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@ l7: stack

@ l6: push_function

@ l0: return address of leavfuction (push pop im speziell)

  stio l0, [l0]

  LDL l0, :start

  LDL l6, :push

  ldi l7, 0x03

  jump 

  lsi l7, 0xff

push:

  st  h0, [l7]

  adi l7, -1

  st  h1, [l7]

  adi l7, -1

  st  h2, [l7]

  adi l7, -1

  st  h3, [l7]

  adi l7, -1

  st  h4, [l7]

  adi l7, -1

  st  h5, [l7]

  adi l7, -1

  st  h6, [l7]

  adi l7, -1

  st  h7, [l7]

  jump 

  adi l7, -1

pop:

  adi l7, 1

  ld  h7, [l7]

  adi l7, 1

  ld  h6, [l7]

  adi l7, 1

  ld  h5, [l7]

  adi l7, 1

  ld  h4, [l7]

  adi l7, 1

  ld  h3, [l7]

  adi l7, 1

  ld  h2, [l7]

  adi l7, 1

  ld  h1, [l7]

  adi l7, 1

  ld  h0, [l7]

  nop

  nop

  jump 

  nop

start:

@ initialize snake

  ldi   h1, 8

l0:

  st    h1, [h1]

  brnz  h1, :l0

  adi   h1, -1

  ldi   h1, 4

  ldi   h2, 0

@ initialize vmem

  LDL   h4, :set_tile

  ldi   l3, 59     @ max y

hloop:

  ldi   l2, 79     @ max x

pixloop:

  call  h0, 

  ldi   l1, 0x6b     @ white=0x7f  (black = 6b)

  brnz  l2, :pixloop

  adi   l2, -1

  brnz  l3, :hloop

  adi   l3, -1

  LDL   h3, :delay

  LDL   h5, :update

loop:

  ldi   l0, 0

  ldio  l0, [l0]

  ldi   l1, 0x01

  and   l2, l0, l1     @ select bit for right

  sub   h2, h2, l2     @ add bit to direction

  shi   l0, -1         @ shift input

  and   l2, l0, l1

  add   h2, h2, l2     @ add bit to direction

  stio  h2, [l0]

  call h0, 

 mov  l1, h2

  call  h0, 
       @ call delay

  ldi   l1, 22         @ 2^20 steps

  brnz  h5, :loop

  nop

update:

  call l0, 

  nop

  ldi   l0, 0

  ld    l1, [l0]

  @ extract tile,y,x  (l1=tile, l2=x, l3=y)

  ldi   l4, 0xff

  and   l2, l1, l4

  shi   l1, -8

  and   l3, l1, l4

  ldi   l1, 0x6b

  LDL   l0, :set_tile

  call  h0, 

  nop

  ldi   l0, 0

  ldi   l2, 1

cloop:

  ld    l1, [l2]      @ src

  adi   l2, 1         @ src + 1

  st    l1, [l0]      @ dst

  cmpl  l3, l0, h1    @ until dst=highest address)

  brnz  l3, :cloop

  adi   l0, 1         @ dst + 1

  ldi   l5, 3

  and   h2, h2, l5    @ trunc direction

  @ extract tile,y,x  (l1=tile, l2=x, l3=y)

  ldi   l4, 0xff

  and   l2, l1, l4

  shi   l1, -8

  and   l3, l1, l4

  shi   l1, -8        @ old direction

  shi   l1, 4         @ old direction << 2

  or    l1, l1, h2    @  00OO 00NN (old new combination)

  adi   l1, 4

  mov   h5, l0

  mov   h6, l2

  mov   h7, l3

  LDL   l4, :set_tile

  call  h0, 

  nop

  mov   l0, h5

  mov   l2, h6

  mov   l3, h7

down:

  xor   l4, h2, l5

  brnz  l4, :up

  nop

  adi   l3, 1

  ldi   l1, 8

up:

  ldi   l5, 1

  xor   l4, h2, l5

  brnz  l4, :right

  nop

  adi   l3, -1

  ldi   l1, 9

right:

  brnz  h2, :left

  ldi   l5, 2

  adi   l2, 1

  ldi   l1, 10

left:

  xor   l4, h2, l5

  brnz  l4, :end_key

  nop

  adi   l2, -1

  ldi   l1, 11

end_key:

  mov   l4, h2

  shi   l4, 8

  or    l4, l4, l3

  shi   l4, 8

  or    l4, l4, l2

  st    l4, [l0]       @ write new head

  LDL   l0, :set_tile

  call  h0, 

  nop

  ldi   l0, 0

  ld    l1, [l0]

  @ extract tail tile,y,x  (l1=tile, l2=x, l3=y)

  ldi   l4, 0xff

  and   l2, l1, l4

  shi   l1, -8

  and   l3, l1, l4

  ldi   l0, 1

  ld    l1, [l0]

  nop

  shi   l1, -16        @ old direction

  adi   l1, 0x0c

  LDL   l0, :set_tile

  call  h0, 

  nop

  adi  h6, l6, 17

  jump 

  nop

set_stile:

  call l0, 

  nop

  ldi  l0, 0xff

  and  l2, l1, l0

  shi  l1, -8

  and  l3, l1, l0

  LDL  h4, :set_tile

  call h0, 

  shi  l1, -8

  adi  h6, l6, 17

  jump 

  nop

set_tile:

  call l0, 

  nop

  ldi   l4, 0xc0   @ vmem starts at 0xc000

  shi   l4, 8

  shi   h3, l3, 7

  add   l0, l2, h3

  add   l0, l0, l4

  stio  l1, [l0]

  adi  h6, l6, 17

  jump 

  nop

delay:

  call l0, 

  nop

  ldi   l0, 1

  sh    l0, l0, l1

delay_loop:

  brnz   l0, :delay_loop

  adi    l0, -1

  adi  h6, l6, 17

  jump 

  nop

stop