"Each block row subdivided into top, middle, and bottom parts Memory stores all top rows first, then middle rows, then bottom rows" Is the key bit. If you are

"Each block row subdivided into top, middle, and bottom parts Memory stores all top rows first, then middle rows, then bottom rows" Is the key bit. If you are at mem location z which represent a point on row y and you want to read the same x position 17 rows down you can't do it by just multiplying a constant by 17 and adding it to your current z.