haha!
Love that pdf spec sheet!
I just HAD to do it, so here’s a proper four bit adder:
==== BEGIN LOGICHEX ====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==== END LOGICHEX ====
Scroll down a bit to see the inputs. They’re separated so the left four bits/inputs specify the first four bit number, and the four bits/inputs to the right specify the other four.
If you scroll way up (to the left in the structure), you’ll find the five output bits.
[edit: Fixed a bug with the first output bit]
Actually, the GBA has some sections of write-only memory. Certain special memory locations are used to poke commands to the tile hardware, but attempting to read these bits of memory always returns the same value (which, oddly enough, isn’t zero).
Assuming the hardware can read the bits, it’s not write only.
Now, a harddrive that can store only 0’s would be useful. 
Reminds of the C64 where you would have a loop that copies memory overtop of itself essentially like
for(int i = 0; i < 16384;i++) mem[i] = mem[i];
but what you were doing was copying the image of the BASIC ROM from ROM to the RAM that lived at the same address, then you would flip a bit somewhere so reads would also come from the RAM and at that point you had a copy of the BASIC interpreter that you could tweak to add more commands, patch things, etc.
The cool thing was that the processor would by default read from teh ROM at that address, but the graphics chip (VIC II rulez!) would read from the underly RAM. So you would store bitmaps and sprites “under” the image of the BASIC interpreter so it took no memory away from what was available to your BASIC program.
Pretty cool trick there, swpalmer 
That would be inefficient. You see, it’s possible to store many more 1’s in the same space because they stack more closely, while lots of space between the 0’s goes to waste. So the way to go would be to use a harddrive that can only store 1’s, then convert them to 0’s when read.