Nand2Tetris CPU containing PC gate: not sure what to do with PC gate output[15] single bit since CPU PC out requires just [0..14]

Question

In the CPU, the PC gate according to the built-in chips of the course files are 16 bit-width in/out.

I've been able to implement the HDL code of the CPU and it makes sense. The only doubt I'm having is the single 16th bit/highest order bit of the PC gate output that I can't find any purpose for. The PC output of the CPU just wants 15 bits and I can't ignore an unused bit. It's gotta go somewhere.

I have a way to "throw it away" into a gate with the reset input of the CPU without harming the reset logic using OR. Just doesn't seem right given how air tight these projects are.

I've been advised to not build the chip myself that would otherwise take 15 bits in/out.

Answer 1

The PC output of the CPU just wants 15 bits and I can't ignore an unused bit. It's gotta go somewhere.

Have you tested this assumption?

You can output individual bits and bit ranges using the [] notation.

For example, if you for some reason needed the low 8 bits and lsb of a 16 bit "out" value of a chip, you could do something like:

CHIP(...,out[0..7]=lobyte,out[0]=lsb)

In this case, bits 8-15 would be unused and discarded.

See A 2.2 Multi-Bit Buses in the textbook for more details.