Computer Science 101

1. Computer Science 101 Control Circuits

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. Fargo North, 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. O0 I1 O1 O2 I0 O3 Decoder • O0: _ _ I1 I0 • O1: _ I1 I0 • O2: _ I1 I0 • O3: I1 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. Multiplexor • 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. C1 C0 Construction of 4-input Mux1. Decoder with controls as inputs

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

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

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. How can I be nostalgicwhen I can’t remember?