noob_asm.c - A Simple Assembly Language Lesson/Demonstration

/**
 * [noob_asm.c]
 * A simple assembly language lesson/demonstration.
 *
 *      (c) Damion 'Phr0z3n.dev' Tapper, 2015.
 *      Email: Phr0z3n.Dev@Gmail.com
 *      Website: http://L337Stuff.Blogspot.com
 **/

#include <stdio.h>

int main(void)
{
    int i;                                  /* Variable declaration. */

        /**
         * The following loop will do 5 things (essentially):
         * 1. Initialize the 'i' variable to 0 (once).
         * 2. Check if the value of 'i' is less than 10.
         * 3. Print 'Hello world!' to the screen if step 2 is true.
         * 4. Increment the value of 'i' by 1.
         * 5. Repeat steps 2-4 until step 2 is false.
         */
        for(i = 0; i < 10; i++)         /* Loop 10 times. */
            puts("Hello World!\n");         /* Put the string to the output. */

    return 0;                               /* Tell OS the program exited without errors. */
}

/**
 * Below is the assembly language syntax corresponding with the previous C-style operations.

    INTEL SYNTAX:
    Dump of assembler code for function main:
       0x080483d4 <+0>:     push   ebp                          ; beginning of function prologue
       0x080483d5 <+1>:     mov    ebp,esp
       0x080483d7 <+3>:     and    esp,0xfffffff0
       0x080483da <+6>:     sub    esp,0x20                     ; end of function prologue
       0x080483dd <+9>:     mov    DWORD PTR [esp+0x1c],0x0     ; initializes the 'i' variable with the value 0
       0x080483e5 <+17>:    jmp    0x80483f8 <main+36>          ; unconditional jump to the for loop section that does the comparison
       0x080483e7 <+19>:    mov    DWORD PTR [esp],0x80484e0    ; copies/prepares the 'Hello world!' string for output
       0x080483ee <+26>:    call   0x80482f0 <puts@plt>         ; calls the puts() function, which prints 'Hello world!' to the screen
       0x080483f3 <+31>:    add    DWORD PTR [esp+0x1c],0x1     ; adds 1 to the value of the 'i' variable
       0x080483f8 <+36>:    cmp    DWORD PTR [esp+0x1c],0x9     ; compares the value of the 'i' variable with 9 (the result of the comparison will set a flag bit accordingly)
       0x080483fd <+41>:    jle    0x80483e7 <main+19>          ; if the previously set flag bit indicates that the value of 'i' is less than or equal to 9, a conditional jump is made to the section in main() that prepares the 'Hello world!' string for output
       0x080483ff <+43>:    mov    eax,0x0                      ; copies/prepares the main() function's return value
       0x08048404 <+48>:    leave                               ; prepares the stack for procedure exit
       0x08048405 <+49>:    ret                                 ; the main() function exits
    End of assembler dump.
*/

/**
 * Below is the same set of instructions as above, only in a different syntax.

    AT&T SYNTAX:
    Dump of assembler code for function main:
       0x080483d4 <+0>:     push   %ebp
       0x080483d5 <+1>:     mov    %esp,%ebp
       0x080483d7 <+3>:     and    $0xfffffff0,%esp
       0x080483da <+6>:     sub    $0x20,%esp
       0x080483dd <+9>:     movl   $0x0,0x1c(%esp)
       0x080483e5 <+17>:    jmp    0x80483f8 <main+36>
       0x080483e7 <+19>:    movl   $0x80484e0,(%esp)
       0x080483ee <+26>:    call   0x80482f0 <puts@plt>
       0x080483f3 <+31>:    addl   $0x1,0x1c(%esp)
       0x080483f8 <+36>:    cmpl   $0x9,0x1c(%esp)
       0x080483fd <+41>:    jle    0x80483e7 <main+19>
       0x080483ff <+43>:    mov    $0x0,%eax
       0x08048404 <+48>:    leave
       0x08048405 <+49>:    ret
    End of assembler dump.
*/