Matrix digital rain

set z, 0x1234 ; rand_seed
set y, z ; cur_rand
set pc, main_loop

:next_rand
mul y, 10061
add y, 1
set pc, pop

:main_loop
set i, 0
set x, y
set y, z
set a, 0x8000

:next_row1
  set j, 0
  :next_char1
    jsr next_rand
    ife [a], 0
    set pc, skip1
    ifb y, 0x7000
    set pc, skip1

    ; mutate char at [a] (j, i)
    set push, y
    set y, x
    jsr next_rand
    and [a], 0xff00 ; reset char
    and y, 0x003f
    bor [a], [code+y] ; set letter
    set x, y
    set y, pop

    :skip1
    add j, 1
    add a, 1
    ifg 32, j
    set pc, next_char1
  add i, 1
  ifg 12, i
  set pc, next_row1
set y, x

; step 2: move all columns down
set i, 12
set a, 0x817f

:next_row2
  set j, 32
  :next_char2
    ife [a], 0
    set pc, empty_char
    ifb [a+32], 0xffff
    set pc, move_down

    ; add new char at [a+32] (j, i+1)
    jsr next_rand
    set [a+32], [a]
    and [a+32], 0xff00
    set x, y
    and x, 0x003f
    bor [a+32], [code+x]
    and [a], 0x7fff
    set pc, skip2

    :empty_char
    set [a+32], 0

    :move_down
    and [a+32], 0x7fff

    :skip2
    sub j, 1
    sub a, 1
    ifg j, 0
    set pc, next_char2
  sub i, 1
  ifg i, 0
  set pc, next_row2
set y, x

; step 3: update top layer
set a, 0x8000
set j, 0
:next_char3
  jsr next_rand
  ifb y, 0x0700
  set pc, skip3
  ifb [a], 0xffff
  set pc, empty_char2

  set [a], 0x2000
  ifb y, 0x0800
  set [a], 0xa000
  set x, y
  and x, 0x003f
  bor [a], [code+x]
  set pc, skip3

  :empty_char2
  set [a], 0

  :skip3
  add j, 1
  add a, 1
  ifg 32, j
  set pc, next_char3

set PC, main_loop

sub PC, 1

:code dat "00112334567889&||!!@==::**##<>>__TYYUDQZJJIX- ~~oiwlrkm//\\'[]^)`"