核心编程关于hookAPI的代码，有些地方不明白，有劳高手释疑

///////////////////////////////////////////////////////////////////////////////





// When an application runs on Windows 98 under a debugger, the debugger

// makes the module's import section point to a stub that calls the desired 

// function. To account for this, the code in this module must do some crazy 

// stuff. These variables are needed to help with the crazy stuff.





// The highest private memory address (used for Windows 98 only)

PVOID CAPIHook::sm_pvMaxAppAddr = NULL;

const BYTE cPushOpCode = 0x68;   // The PUSH opcode on x86 platforms





///////////////////////////////////////////////////////////////////////////////





// The head of the linked-list of CAPIHook objects

CAPIHook* CAPIHook::sm_pHead = NULL;





///////////////////////////////////////////////////////////////////////////////



CAPIHook::CAPIHook(PSTR pszCalleeModName, PSTR pszFuncName, PROC pfnHook, 

   BOOL fExcludeAPIHookMod) {



   if (sm_pvMaxAppAddr == NULL) {

      // Functions with address above lpMaximumApplicationAddress require

      // special processing (Windows 98 only)

      SYSTEM_INFO si;

      GetSystemInfo(&si);

      sm_pvMaxAppAddr = si.lpMaximumApplicationAddress;

   }



   m_pNext  = sm_pHead;    // The next node was at the head

   sm_pHead = this;        // This node is now at the head



   // Save information about this hooked function

   m_pszCalleeModName   = pszCalleeModName;

   m_pszFuncName        = pszFuncName;

   m_pfnHook            = pfnHook;

   m_fExcludeAPIHookMod = fExcludeAPIHookMod;

   m_pfnOrig            = GetProcAddressRaw(

      GetModuleHandleA(pszCalleeModName), m_pszFuncName);

   chASSERT(m_pfnOrig != NULL);  // Function doesn't exist



   if (m_pfnOrig > sm_pvMaxAppAddr) {

      // The address is in a shared DLL; the address needs fixing up 

      PBYTE pb = (PBYTE) m_pfnOrig;

      if (pb[0] == cPushOpCode) {

         // Skip over the PUSH op code and grab the real address

         PVOID pv = * (PVOID*) &pb[1];

         m_pfnOrig = (PROC) pv;

      }

   }



   // Hook this function in all currently loaded modules

   ReplaceIATEntryInAllMods(m_pszCalleeModName, m_pfnOrig, m_pfnHook, 

      m_fExcludeAPIHookMod);

}