16 Bits of Memory

This prototype memory unit consists of 16, 1-bit registers arranged such that we have 4 memory locations, each 4 bits long. The unit is addressable using 4 decoders, which make up the top of the structure. The decoders are set using 3-bit; 2 for the address and one to determine reading or writing.
After developing the unit, we actually decided to scrap this format for a number of reasons. The first is that we want something more vertical, so we can put it directly on top of the adder-subtracter. The other reason is that the decoders were difficult to set using balls.

We have already developed the new memory and are in the process of making all 16 bits.

4-Bit Adder

If you look closely, you can see that this 4-bit adder is 4, 1-bit full adders connected together. The carry-out of one adder drops into the carry-in of the next, moving from right to left.


Since these pictures were taken, we modified the adder to be an adder-subtracter. We also added a zero flag.