The leap year determination is written by inline assembler.

leap_year_assembler.c

#include <stdio.h>

int main (int argc, char **argv) {
    int year = 0, check = 0;
    scanf("%d", &year);
    char ok[] = "leap", ng[] = "not leap";

    __asm__ volatile("mov %%rcx, %%rax;\n\t"
                     "mov $400, %%rbx;\n\t"
                     "idiv %%rbx;\n\t"
                     "cmp $0, %%rdx\n\t"
                     "jz ok\n\t"
                     "mov %%rcx, %%rax;\n\t"
                     "mov $100, %%rbx;\n\t"
                     "mov $0, %%rdx;\n\t"
                     "idiv %%rbx;\n\t"
                     "cmp $0, %%rdx\n\t"
                     "jz ng\n\t"
                     "mov %%rcx, %%rax;\n\t"
                     "mov $4, %%rbx;\n\t"
                     "mov $0, %%rdx;\n\t"
                     "idiv %%rbx;\n\t"
                     "cmp $0, %%rdx\n\t"
                     "jz ok\n\t"
                     "jmp ng\n\t"
                     "ok:\n\t"
                     "mov $1, %%rcx\n\t"
                     "jmp exit\n\t"
                     "ng:\n\t"
                     "mov $0, %%rcx\n\t"
                     "jmp exit\n\t"
                     "exit:\n\t"
                     : "=c"(check)
                     : "c" (year)
                     : "%rax", "%rbx", "%rdx"
                     );
    printf("%s\n", (check ? ok : ng));
}