1. More Devices: Control (Making Choices)
Computer Science 101
More Devices:
Control (Making Choices)

2. 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

3. 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.

4. Decoder O0: _ _ I1 I0 O1: _ I1 I0 O2: _ I1 I0 O3: I1 I0 O0 I1 O1 O2 I0

5. 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.

6. 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.

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

8. 2. Add an input to each AND gate

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

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

11. Arithmetic and Logic Unit (ALU)p e r a t i o n C a r r y I n
Three operations (AND, OR, +) are run simultaneously
A mux selects the output of one of them, based on control codes a 1 R e s u l t 2 b C a r r y O u t