Why do copyright symbols appear as question marks

I have encountered a problem before, when running some old programs, found that copyright and some of the watchmaking symbols do not appear. I don't know why. In the download of a plain text intel386 description of the time, with a text editor opened, found that some characters are not displayed, displayed as a repeating Chinese characters "which", with the hex editor to open the character code for the repeated c4.

So with the Dosbox,type text file, it is a horizontal line, as if it is a line of watchmaking. The Search DOS code page found the problem.

The code page used for DOS is 437, and the code page for the Chinese version of the system is 936.

ASCII each character code 8 bits, the first bit is not used. But in the 437 code page, 8 bits are all used. Therefore, the characters starting from 80h (1000 0000) are used for other characters in the system with code page 936, and the characters of natural character code C4 are not displayed.

Another: In the learning Assembly interrupted, found that the written interrupt exercises in DOSBox can not run. DOSBox also has no division overflow interrupt. Go to the virtual machine and it's done.

Why do copyright symbols appear as question marks