1

• 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

Tri-state buffer

e2e3 e1 e0

d0d3 d2 d1

2

• 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

Sharing bus: Writing result back to registers

ALU

4-bitReg

4-bitReg

MUX

MUX

LD1

M2

M1

LD2

OpCode

enable

Keyboard

enable

3

• 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

Designing instruction