1. Tri-state buffer
A circuit which allows an input to go to output when desired
Otherwise it behaves as if “nothing” is connected to the wire
An equivalent technology is open-collector design
Tri-state buffers can be used to design multiplexers
An example of a tri-state buffer and a 4 to one multiplexer is shown below e2 e3 e1 e0 d0 d3 d2 d1

2. Sharing bus: Writing result back to registers
Output of ALU may be multiplexed with other inputs to write back
For example a keyboard input and output of ALU is shown
One of the two outputs is selected to be written back in register
Either of the registers can be loaded with data using LD signals A L U
4-bit
Reg M X
LD1 M2 M1
LD2
OpCode
enable
Keyboard

3. Designing instruction
An instruction consists of
Specification of function to be carried out
Specification of two operands
Specification of output destination
Operation code decides what output is selected by ALU
Specification of each operand is used to select correct operand sources like memory or a register
Specification of output destination is used to write the result at the correct place
Control signals are generated from an instruction to control various units
A state machine is used to control the timing of control signals