c语言字节范围理解

unsigned char a = 100;
unsigned char b = 200;
unsigned char ab = a + b;
char c = 100;
char d = 200;
printf("a = %d, b = %d, c = %d, d = %d\n", a, b, c, d);
printf("sizeof(a) = %d\n", sizeof(a));
printf("a + b = %d\n", a + b);
printf("(unsigned char)(a + b) = %d\n", (unsigned char)(a + b));
printf("ab = %d\n", ab);
printf("c + d = %d\n", c + d);