org 0x7C00

init:
  mov word [body_start], 2000
  mov word [body_end], 2000
  mov word [head_dir], -160
  ; clear the screen
  mov ax, 0xB800
  mov es, ax
  mov cx, (80 * 25)
  xor ax, ax
  mov di, ax
  rep stosw
  ; clear the game memory
  mov es, ax
  mov di, 0x0500
  mov cx, (80 * 25)
  rep stosw

  call game_tick

  ; store the original timer isr
  cli
  xor ax, ax
  mov ax, [32]
  mov [orig_isr8], ax
  mov ax, [34]
  mov [orig_isr8 + 2], ax
  ; install our timer isr
  mov word [32], timer_tick
  mov word [34], 0
  sti

.loop:
  jmp .loop

orig_isr8: dd 0
tick_count: db 0
timer_tick:
  mov al, [tick_count]
  inc al
  cmp al, 9
  jl .nogametick
  call game_tick
.nogametick:
  mov [tick_count], al
  jmp far [orig_isr8]

game_tick:
  ; grab some vars before switching to game seg
  mov si, [body_start]
  mov di, [body_end]
  mov dx, [head_dir]
  mov ax, 0x0050
  mov ds, ax
  mov es, ax

.noinit:
  ; update head position
  mov ax, si
  add ax, dx
  mov [si], ax
  mov si, ax
  mov bx, [di]
  mov word [di], 0
  mov di, bx

  ; check if body overlapping
  cmp word [si], 0
  je .nodie
  add sp, 2
  jmp init
.nodie:

.end:
  xor ax, ax
  mov ds, ax
  mov es, ax
  mov [body_start], si
  mov [body_end], di
  ret

head_dir:   dw 0
body_start: dw 0
body_end:   dw 0

; Pad out the rest of the boot sector and add magic bytes
times 510 - ($ - $$) db 0
dw 0xAA55

; Pad out the rest of a 1.44MB floppy image
times (2880 * 512) - ($ - $$) db 0