$2100 - screen on/off + brightness

$2105 - screen format register, changes size of tiles

$2107 - set starting tilemap location

$2121 - stores the index into CGRAM to write a color value via $2122

$2122 - write a 2-byte SNES color value to the CGRAM. this auto-increments after writing,
which is why $2121 needs to be written to reset this. it's common to write `stz $2121`