思考：
每个线程在执行的时候，都会独自占用一个CPU,当系统中的线程数量 > CPU的数量时，就会存在多个线程共用一个CPU
的情况。但CPU每次只能运行一个线程，Windows每隔20毫秒会进行线程的切换，那比如线程A执行到地址：0x2345678
eax:1 ecx:2 edx:3 ebx:4...还有eflag标志寄存器中的值等等。。。
此时，线程执行时间到了，被切换到了线程B。。。。当线程B的时间片也到了，再切换会线程A时，系统是如何知道该
从哪个地址开始执行呢？被切换前用到的各种寄存器的值该如何恢复呢？
CONTEXT：
该结构包含了特定处理器的寄存器数据。
typedef struct _CONTEXT {
//
// The flags values within this flag control the contents of
// a CONTEXT record.
//
// If the context record is used as an input parameter, then
// for each portion of the context record controlled by a flag
// whose value is set, it is assumed that that portion of the
// context record contains valid context. If the context record
// is being used to modify a threads context, then only that
// portion of the threads context will be modified.
//
// If the context record is used as an IN OUT parameter to capture
// the context of a thread, then only those portions of the thread's
// context corresponding to set flags will be returned.
//
// The context record is never used as an OUT only parameter.
//
DWORD ContextFlags;
//
// This section is specified/returned if CONTEXT_DEBUG_REGISTERS is
// set in ContextFlags. Note that CONTEXT_DEBUG_REGISTERS is NOT
// included in CONTEXT_FULL.
//
DWORD Dr0;
DWORD Dr1;
DWORD Dr2;
DWORD Dr3;
DWORD Dr6;
DWORD Dr7;
//
// This section is specified/returned if the
// ContextFlags word contians the flag CONTEXT_FLOATING_POINT.
//
FLOATING_SAVE_AREA FloatSave;
//
// This section is specified/returned if the
// ContextFlags word contians the flag CONTEXT_SEGMENTS.
//
DWORD SegGs;
DWORD SegFs;
DWORD SegEs;
DWORD SegDs;
//
// This section is specified/returned if the
// ContextFlags word contians the flag CONTEXT_INTEGER.
//
DWORD Edi;
DWORD Esi;
DWORD Ebx;
DWORD Edx;
DWORD Ecx;
DWORD Eax;
//
// This section is specified/returned if the
// ContextFlags word contians the flag CONTEXT_CONTROL.
//
DWORD Ebp;
DWORD Eip;
DWORD SegCs; // MUST BE SANITIZED
DWORD EFlags; // MUST BE SANITIZED
DWORD Esp;
DWORD SegSs;
//
// This section is specified/returned if the ContextFlags word
// contains the flag CONTEXT_EXTENDED_REGISTERS.
// The format and contexts are processor specific
//
BYTE ExtendedRegisters[MAXIMUM_SUPPORTED_EXTENSION];
} CONTEXT;
获取线程CONTEXT结构：
//挂起线程
SuspendThread(线程句柄);
CONTEXT context
//设置要获取的类型
context.ContextFlags = CONTEXT_CONTROL;
//获取
BOOL ok = ::GetThreadContext(hThread,&context);
//设置
context.Eip = 0x401000;
SetThreadContext(hThread,&context);
ResumeThread(hThead);