If you want to figure it out the first step I would do is lay out the ranges as bits
1111 11 5432 1098 7654 3210 ------------------- 0x1000 = 0001 0000 0000 0000 serial 0x100F = 0001 0000 0000 1111 0x1010 = 0001 0000 0001 0000 parallel 0x101F = 0001 0000 0001 1111 0x8000 = 1000 0000 0000 0000 memory 0xFFFF = 1111 1111 1111 1111
Now based on the comment you made "A15 - A4 are the bits that matter and A3-A0 are dont cares." It makes me think that you only need to select whether it goes to memory, serial or parallel. We don't care about telling the difference between the min or max of a range, only that we are inside the range. So what we need to do is detect if bits that are unique to the entire range.
If you look at the above chart, we can see we can tell the difference between a Serial and parallel by checking if bit 4 is set. Now to tell if we are using one of the IO or if we are using memory we need to see if bit 15 is set.
We can see this easier if we mask off the bits that don't effect us
1111 11 5432 1098 7654 3210 ------------------- 0x1000 = 0XXX XXXX XXX0 XXXX serial 0x100F = 0XXX XXXX XXX0 XXXX 0x1010 = 0XXX XXXX XXX1 XXXX parallel 0x101F = 0XXX XXXX XXX1 XXXX 0x8000 = 1XXX XXXX XXXX XXXX memory 0xFFFF = 1XXX XXXX XXXX XXXX
So we just need 2 bits to be able to choose the device we want, your multiplexer would look like this
MUX ----------- A4 --| 0 00 |--- Serial | | | 01 |--- Parallel | | ___ A15 --| 1 10 |---\ \ | | | >--- Memory | 11 |---/___/ ----------- (^--- That is a OR gate)
Accessing something not in one of the 3 defined ranges will give "undefined behavior". If you don't want undefined behavior, then you need to define it! What will happen when I attempt to access a address in the range of 0x0020 to 0x7FFF?
Here is a full schematic of the original setup, I could not make thicker lines for the buses, Just assume the A0 on one side matches to A0 on the other. CS is Chip Select.
