Computer Systems Fundamentals

#include <stdio.h>

int main(int argc, char *argv[]) {
    int num;
    int rem;

    while (1) {  // forever
        // get the number
        printf("Integer? ");
        if (scanf("%d", &num) != 1) break;

        // extract the digits
        rem = num;
        do {
            printf("%d\n", rem % 10);
            rem = rem / 10;
        } while (rem != 0);
    }
    return 0;
}

#include <stdio.h>

#define MAXBITS 32

int main(int argc, char *argv[]) {
    int num;
    unsigned int rem;
    int bits[MAXBITS];
    int nbits;

    while (1) {  // forever
        // get the number
        printf("Integer? ");
        if (scanf("%d", &num) != 1) break;

        // extract the digits
        rem = num;
        nbits = 0;
        do {
            bits[nbits] = rem % 2;
            nbits++;
            rem = rem / 2;
        } while (rem != 0);

        printf("%d = %08x = ", num, num);
        for (int i = nbits-1; i >= 0; i--) {
            printf("%d", bits[i]);
        }
        putchar('\n');
    }
    return 0;
}

#include <stdio.h>

int main(void){
  
    unsigned int a = 938271;        //decimal                          
    unsigned int b = 0xAF12;        //hexadecimal
    unsigned int c = 0b00101010;    //binary (may not work on all systems)
    unsigned int d = 0123;          //0 octal                 
    
    printf("Dec: %u %u %u %u\n", a, b, c, d); //printing in decimal
    printf("Hex: %x %X %X %X\n", a, b, c, d); //printing in hex
    printf("Oct: %o %o %o %o\n", a, b, c, d); //printing in octal
    
    return 0;
}

#include <stdio.h>
#include <limits.h>

int main(void) {

    char c;
    signed char sc;
    unsigned char uc;
    short s;
    unsigned short us;
    int i;
    unsigned int ui;
    long l;
    unsigned long ul;
    long long ll;
    unsigned long long ull;

    printf("%18s %5s %4s\n", "Type", "Bytes", "Bits");

    printf("%18s %5lu %4lu\n", "char",               sizeof c,   8 * sizeof c);

    printf("%18s %5lu %4lu\n", "signed char",        sizeof sc,  8 * sizeof sc);
    printf("%18s %5lu %4lu\n", "unsigned char",      sizeof uc,  8 * sizeof uc);

    printf("%18s %5lu %4lu\n", "short",              sizeof s,   8 * sizeof s);
    printf("%18s %5lu %4lu\n", "unsigned short",     sizeof us,  8 * sizeof us);

    printf("%18s %5lu %4lu\n", "int",                sizeof i,   8 * sizeof i);
    printf("%18s %5lu %4lu\n", "unsigned int",       sizeof ui,  8 * sizeof ui);

    printf("%18s %5lu %4lu\n", "long",               sizeof l,   8 * sizeof l);
    printf("%18s %5lu %4lu\n", "unsigned long",      sizeof ul,  8 * sizeof ul);

    printf("%18s %5lu %4lu\n", "long long",          sizeof ll,  8 * sizeof ll);
    printf("%18s %5lu %4lu\n", "unsigned long long", sizeof ull, 8 * sizeof ull);

    printf("\n");

    printf("%18s %20s %20s\n", "Type", "Min", "Max");

#ifdef __CHAR_UNSIGNED__
    printf("%18s %20hhu %20hhu\n", "char",               (char)CHAR_MIN,         (char)CHAR_MAX);
#else
    printf("%18s %20hhd %20hhd\n", "char",               (char)CHAR_MIN,         (char)CHAR_MAX);
#endif

    printf("%18s %20hhd %20hhd\n", "signed char",        (signed char)SCHAR_MIN, (signed char)SCHAR_MAX);
    printf("%18s %20hhu %20hhu\n", "unsigned char",      (unsigned char)0,       (unsigned char)UCHAR_MAX);

    printf("%18s %20hd %20hd\n",   "short",              (short)SHRT_MIN,        (short)SHRT_MAX);
    printf("%18s %20hu %20hu\n",   "unsigned short",     (unsigned short)0,      (unsigned short)USHRT_MAX);

    printf("%18s %20d %20d\n",     "int",                INT_MIN,                INT_MAX);
    printf("%18s %20u %20u\n",     "unsigned int",       (unsigned int)0,        UINT_MAX);

