The short answer is that he's doing low-level stack-twiddly stuff using inline assembler in C. It's one of the standard perils of mixing languages: the C comp

The short answer is that he's doing low-level stack-twiddly stuff using inline assembler in C. It's one of the standard perils of mixing languages: the C compiler assumes (and attempts to ensure) that it's the only thing manipulating the stack pointer, and the inline assembler violates that assumption.