One-hot selector
A set of columns where exactly one is 1 and the rest are 0, used to encode “which of N options is active” in a way that's cheap to check and multiply against algebraically (multiplying by a 0/1 selector acts like a conditional).
In this repo: two uses — seven columns selecting the opcode (
s_add...s_store), and NUM_REGISTERS columns (reg_sel) selecting which register a LOAD/STORE addresses. See A Register File Without an Algebraic Mux.