Notes:

Memory location $fe contains a new random byte on every instruction.
Memory location $ff contains the ascii code of the last key pressed, $0d is Enter, $08 is backspace.

Memory locations $200 to $5ff map to the screen pixels. Different
values will draw different colour pixels. The colours are:

    $0: Black
    $1: White
    $2: Red
    $3: Cyan
    $4: Purple
    $5: Green
    $6: Blue
    $7: Yellow
    $8: Orange
    $9: Brown
    $a: Light red
    $b: Dark grey
    $c: Grey
    $d: Light green
    $e: Light blue
    $f: Light grey

Reference:
    6502.org (preferred)
    Obelisk
    Easy 6502
    Assembly in One Step
    6502 Adressing Modes
    6502 Registers

Addressing:
    ZP, X   -   ZP = $80 ; X = $0F ; Result = $8F
    ZP, Y   -   ZP = $80 ; Y = $0F ; Result = $8F
    ABSO, X -   ABSO = $2000 ; X = $92 ; Result = $2092
    ABSO, Y -   ABSO = $2000 ; Y = $92 ; Result = $2092
    (ABSO)  -   ABSO = $2000 ; Result = address @ location $2000
    (ZP, X) -   ZP = $80 ; X = $0F ; Result = (address @ location $80 + $0F)
    (ZP), Y -   ZP = $80 ; Y = $0F ; Result = address @ location $80 + $0F