需要帮助来理解绘制棋盘的代码

The book uses this code to draw a checkerboard. The only problem I'm having is understanding the sequence of how the individual squares on drawn on the checkerboard. When I do my calculations it only fills the checkerboard with squares in a diagonal direction from top left to bottom right. The book says the code does this loop 64 times for every square but I'm not sure how it does this 64 times. The book uses this code to draw a checkerboard.