Timing and control unit
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The control unit generates relevant timing and control signals to coordinate all operations in the computer. It directs the entire system to carry out instructions by communicating with the ALU and memory. Control units are implemented with either a hardwired or microprogrammed design. A hardwired control unit uses logic circuits to generate signals but is complex and difficult to modify. It works fast but changes require rewiring. A microprogrammed control unit uses a microprogram to generate signals and is easier to modify but slower.
Timing and control unit
- 2. CONTROL UNIT • CPU is partitioned into Arithmetic Logic Unit (ALU) and Control Unit (CU) • The function of control unit is to generate relevant timing and control signals to all operations in the computer. • It controls the flow of data between the processor and memory and peripherals
- 3. FUNCTIONS OF CONTROL UNIT • The control unit directs the entire computer system to carry out stored program instructions. • The control unit must communicate with both the arithmetic logic unit (ALU) and main memory. • The control unit instructs the arithmetic logic unit that which logical or arithmetic operation is to be performed. • The control unit co-ordinates the activities of the other two units as well as all peripherals and auxiliary storage devices linked to the computer.
- 4. DESIGN OF CONTROL UNIT • Control unit generates control signals using one of the two organizations: • Hardwired Control Unit • Micro-programmed Control
- 5. HARDWIRED CONTROL UNIT • It is implemented as logic circuits (gates, flip-flops, decoders etc.) in the hardware. • This organization is very complicated if we have a large control unit. • In this organization, if the design has to be modified or changed, requires changes in the wiring among the various components. Thus the modification of all the combinational circuits may be very difficult.
- 6. HARDWIRED CONTROL UNIT ADVANTAGES • Hardwired Control Unit is fast because control signals are generated by combinational circuits. • The delay in generation of control signals depends upon the number of gates.
- 7. HARDWIRED CONTROL UNIT DISADVANTAGES • More is the control signals required by CPU; more complex will be the design of control unit. • Modifications in control signal are very difficult. That means it requires rearranging of wires in the hardware circuit. • It is difficult to correct mistake in original design or adding new feature in existing design of control unit.
- 8. CONTROL UNIT(HARDWIRED) Control unit consist of a: - Instruction Register - Number of Control Logic Gates, - Two Decoders - 4-bit Sequence Counter
- 10. CONTD… • An instruction read from memory is placed in the instruction register (IR). • The instruction register is divided into three parts: the I bit, operation code, and address part. • First 12-bits (0-11) to specify an address, next 3-bits specify the operation code (opcode) field of the instruction and last left most bit specify the addressing mode I. I = 0 for direct address I = 1 for indirect address
- 11. CONTD.. • First 12-bits (0-11) are applied to the control logic gates. • The operation code bits (12 – 14) are decoded with a 3 x 8 decoder. • The eight outputs ( D0 through D7) from a decoder goes to the control logic gates to perform specific operation. • Last bit 15 is transferred to a I flip-flop designated by symbol I.
- 12. CONTD… • The 4-bit sequence counter SC can count in binary from 0 through 15. • The counter output is decoded into 16 timing pulses T0 through T15. • The sequence counter can be incremented by INR input or clear by CLR input synchronously.
- 14. CONTD… • For example: Consider the case where SC is incremented to provide timing signals T0, T1, T 2 , T3, and T4 in sequence. At time T4 , SC is cleared to 0 if decoder output D3 is active. This is expressed symbolically by the statement: D3 T4 : SC ← 0 • The timing diagram shows the time relationship of the control signals.