Directx10和Directx11很多resource类都有个Map()函数，个人感觉它是一个蛮重要的CPU和GPU沟通桥梁。下面以ID3D11DeviceContext::Map()为例讲下。

ID3D11DeviceContext::Map官方解释是：ID3D11DeviceContext::Map Method
  Get a pointer to the data contained in a subresource, and deny the GPU access to that subresource.

Syntax
HRESULT Map(
    ID3D11Resource *pResource,
    UINT Subresource,
    D3D11_MAP MapType,
    UINT MapFlags,
    D3D11_MAPPED_SUBRESOURCE *pMappedResource
);

个人理解总结是当需要用CPU读写（GPU的）subresouce（最常用如buffer）时，就用Map()得到该subresource的pointer（D3D11_MAPPED_SUBRESOURCE*），然后将D3D11_MAPPED_SUBRESOURCE::pData强制转换成CPU理解的类型（struct，class），CPU就可以对得到的struct*或class*读写了。

附录D3D11_MAPPED_SUBRESOURCE结构体
typedef struct D3D11_MAPPED_SUBRESOURCE
    {
    void *pData;
    UINT RowPitch;
    UINT DepthPitch;
    }  D3D11_MAPPED_SUBRESOURCE;

Map函数使用的几个经典场景1：debug时用CPU读写GPU之前读写的buffer（比如GPU计算输出的结果），然后再通过CPU输出来检验对错。
//将GPU读或读写的buffer *pGBuffer拷贝到可给CPU读的buffer并返回，以便调试
ID3D11Buffer* GPUBaseApp::CreateAndCopyToDebugBuf(ID3D11Buffer* pGBuffer)
{
 D3D11_BUFFER_DESC bufferDesc;
 ZeroMemory(&bufferDesc,sizeof(bufferDesc));
 pGBuffer->GetDesc(&bufferDesc);
 bufferDesc.CPUAccessFlags=D3D11_CPU_ACCESS_READ;
 bufferDesc.Usage=D3D11_USAGE_STAGING;
 bufferDesc.BindFlags=0;
 bufferDesc.MiscFlags=0;
 
 ID3D11Buffer* pDebugBuffer=NULL;

if( SUCCEEDED(m_pDevice->CreateBuffer(&bufferDesc,NULL,&pDebugBuffer)) )
 {
  m_pDeviceContext->CopyResource(pDebugBuffer,pGBuffer);
 }
 return pDebugBuffer;
}

HRESULT Function()
{
      
       //==============================================
      /

//===============================================
 //看Direct Compute的计算结果
 ID3D11Buffer* debugOutBuffer=NULL;
 ID3D11Buffer* debugInBuffer=NULL;

debugOutBuffer=CreateAndCopyToDebugBuf(m_pOutBuffer);
 debugInBuffer=CreateAndCopyToDebugBuf(m_pInputBuffer);

VoronoiOutBufType* pOut=NULL;
 VoronoiInBufType* pIn=NULL;

D3D11_MAPPED_SUBRESOURCE mappedOutResource;
 D3D11_MAPPED_SUBRESOURCE mappedInResource;

m_pDeviceContext->Map(debugOutBuffer,0,D3D11_MAP_READ,0,&mappedOutResource);
 pOut=(VoronoiOutBufType*)mappedOutResource.pData;

m_pDeviceContext->Map(debugInBuffer,0,D3D11_MAP_READ,0,&mappedInResource);
 pIn=(VoronoiInBufType*)mappedInResource.pData;

//printf("OutBuffer的结果如下:/n");
 //for(int i=0;i<30;i++)
 // printf("%d %f/n",pOut[i].index,pOut[i].dist);

m_pDeviceContext->Unmap(debugOutBuffer,0);
 SAFE_RELEASE(debugOutBuffer);

m_pDeviceContext->Unmap(debugInBuffer,0);
 SAFE_RELEASE(debugInBuffer);

//===============================================

//==============================================
      /

}

Map函数使用的几个经典场景2：更新GPU Shader中Constant buffer，把CPU中新的数据struct CB_MouseInfo mouseInfo写入GPU中Constant Buffer。
struct CB_MouseInfo
{
 UINT x;
 UINT y;
 UINT btDown;
};

CB_MouseInfo mouseInfo；

HRESULT GPUBaseApp::UpDateMouseInfoCB()
{
 HRESULT hr=S_OK;

D3D11_MAPPED_SUBRESOURCE subData;

ZeroMemory(&subData,sizeof(subData));

m_pDeviceContext->Map(m_pConstantBuffer,0,D3D11_MAP_WRITE_DISCARD,0,&subData);

memcpy(subData.pData,&mouseInfo,sizeof(CB_MouseInfo));

m_pDeviceContext->Unmap(&subData,0);

return hr;
}

注意点，当调用了Map()，最后记得调用ID3D11DeviceContext::Unmap Method使得GPU重新获取对该resouce的读写权。
Invalidate the pointer to a resource and re-enable the GPU's access to that resource.

Syntax
void Unmap(
    ID3D11Resource *pResource,
    UINT Subresource
);

Parameters：

pResource 
ID3D11Resource 
A pointer to a ID3D11Resource interface.

Subresource 
UINT 
A subresource to be unmapped.