达夫的设备是如何工作的? [英] How does Duff's device work?

问题描述

我在 Duff 的设备上阅读了 维基百科上的文章，但我不知道不明白.我真的很感兴趣，但我已经读了几次那里的解释，但我仍然不明白 Duff 的设备是如何工作的.

I've read the article on Wikipedia on the Duff's device, and I don't get it. I am really interested, but I've read the explanation there a couple of times and I still don't get it how the Duff's device works.

更详细的解释是什么?

推荐答案

其他地方有一些很好的解释，但让我尝试一下.(这在白板上容易得多！)这是带有一些符号的维基百科示例.

There are some good explanations elsewhere, but let me give it a try. (This is a lot easier on a whiteboard!) Here's the Wikipedia example with some notations.

假设您要复制 20 个字节.第一遍程序的流程控制为:

Let's say you're copying 20 bytes. The flow control of the program for the first pass is:

int count;                        // Set to 20
{
    int n = (count + 7) / 8;      // n is now 3.  (The "while" is going
                                  //              to be run three times.)

    switch (count % 8) {          // The remainder is 4 (20 modulo 8) so
                                  // jump to the case 4

    case 0:                       // [skipped]
             do {                 // [skipped]
                 *to = *from++;   // [skipped]
    case 7:      *to = *from++;   // [skipped]
    case 6:      *to = *from++;   // [skipped]
    case 5:      *to = *from++;   // [skipped]
    case 4:      *to = *from++;   // Start here.  Copy 1 byte  (total 1)
    case 3:      *to = *from++;   // Copy 1 byte (total 2)
    case 2:      *to = *from++;   // Copy 1 byte (total 3)
    case 1:      *to = *from++;   // Copy 1 byte (total 4)
           } while (--n > 0);     // N = 3 Reduce N by 1, then jump up
                                  //       to the "do" if it's still
    }                             //        greater than 0 (and it is)
}

现在，开始第二遍，我们只运行指定的代码:

Now, start the second pass, we run just the indicated code:

int count;                        //
{
    int n = (count + 7) / 8;      //
                                  //

    switch (count % 8) {          //
                                  //

    case 0:                       //
             do {                 // The while jumps to here.
                 *to = *from++;   // Copy 1 byte (total 5)
    case 7:      *to = *from++;   // Copy 1 byte (total 6)
    case 6:      *to = *from++;   // Copy 1 byte (total 7)
    case 5:      *to = *from++;   // Copy 1 byte (total 8)
    case 4:      *to = *from++;   // Copy 1 byte (total 9)
    case 3:      *to = *from++;   // Copy 1 byte (total 10)
    case 2:      *to = *from++;   // Copy 1 byte (total 11)
    case 1:      *to = *from++;   // Copy 1 byte (total 12)
           } while (--n > 0);     // N = 2 Reduce N by 1, then jump up
                                  //       to the "do" if it's still
    }                             //       greater than 0 (and it is)
}

现在，开始第三遍:

int count;                        //
{
    int n = (count + 7) / 8;      //
                                  //

    switch (count % 8) {          //
                                  //

    case 0:                       //
             do {                 // The while jumps to here.
                 *to = *from++;   // Copy 1 byte (total 13)
    case 7:      *to = *from++;   // Copy 1 byte (total 14)
    case 6:      *to = *from++;   // Copy 1 byte (total 15)
    case 5:      *to = *from++;   // Copy 1 byte (total 16)
    case 4:      *to = *from++;   // Copy 1 byte (total 17)
    case 3:      *to = *from++;   // Copy 1 byte (total 18)
    case 2:      *to = *from++;   // Copy 1 byte (total 19)
    case 1:      *to = *from++;   // Copy 1 byte (total 20)
           } while (--n > 0);     // N = 1  Reduce N by 1, then jump up
                                  //       to the "do" if it's still
    }                             //       greater than 0 (and it's not, so bail)
}                                 // continue here...

现在复制了 20 个字节.

20 bytes are now copied.

注意:原始 Duff 的设备(如上所示)复制到位于 to 地址的 I/O 设备.因此，没有必要增加指针*to.在两个内存缓冲区之间复制时，您需要使用 *to++.

Note: The original Duff's Device (shown above) copied to an I/O device at the to address. Thus, it wasn't necessary to increment the pointer *to. When copying between two memory buffers you'd need to use *to++.

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