I've added a diagram of the ALU to the Details section. It has 5 functions: bitwise AND, bitwise NOT, Add with carry, decrement, and passthrough. The first 4 fuctions are obvious enough and map directly to opcodes. The last, passthrough, may require some explanation. Passthrough makes the output the same as the A input. This function, in combination with the 4-way mux on the A input, supports the @ and ! memory storage opcodes, and the >R and R> opcodes which shift data between the Data and Return stacks.

For example, set the multiplexer to [T] and the ALU function to Passthrough. [T] is the data from RAM or I/O (depending on the high address bits) at the address held in T. This data is now passed through and the address is replaced by the memory contents held at that address (similar to how the Forth '@' command works). The Passthrough function in this case avoids the need for an additional register to address the memory.