|This is a 2-bit decoder. It is used to access a desired memory location. A regular decoder takes a binary number as an input, and "activates" the corresponding bit of output. In our case, we first reset the decoder such that all output bits are active, and then as we enter the binary input, the incorrect outputs are deactivated.|
The first ball that goes through is the reset ball. The other two balls are the binary input. The red squares are the output bits, and the one which remains on the right is the active one.
This was Jeff's baby. So maybe he will describe it more.