#include <stdint.h>

#define kDataAddr ((uint8_t*) 0x0000)

uint16_t crc16(int data_len) {
    const uint8_t* data = kDataAddr;

    uint16_t crc = 0xFFFF;
    for (unsigned int i = 0; i < data_len; ++i) {
        uint16_t dbyte = data[i];
        crc ^= dbyte << 8;
        for (unsigned char j = 0; j < 8; ++j) {
            uint16_t mix = crc & 0x8000;
            crc = (crc << 1);
            if (mix)
                crc = crc ^ 0x1021;
        }
    }
    return crc;
}

asm (
        ".global main      \n"
        "main:             \n"
        "  push %bp        \n"
        "  mov %sp, %bp    \n"
        "  mov 6(%bp), %si \n"
        "  push (%si)      \n"
        "  call crc16      \n"
        "  mov 8(%bp), %di \n"
        "  mov %ax, (%di)  \n"
        "  pop %bp         \n"
        "  lret $4         \n"
        );