I used to draw directly to the LCD memory since I did not have enough RAM.

In my case, setting a pixel in LCD memory is not different than modifing a whole byte in the LCD memory.

So, I'd keep a modified flag for each byte/byte-block, and set it when I set/reset any pixel in byte/byte-block.

A simple check will be adequate to decide which byte(s) to update in LCD.

wrong?