Implementation of a 8-to-256 decoder using BJTs
If you want to implement a decoder by discrete components you can use DTL and a diode matrix:

Here is an example of a 2-to-4 decoder that explains the basic principle:


for each address line create an inverted signal
for each output create an AND gate that connects to the inverted address line for a 0 address bit and to the non-inverted address line for 1 address bit. 


Probably you have to buffer the outputs.
I't won't be very high speed (depending on the pull up resistors).
Yes, you need lots of diodes (8 × 256 = 2048). 







simulate this circuit &ndash; Schematic created using CircuitLab