I don't know if wrapping rules are defined by the standard or implementation defined. But the easiest thing for a compiler to implement is simple truncation.

I don't know if wrapping rules are defined by the standard or implementation defined. But the easiest thing for a compiler to implement is simple truncation. A space is 0x20 (32 decimal) in most C compilers, so multiplying it by 13 is 416. Truncating that to 8 bits, the size of char on most compilers, is 160 (0xa0). If char is signed, the upper bit being set will cause it to be a negative number -96. Promotion of the char to int won't change its value. There are a huge number of assumptions in that simple chain of events, and if any of them are wrong you get a different answer.