如何快速访问ISA总线端口

精简ISA总线是英创工控主板的特色之一，我们基于ISA总线推出了多串口、多网口、多CAN接口等扩展模块，我们的很多用户也基于ISA总线设计了自己的专有扩展单元并取得了成功。为了充分发挥ISA总线的性能，对于ISA总线的访问，我们除了提供最基本的设备驱动API操作函数外，还先后增加了ISA总线的块读写操作方法和ISA总线的DMA操作方法。当ISA总线以DMA方式进行数据传输时，在最大限度的利用ISA总线带宽的同时，又减少了ISA操作占用CPU的时间，所以利用DMA是进行批量数据传输时的首选方式。

在实际的应用中，除了成批量的数据访问外，也可能会存在对外设进行频繁的字或字节访问。英创主板的ISA总线周期通常在200ns左右，而应用程序调用一次设备驱动程序API(WriteFile，ReadFile)花费的时间却需要数微秒的时间，这显然大大降低了对外设单字（或单字节）的访问效率。为了解决这一问题，我们利用了WinCE的虚拟地址映射技术，在ISA驱动程序中实现了在使用ISA的应用进程地址空间内分配一段虚拟地址空间，并将其与ISA接口的物理地址空间进行了绑定。简单来讲就是实现了在WinCE应用程序中可以直接访问ISA总线的外设地址空间，从使用的角度看，我们实现了以下5个函数。ISAMmMap用于获得ISA总线的基地址，其余4个函数分别为字读/写和字节读/写操作函数。

HANDLE       ISAMmMap(HANDLE hISA);                                    // 映射ISA总线物理地址空间 BYTE          ISARead8(HANDLE hMmMap, DWORD dwPortOffset);               // 读单字节 void           ISAWrite8(HANDLE hMmMap, DWORD dwPortOffset, BYTE ucValue);   // 写单字节 WORD       ISARead16(HANDLE hMmMap, DWORD dwPortOffset);                    // 读单字 void           ISAWrite16(HANDLE hMmMap, DWORD dwPortOffset, WORD wValue);       // 写单字

下面是上述5个函数实现的源代码，在ISAMmMap函数中调用DeviceIoControl，通过IOCTL_VIRTUAL_COPY_EX命令获取ISA总线的基地址。

// Function: Get the base address of ISA Port // Input:    hISA: Handle of ISA1: // Return:   Base address of the ISA Port HANDLE ISAMmMap(HANDLE hISA) {        DWORD       dwMemBase;        if(DeviceIoControl(hISA,         //打开“ISA1:”返回的Handler               IOCTL_VIRTUAL_COPY_EX,      // IOCTL命令码                   NULL,0,                                // 不使用输入参数               &dwMemBase, sizeof(DWORD),  // 得到ISA基地址                  NULL, NULL))               return (HANDLE)dwMemBase;        return NULL; } // Function: read a byte from a port on ISA bus // Input: hMmMap: Base address of the ISA Port //         dwPortOffset = 0, 1, .. 255, address of port on ISA // Return: the byte data read BYTE ISARead8(HANDLE hMmMap, DWORD dwPortOffset) {        WORD   *pPortAddr;        WORD   wValue;        dwPortOffset &= 0xff;        dwPortOffset <<= 1;               // D[0..7] <=> A[1..8] in AD-muxed mode        pPortAddr = (WORD*)((DWORD)hMmMap + dwPortOffset);        wValue = *pPortAddr;        return (BYTE)wValue; } // Function: write a byte to a port on ISA bus // Input: hMmMap: Base address of the ISA Port// //        dwPortOffset = 0, 1, .. 255, address of port on ISA //        ucValue = the byte data to be written void ISAWrite8(HANDLE hMmMap, DWORD dwPortOffset, BYTE ucValue) {        WORD   *pPortAddr;        dwPortOffset &= 0xff;        dwPortOffset <<= 1;               // D[0..7] <=> A[1..8] in AD-muxed mode        pPortAddr = (WORD*)((DWORD)hMmMap + dwPortOffset);        *pPortAddr = (WORD)ucValue;    } // Function: read a word from a port on ISA bus // Input: hMmMap: Base address of the ISA Port //        dwPortOffset = 0, 2, 4, .. 254, address of port on ISA // Return: the word data read WORD ISARead16(HANDLE hMmMap, DWORD dwPortOffset) {        DWORD       *pPortAddr;        DWORD       dwValue;        dwPortOffset &= 0xFE;           // 2-byte alignment        dwPortOffset <<= 1;               // D[0..7] <=> A[1..8] in AD-muxed mode        pPortAddr = (DWORD*)((DWORD)hMmMap + dwPortOffset);        dwValue = *pPortAddr;        // the high-byte of data is at value[23..16]        return (WORD)(((dwValue >> 8) & 0xFF00) | (dwValue & 0xFF));      } // Function: write a word to a port on ISA bus // Input: hMmMap: Base address of the ISA Port //        dwPortOffset = 0, 2, 4, .. 254, address of port on ISA //        wValue = the word data to be written void ISAWrite16(HANDLE hMmMap, DWORD dwPortOffset, WORD wValue) {        DWORD       *pPortAddr;        DWORD       dwValue;        dwPortOffset &= 0xFE;           // 2-byte alignment        dwPortOffset <<= 1;               // D[0..7] <=> A[1..8] in AD-muxed mode        pPortAddr = (DWORD*)((DWORD)hMmMap + dwPortOffset);        dwValue = wValue;        // dispatch high-byte of data to value[23..16]        *pPortAddr = ((dwValue << 8) & 0x00ff0000) | (dwValue & 0x000000ff); }

