I'd just like to jump in here. I don't know what you guys are talking about 
with functions and procedures with C/C++ etc. as I only program in Java. 
I would just like to explain the VDU that I built (and designed) without the 
nervousness that I had as a beginning commentator.

Here goes:

First of all, I built this horribly. I could have built it much more compact. 
The images are stored in D-flip flops that you edit with the levers. The 
save button activates all the D-flip flops in one image to save the image 
into them. Each D-flip flop controls one pixel, being one bit of memory. 
That makes each image 256 bits, or 32 bytes. The screen also stores its 
memory in an array of D-flip flops, almost exactly the same as the ones 
that store the images permanently. The "save" feature on the screen 
memory, however, is activated every time you load an image. What 
happens is when you press a load button a signal is sent through from the 
array of D-flip flops that contains the image into the array for the screen. 
The screen then saves it until the next time an image is loaded. The clear-
screen button saves the status of the screen's D-flip flops without loading 
an image, which in most cases will clear the screen. If you load an image 
then quickly press the clear screen button though, it won't clear the 
screen. That's about all there is to it, although I may also add that during 
the debugging after I built it, I encountered a problem in the screen in 
which sometimes the D-flip flops would emit a rapid pulse and cause the 
sticky pistons to push out and pull in too quickly to pull its block back 
with it. I fixed this by simply changing all the repeaters hooked to the 
screen to a three tick delay instead of one. This took me a week to build, I 
spent 1-2 hours a day on it. I thought of the plans in my head, and despite 
what many people seem to think, it was extremely simple. I built all the 
parts separately then assembled them in MCEdit, as you can see if you 
download the map and fly over the enormous dirt wall. That's all. I'll 
probably use pastebin to post this so that I don't use like 5 comments.