BusMUX

Inputs

Outputs

How It Works

BusMUX is a railway switch for bytes. Two 8-bit buses arrive at inputs A and B, while a single selector bit comes to the control pin Sel: Sel = 0 routes bus A to the output whole, Sel = 1 routes bus B. Inside, it is eight ordinary one-bit multiplexers driven in sync by the same Sel — so the selection happens for all bits at once, with no skew between digits.

The key property: the selected bus passes to the output unchanged, all 8 bits at once, while the unselected one is ignored completely — the bytes do not mix and do not add up. A byte example: A = 00001111 (15), B = 11110000 (240). With Sel = 0 the output is 00001111; with Sel = 1 it is 11110000. No intermediate value between them ever appears.

A subtlety: BusMUX is combinational logic — it remembers nothing. Sel changes and the output instantly switches to the other source. Also, you cannot "see" both inputs through a BusMUX at the same time: to compare two buses, run them through one at a time or feed them into a circuit that honestly reads both inputs — such as BusXOR.

8-bit Examples

Usage

BusMUX is the key element of the processor datapath. In the ALU (level 13) it selects the second operand: sometimes a value from a register, sometimes a constant from the instruction. A multiplexer of its own stands at the input of each register — deciding what to memorize on the next tick: the ALU result, a data byte from memory, or the previous value for rewriting.

In the Harvard architecture (level 16 and beyond) the data bus is shared, but it has several sources, and BusMUX makes sure exactly one of them gets through: an instruction from ROM or a data byte from RAM, the ALU result or gamepad input. The selector is driven by the instruction decoder according to the opcode. The same trick saves hardware: instead of duplicating circuits, one multiplexer feeds different sources to a shared input in turn.