    printf("%18s %20ld %20ld\n",   "long",               LONG_MIN,               LONG_MAX);
    printf("%18s %20lu %20lu\n",   "unsigned long",      (unsigned long)0,       ULONG_MAX);

    printf("%18s %20lld %20lld\n", "long long",          LLONG_MIN,              LLONG_MAX);
    printf("%18s %20llu %20llu\n", "unsigned long long", (unsigned long long)0,  ULLONG_MAX);

    return 0;
}

#include <stdio.h>
#include <limits.h>

int main(void){
    int x = INT_MAX;
    printf("%d + 1 = ", x);
    x = x + 1;   //overflow undefined behaviour
    printf("%d\n", x); 
    
    int y = INT_MIN;
    printf("%d - 1 = ", y);
    y = y - 1;   //overflow undefined behaviour
    printf("%d\n", y);

    return 0;
}

#include <stdio.h>
#include <limits.h>

int main(void){
    unsigned int x = UINT_MAX;
    printf("%u + 1 = %u\n", x, x + 1);  

    unsigned int y = -1;
    printf("-1 = %u\n", y);
   
    // infinite loop. Uncomment to try it out
    // z can never be < 0 as it is an unsigned type!
    // for(unsigned int z = 3; z >= 0; z--){
    //    printf("%u\n",z);
    // }
    
    return 0;
}

#include <stdint.h>

int main(void) {

                 // range of values for type
                 //             minimum               maximum
    int8_t   i1; //                 -128                  127
    uint8_t  i2; //                    0                  255
    int16_t  i3; //               -32768                32767
    uint16_t i4; //                    0                65535
    int32_t  i5; //          -2147483648           2147483647
    uint32_t i6; //                    0           4294967295
    int64_t  i7; // -9223372036854775808  9223372036854775807
    uint64_t i8; //                    0 18446744073709551615

    return 0;
}

#include <stdio.h>

int main(void) {

    // Common C bug:

    char c;  // c should be declared int   (int16_t would work, int is better)
    while ((c = getchar()) != EOF) {
        putchar(c);
    }

    // Typically `stdio.h` contains:
    // ```c
    // #define EOF -1
    // ```
    //
    // - most platforms: char is signed (-128..127)
    //   - loop will incorrectly exit for a byte containing 0xFF
    //
    // - rare platforms: char is unsigned (0..255)
    //   - loop will never exit

    return 0;
}

// header file so we use print_bits in several examples
#ifndef PRINT_BITS_H
#define PRINT_BITS_H

#include <stdint.h>
void print_bits(uint64_t value, int how_many_bits);

#endif

#include <stdio.h>
#include <stdint.h>

#include "print_bits.h"

// extract the nth bit from a value
int get_nth_bit(uint64_t value, int n) {
    // shift the bit right n bits
    // this leaves the n-th bit as the least significant bit
    uint64_t shifted_value = value >> n;

    // zero all bits except the the least significant bit
    int bit = shifted_value & 1;

    return bit;
}

// print the bottom how_many_bits bits of value
void print_bits(uint64_t value, int how_many_bits) {
    // print bits from most significant to least significant

    for (int i = how_many_bits - 1; i >= 0; i--) {
        int bit = get_nth_bit(value, i);
        printf("%d", bit);
    }
}

#include <stdio.h>
#include <stdint.h>
#include "print_bits.h"

int main(void) {
    int a = 0;
    printf("Enter an int: ");
    scanf("%d", &a);

    // sizeof returns number of bytes, a byte has 8 bits
    int n_bits = 8 * sizeof a;

    print_bits(a, n_bits);
    printf("\n");

    return 0;
}

#include <stdio.h>
#include <stdint.h>
#include "print_bits.h"

int main(void) {

    for (int i = -128; i < 128; i++) {
        printf("%4d ", i);
        print_bits(i, 8);
        printf("\n");
    }

    return 0;
}

#include <stdio.h>
#include <stdint.h>

int main(void) {
    uint8_t b;
    uint32_t u;

    u = 0x03040506;
    // load first byte of u
    b = *(uint8_t *)&u;
    // prints 6 if little-endian
    // and 3 if big-endian
    printf("%d\n", b);
}