ASM Tutorial Notes

;========================================================================;
;Authored by H4x0                                                        ;
;Using samples from the 'blackfireize' Youtube tutorial series           ;
;http://www.youtube.com/watch?v=GFkrYM7mbTc                              ;
;http://www.h4x0.com/lib/Snippets/Tutorial_Notes.asm                     ;
;16Bit Assembly (DOS) - emu8086 & FASM                                   ;
;========================================================================;
#fasm#
name "mycode"
org 100h                        ;DOS FTW!

call one                        ;Used for calling a function or "label"
                                ;Change to lesson function you want to see

call return_os                  ;Call our helper function to return control
                                ;to the operating system

;========================================================================;
;Variables                                                               ;
;========================================================================;
msgOne db 'Hello World$'        ;Define a string called msgOne
msgThr db 'Look At Source$'

;========================================================================;
;Lesson functions                                                        ;
;========================================================================;
one:
  mov ah, 9h
  mov dx, msgOne                ;Move msgOne to dx register
  int 21h                       ;Print (Execute interupt)
  ret                           ;Return from function

two:
  mov ax, 123d                  ;Move 123 (Integer) to ax register
  call print_decimal            ;Call the print_decimal helper function
  ret                           ;Return from function

three:
  ;Informational Lesson

  ;AX                           ;Acumulator Register (Mathmatical Operations)
    ;AH                           ;A (Register) H (High)
    ;AL                           ;A (Register) L (Low)

  ;BX                           ;Base Register (Counting, Manipulation, General)
    ;BH                           ;B (Register) H (High)
    ;BL                           ;B (Register) L (Low)

  ;CX                           ;Unknown Name (Counting Operations)
    ;CH                           ;C (Register) H (High)
    ;CL                           ;C (Register) L (Low)

  ;DX                           ;Data Register (Holding Memory or Data Pointers)
    ;DH                           ;D (Register) H (High)
    ;DL                           ;D (Register) L (Low)

  ;Segment Registers (Point to 64kb memory segments)
  ;CS                           ;Code Segment
  ;DS                           ;Data Segment
  ;ES                           ;Extra Segment
  ;FS                           ;Extra Segment
  ;GS                           ;Extra Segment
  ;SS                           ;Stack Segment

  ;ESI                          ;Extended Source Index
    ;SI                           ;Source Index (String Operations)

  ;EDI                          ;Extended Destination Index
    ;DI                           ;Destination Index (General Purposse)

  ;EBP                          ;Extended Base Pointer
    ;BP                           ;Base Pointer

  ;ESP                          ;Extended Stack Pointer
    ;SP                           ;Stack Pointer

  ;EFLAGS                       ;Never Change (at least directly)
    ;FLAGS                        ;Never Change (at least directly)
      ;ZF                           ;Zero Flag
      ;PF                           ;Parody Flag

  mov ah, 9h
  mov dx, msgThr                ;Move msgThr to dx register
  int 21h                       ;Print (Execute interupt)
  ret                           ;Return from function


;========================================================================;
;Helper functions                                                        ;
;========================================================================;
return_os:                      ;Return control to OS
  mov ah, 4ch
  int 21h                       ;Execute interupt

print_decimal:                  ;AX -> Decimal Number
  pusha                         ;Push all to the stack
  mov bx, 10d                   ;Move 10 (Integer) to bx register
  mov cx, 0                     ;Move 0 (Integer) to cx register
  mov dx, 0                     ;Move 0 (Integer) to dx register

  .tryloop1:                    ;Loop
    inc cx                      ;Register cx++ or cx=cx+1 (Increment)
    mov dx, 0                   ;Move 0 (Integer) to dx register
    div bx                      ;Divide bx register
    push dx                     ;Push dx to stack
    cmp dx, 0                   ;Compare dx register with 0
  jne .tryloop1                 ;Jump back if not equal

  mov ah, 02h

  .tryloop2:                    ;Loop
    pop dx                      ;Clear dx register
    add dl, 48                  ;Get ASCII char from integer by adding 48
    int 21h                     ;Print (Execute interupt)
  loop .tryloop2                ;

  popa                          ;Pop all from stack
  ret                           ;Return from function