What's the purpose of using assembly language inside a C program? Compilers are able to generate assembly language already. In what cases would it be better to write assembly than C? Is performance a consideration?
There were various reasons to write inline assemblies in C. We can simply categorize the reasons into necessary and unnecessary.
For the reasons of unnecessary, possibly be:
I consider above as unnecessary because sometime they can be discard or implemented through pure C. For example of platform compatibility, you can totally implement particular version for each platform, however, use inline assemblies might reduce the effort. Here we are not going to talk too much about the unnecessary reasons.
For necessary reasons, they possibly be:
These reasons considered necessary, because of they are almost not possibly done with pure C language. For example, in old DOSes, software interrupt INT21 was not reentrantable. If you want to write a Virtual Dirve fully use INT21 supported by the compiler, it was impossible to do. In this situation, you would need to hook the original INT21, and make it reentrantable. However, the compiled code wraps your every call with prolog/epilog. Thus, you can never break something restricted, or you just crashed the code. You can try any of trick by using the pure language of C with libraries; but even you can successfully find a trick, that would mean you found a particular order that the compiler generates the machine code; this is implying: you tried to let the compiler compiles your code to exactly machine code. So, why not just write inline assemblies directly?
This example explained all above of necessary reasons except instruction set not supported, but I think that was easy to think about.
In fact, there're more reasons to write inline assemblies, but now you have some ideas of them, and so on.