Computer Science 210Computer 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

• 2N output lines

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

DecoderI1 I0 O0 O1 O2 O30 0 1 0 0 00 1 0 1 0 01 0 0 0 1 01 1 0 0 0 1

• O0: _ _ I1 I0

• O1: _ I1 I0

• O2: _ I1 I0

• O3: I1 I0

I1

I0

O0

O1

O3

O2

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)

• 2N 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 Mux1. 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

C1 C0 O0 O1 O2 O30 0 I0 0 0 00 1 0 I1 0 01 0 0 0 I2 01 1 0 0 0 I3

3. Send outputs to an OR gate.

C1 C0

I0

I1

I2

I3

O

C1 C0 O

0 0 I00 1 I11 0 I21 1 I3

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.