Computer Science 210 Computer Organization Control Circuits Decoders and Multiplexers

Control Circuits Control circuits are special circuits that are used to control other circuit components. – Enable or disable components. – Select one of many components. – Determine which operation is performed. Two types we’ll study now: – Decoders – Multiplexors

Decoder N input lines 2 N output lines Each setting of the input lines “selects” one of the output lines to be 1 with the other output lines being 0.

Decoder O 0 : _ _ I 1 I 0 O 1 : _ I 1 I 0 O 2 : _ I 1 I 0 O 3 : I 1 I 0 I1I1 I0I0 O0O0 O1O1 O3O3 O2O2

Some uses of decoders The inputs could be bits of a memory address, and the decoder could select the memory cell to activate. The inputs could be bits in an instruction telling what type of operation is to be performed, and the decoder could activate the appropriate machine components.

Multiplexer (Mux) 2 N input lines N control lines 1 output line The control lines “select” which of the input lines is “passed on” to the output.

Construction of 4-input Mux 1. Decoder with controls as inputs C1 C0

2. Add an input to each AND gate C1 C0 I0 I1 I2 I3 O0 O1 O2 O3

3. Send outputs to an OR gate. C1 C0 I0 I1 I2 I3 O

Some uses of multiplexers Select which of several computers connected to single printer can print. Decide which of several completed operations should be sent to the output.