simple.asm with more comments

simple.asm

; a simple hello world .COM

; compile with: yasm -o simple.com simple.asm

; Ange Albertini, BSD Licence, 2013
; w/ some dummy comments by Rich Seymour 

org 100h ; standard loaded address
bits 16 ; ouch :p

DISPLAY_STRING equ 9h ; 9h is DISPLAY_STRING equ means equals foreva, 9h is an interrupt to display
TERMINATE_WITH_RETURN_CODE equ 4ch ; same. interrupt to return from program

MAIN_DOS_API equ 21h ; aka the main dos interrupt

    ; in a .COM file, DATA is with mixed with CODE
    ; not strictly required, as by default, ds=cs on initialization, but safer
    ; cs: code segment, ds: data segment
    push cs  ; store cs on sp, the stack pointer
    pop  ds  ; pop the sp into ds, the data segment

    ; print("Hello World!\r\n\r")
    ; now remember mov moves right to left
    ; I still haven't found a reason for Intel syntax being right to left.
    ; But other parts of it make a lot more sense. you could perhaps think of this as mov to dx from msg
    ; the other syntax AT&T has its own problems so we needn't concern ourselves with it

    mov  dx, msg ; msg defined at the end is stored in data register DX (ax,bx,cx,dx were the 4 16 bit registers)
    mov  ah, DISPLAY_STRING ; store the interrupt to actually display a string
    int  MAIN_DOS_API ; this calls the dos api to do what's in AH

    ; return 1;
    ERRORCODE equ 1
    mov  ax, (TERMINATE_WITH_RETURN_CODE << 8) | ERRORCODE
    int  MAIN_DOS_API ; similar as above, but AX because AH has the 4ch and the errorcode is down in AL

msg db 'Hello World!', 0dh, 0dh, 0ah, '$' ; DOS string are $-terminated. db is a pseudo op to 'assemble' bytes into a variable