Tristate buffer A circuit which allows an input

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 d 2 d 3 e 2 d 1 e 1 d 0 e 0 1

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 LD 1 M 1 Op. Code 4 -bit Reg M U X LD 2 Keyboard enable A L U M 2 2

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 3