Computer Systems Fundamentals

#include <stdio.h>

int g = 10; //global variable: data section on memory

void f(int x);

int main(void){
    int i = 10;             // local variable stack
    char v[] = "Hello";     // v is a local variable on the stack of size 6 bytes                          
    char *s = "Hello";	    // s is a local variable on the stack
    			            // that points to constant read only string in the data section	       
    int *n = malloc(10);    // n is a locat variable on the stack
                            // that points to a chunk of malloced memory on the heap

    return 0;
}

1) convert 1101 from binary to decimal
1101 = 1*2^3 + 1*2^2 + 1*2^0 = 8+4+1 = 13

2) convert 29 from decimal to binary

29/2 = 14 R 1
14/2 = 7  R 0
 7/2 = 3  R 1
 3/2 = 1  R 1
 1/2 = 0  R 1

 Answer = 11101

3) Convert 1011 1110 0010 1001 from binary to hex
             B   E     2    9

4) Convert 1CED from hex to binary  
    0001 1100 1110 1101

Remember in Hex:
A 10
B 11
C 12
D 13
E 14
F 15

#include <stdio.h>

int main(void){
  
    unsigned int a = 938271;        //decimal                          
    unsigned int b = 0xAF12;        //hexadecimal
    unsigned int c = 0b00101010;    //binary may not work with all compilers
    unsigned int d = 0123;          //0 octal                 
    
    printf("%u %u %u %u\n",a,b,c,d); //printing in decimal
    printf("%X %X %X %X\n",a,b,c,d); //printing in hex
    printf("%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 <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;
}

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

//You need to compile and run with gcc! dcc will give a run time error
//due to the overflow

int main(void){
    //INT_MIN..INT_MAX
    int x = INT_MAX;
    printf("%d + 1 = ",x);
    x = x + 1;
    printf("%d\n",x);
    
    //0...UNINT_MAX
    unsigned y = UINT_MAX;
    printf("%u + 1 = ",y);
    y = y + 1;
    printf("%u\n",y);
    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;
}