Your POST_ loops are missing to very important things.
1) The don't ever set the screen to white or black,
2) The don't increment screen_address.
Your earlier code had both of these, but they seem to have gone AWOL.
M=0 //clear in current address
M=M+1 //increment address to be filled in
Fix these two bugs and your code works.
It can be quite hard to tell if your program is turning the white screen white 8-). A simple thing that you can do for debugging is to change the M=0 to M=1. This will make the white loop draw vertical lines instead of all white. You can change it back to M=0 when the program is working.