我们在ESM3352上，分别测试了利用地址映射方法直接访问ISA外设地址和调用设备驱动API函数读写ISA地址，测得的结果是当进行字(WORD)读写时，应用程序直接访问ISA外设地址比调用设备驱动API函数快10倍以上，当时行字节(BYTE)读写时，应用程序直接访问ISA外设地址比调用设备驱动API函数快15倍以上。

下面是测试程序源代码：

int _tmain(int argc, _TCHAR* argv[]) {        HANDLE       hISA, hMmMap;        BYTE             ucValue;        WORD          wValue;        DWORD       dwStartTick, dwEndTick, i, k, cnt=2000;        hISA = CreateFile(_T("ISA1:"),        // name of device               GENERIC_READ|GENERIC_WRITE,         // desired access               FILE_SHARE_READ|FILE_SHARE_WRITE,      // sharing mode               NULL,                                               // security attributes (ignored)               OPEN_EXISTING,                             // creation disposition               FILE_FLAG_RANDOM_ACCESS,                    // flags/attributes               NULL);        hMmMap = ISAMmMap(hISA);        if(hMmMap == NULL)               return -1;        printf("ISA read/write speed test\r\n");        printf("Memory Map VS Device Driver API\r\n");        dwStartTick = GetTickCount();        for(i=0; i               for(k=0; k<256; k++)               {                      ISAWrite8(hMmMap, k, 0x55);                      ucValue = ISARead8(hMmMap, k);                     }        dwEndTick = GetTickCount();        printf("1. Memory map BYTE read/write %d KBytes, Take time:%d ms\r\n", 256 * 2 *cnt / 1024, dwEndTick - dwStartTick);        dwStartTick = GetTickCount();        for(i=0; i               for(k=0; k<256; k++)               {                      ISA_WriteUchar( hISA, k, 0x55 );                      ISA_ReadUchar( hISA, k, &ucValue );               }               dwEndTick = GetTickCount();        printf("2. Device driver API BYTE read/write %d KBytes, Take time:%d ms\r\n", 256 * 2 * cnt/ 1024, dwEndTick - dwStartTick);        cnt /= 2;        dwStartTick = GetTickCount();        for(i=0; i               for(k=0; k<256; k++)               {                      ISAWrite16(hMmMap, k, 0x55AA);                      wValue = ISARead16(hMmMap, k);                    }        dwEndTick = GetTickCount();        printf("3. Memory map WORD read/write %d KBytes, Take time:%d ms\r\n", 256 * 4 * cnt / 1024, dwEndTick - dwStartTick);        dwStartTick = GetTickCount();        for(i=0; i               for(k=0; k<256; k++)               {                      ISA_WriteWord( hISA, k, 0x55AA );                      ISA_ReadWord( hISA, k, &wValue );               }               dwEndTick = GetTickCount();        printf("4. Device driver API WORD read/write %d KBytes, Take time:%d ms\r\n", 256 * 4 * cnt / 1024, dwEndTick - dwStartTick);        CloseHandle(hISA);        return 0; }

　　下面是在ESM3352上两种访问方式的测试结果：

要实现应用程序直接访问ISA外设地址空间需要更新ISA驱动程序，需要的用户可与英创联系。