10 C

#include <stdio.h>  // needed for fgets below
#include <stdlib.h> // needed for atoi below

// update sum of signals
int upd_sum(int cycle, int sum, int reg) {
    // update only for some cycles
    switch (cycle) {
        case 20:
        case 60:
        case 100:
        case 140:
        case 180:
        case 220:
            return sum + cycle*reg;
        default:
            break;
    }
    return sum;
}

// main function which is run when executed
int main() {
    const int MAXL = 100; // max line length, auxiliary variable
    FILE* fp;             // file descriptor
    char line[MAXL];      // line string
    int sum = 0;          // cummulative signal at selected cycles
    int signal = 0;       // signal from the instruction
    int reg = 1;          // register value
    int cycle = 0;        // clock

    // open file but do not check if it fails
    fp = fopen("10.input", "r");

    // read input file until the EOF (end of file)
    while ((fgets(line, MAXL, fp)) != NULL) {
        // noop instruction, do nothing
        if (line[0] == 'n') {
            cycle += 1;
            sum = upd_sum(cycle, sum, reg);
        }
        // addx instruction, 2 cycles
        else {
            signal = atoi(&line[5]);  // convert from position 5 to int
            cycle += 1;
            sum = upd_sum(cycle, sum, reg);
            cycle += 1;
            sum = upd_sum(cycle, sum, reg);
            reg += signal;
        }
    }

    printf("Sum of signals %d\n", sum);

    fclose(fp);     // better close the file
    return 0;       // we are fine!
}

#include <stdio.h>
#include <stdlib.h>

const int MAXL = 100;
const int CRTW = 40;  // width of CRT

// init CRT screen line with '.'
void init_crt(char *crt_row) {
    for (int i=0; i<CRTW; i++) {
        crt_row[i] = '.';
    }
    // end of the string
    crt_row[CRTW] = '\0';
}

// draw pixel if sprint is at the right position
void draw_px(int cycle, int sprite, char *crt_row) {
    // get the right position from the cycle
    int c = (cycle-1) % CRTW;
    if (c == sprite || c+1 == sprite || c-1 == sprite) {
        crt_row[c] = '#';
    }
}

void upd_crt(int cycle, int sprite, char *crt_row) {
    switch (cycle) {
        // now the cycles are shifted
        case 40:
        case 80:
        case 120:
        case 160:
        case 200:
        case 240:
            draw_px(cycle, sprite, crt_row);
            // print CRT screen line
            printf("%s\n", crt_row);
            init_crt(crt_row);
            break;
        default:
            draw_px(cycle, sprite, crt_row);
            break;
    }
}

int main() {
    FILE* fp;
    char line[MAXL];
    int sprite = 1;       // position of sprint on CRT
    int cycle = 0;        // cycle number
    char crt_row[CRTW+1]; // CRT screen line

    fp = fopen("10.input", "r");
    init_crt(crt_row);  // initialize with

    while ((fgets(line, MAXL, fp)) != NULL) {
        if (line[0] == 'n') {
            cycle += 1;
            upd_crt(cycle, sprite, crt_row);
        }
        else {
            cycle += 1;
            upd_crt(cycle, sprite, crt_row);
            cycle += 1;
            upd_crt(cycle, sprite, crt_row);
            sprite += atoi(&line[5]);
        }
    }

    fclose(fp);
    return 0;

$ sudo apt install nasm

nasm -f elf 10.asm  # compiles
ld -m elf_i386 -s -o hello 10.o  # links into hello executable
./hello  # runs