Aleluya, a simple 44byte dos asm intro.

aleluya.asm

;Aleluya, a simple 44byte dos asm intro. Compile  with "nasm aleluya.asm -o aleluya.com" . 
BITS 16                     ; 16bit asm code
org 100h                    ; 'boilerplate' assembly directive for offest of dos com programs
section .text               ; starting the code section, memory variables and data are different
push 0xA000                 ; This is the base location of video memory in mode 13h
pop es                      ; ES register cannot be set directly by an immediate (a direct number)
mov ax, 13h                 ; these two lines set a simple flat video mode  called 13h
int 10h                     ; with 320x200 pixels and 256 color palette 
mov dx,0                    ; we will use DX as a frame counter
frame_loop_aleluya:         ; our label for the start of drawing a frame, aleluya!
  mov di, 320*200           ; DI will both be used to countdown to 0 and to point to the pixel in memory
  pixel_loop_aleluya:       ; label for start of a pixel
    mov bx, di              ; we will operate on the counter within BX 
    add bx, dx              ; move the bars every frame
    and bl, 0xf             ; modulus 16 of BL, the lowest 8 bits of BX (we write a byte=pixel at a time)
    add bl, 16              ; add 16 so it is in a grayscale range of the default 256 color palette
    mov [es:di], bl         ; place from byte register al to [0xA000:di]
    dec di                  ; countdown to 0 to finish frame
  jnz pixel_loop_aleluya    ; if last operation did not set 0 flag, the write next pixel
  inc dx                    ; increment frame counter
  mov ah, 1                 ; these two lines detect the keypress
  int 16h                   ; if a key is *not* pressed, the zero flag will be set
jz frame_loop_aleluya       ; render another frame if zero flag not set
mov ax, 3h                  ; these two lines go back to standard dos text mode
int 10h
ret                         ; end program, but your life doesn't need to end, For God so loved the world, 
; That He gave His only begotten Son, that all who believe in him should not perish but have everlasting life