; Control the snake using arrow keys.
;
; Any other keys stop the snake.
;
; Press Esc to exit.


name "snake"

org     100h

; jump over data section:
jmp     start

; ------ data section ------

s_size  equ     7

; the snake coordinates
; (from head to tail)
; low byte is left, high byte
; is top - [top, left]
snake dw s_size dup(0)

tail    dw      ?

; direction constants
;          (bios key codes):
left    equ     4bh
right   equ     4dh
up      equ     48h
down    equ     50h

; current snake direction:
cur_dir db      right

wait_time dw    0

; welcome message
msg 	db "==== How To Play ====", 0dh,0ah
	db "Control the snake using arrow keys", 0dh,0ah	
	db "Any other keys stop the snake.", 0dh,0ah, 0ah
	db "Press Esc to exit.", 0dh,0ah
	db "====================", 0dh,0ah, 0ah
	db "Press any Key to start!$"

; ------ code section ------

start:

; print welcome message:
mov dx, offset msg
mov ah, 9 
int 21h


; wait for any key:
mov ah, 00h
int 16h


; hide text cursor:
mov     ah, 1
mov     ch, 2bh
mov     cl, 0bh
int     10h           


game_loop:

; === select first video page
mov     al, 0  ; page number.
mov     ah, 05h
int     10h

; === show new head:
mov     dx, snake[0]

; set cursor at dl,dh
mov     ah, 02h
int     10h

; print '*' at the location:
mov     al, '*'
mov     ah, 09h
mov     bl, 0eh ; attribute.
mov     cx, 1   ; single char.
int     10h

; === keep the tail:
mov     ax, snake[s_size * 2 - 2]
mov     tail, ax

call    move_snake


; === hide old tail:
mov     dx, tail

; set cursor at dl,dh
mov     ah, 02h
int     10h

; print ' ' at the location:
mov     al, ' '
mov     ah, 09h
mov     bl, 0eh ; attribute.
mov     cx, 1   ; single char.
int     10h



check_for_key:

; === check for player commands:
mov     ah, 01h
int     16h
jz      no_key

mov     ah, 00h
int     16h

cmp     al, 1bh    ; esc - key?
je      stop_game  ;

mov     cur_dir, ah

no_key:



; === wait a few moments here:
; get number of clock ticks
; (about 18 per second)
; since midnight into cx:dx
mov     ah, 00h
int     1ah
cmp     dx, wait_time
jb      check_for_key
add     dx, 4
mov     wait_time, dx



; === eternal game loop:
jmp     game_loop


stop_game:

; show cursor back:
mov     ah, 1
mov     ch, 0bh
mov     cl, 0bh
int     10h

ret

; ------ functions section ------

; this procedure creates the
; animation by moving all snake
; body parts one step to tail,
; the old tail goes away:
; [last part (tail)]-> goes away
; [part i] -> [part i+1]
; ....

move_snake proc near

; set es to bios info segment:  
mov     ax, 40h
mov     es, ax

  ; point di to tail
  mov   di, s_size * 2 - 2
  ; move all body parts
  ; (last one simply goes away)
  mov   cx, s_size-1
move_array:
  mov   ax, snake[di-2]
  mov   snake[di], ax
  sub   di, 2
  loop  move_array


cmp     cur_dir, left
  je    move_left
cmp     cur_dir, right
  je    move_right
cmp     cur_dir, up
  je    move_up
cmp     cur_dir, down
  je    move_down

jmp     stop_move       ; no direction.


move_left:
  mov   al, b.snake[0]
  dec   al
  mov   b.snake[0], al
  cmp   al, -1
  jne   stop_move       
  mov   al, es:[4ah]    ; col number.
  dec   al
  mov   b.snake[0], al  ; return to right.
  jmp   stop_move

move_right:
  mov   al, b.snake[0]
  inc   al
  mov   b.snake[0], al
  cmp   al, es:[4ah]    ; col number.   
  jb    stop_move
  mov   b.snake[0], 0   ; return to left.
  jmp   stop_move

move_up:
  mov   al, b.snake[1]
  dec   al
  mov   b.snake[1], al
  cmp   al, -1
  jne   stop_move
  mov   al, es:[84h]    ; row number -1.
  mov   b.snake[1], al  ; return to bottom.
  jmp   stop_move

move_down:
  mov   al, b.snake[1]
  inc   al
  mov   b.snake[1], al
  cmp   al, es:[84h]    ; row number -1.
  jbe   stop_move
  mov   b.snake[1], 0   ; return to top.
  jmp   stop_move

stop_move:
  ret
move_snake endp