2V, but without casts
  