Duff 的设备是如何工作的? [英] How does Duff's device work?
问题描述
我在 Duff 的设备上阅读了关于维基百科的文章,但我没有不明白.我真的很感兴趣,但我已经阅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,但让我试一试.(这在白板上要容易得多!)这是带有一些符号的 Wikipedia 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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