Control Unit | CSO

Control Unit

There are two type of control organisation:

1. Hardwired control
2. Micro Programmed control

Hard wired control

In hardwired organization the control logic is implemented with gates,Flip-Flops, Decoders, and other digital circuits.In this type we can optimize to produce fast mode operation.In hardwired control we can change the wiring among component if needed.

Micro Programmed control

In micro programmed organization the control information is stored in a control memory.The control memory is programmed to initiate the required sequence of micro operations.In micro programmed control any modification is done by updating micro program in control memory.

In the above diagram we have two decoders, a sequence counter,number of control logic gates.Here an instruction is read from memory is placed in (IR). The operation code in IR i.e bit 12 through 14 are decoded with a 3X8 decoder. The 8 output of decoder is D0 to D7.
Bit 15 of IR is transferred to a flip flop designated by symbol I.
Bit 0 to 11 are applied to the control logic gates.
The 4-bit sequence counter can count in binary from 0-15.The output of the counter are decoded into 16 timing single T0-T15.
The sequence counter(SC) can be incremented or cleared. synchronously.Most of the timing signals out of the 4 X 16 decoder.
Once the counter is cleared to 0 causing the next active time signal to be T0.

Instructions

There are basically three instruction format such as

1. Memory-Reference Instruction

Range of opcode=(000 to 110)
I=0 - Direct addressing mode
I=1 - Indirect addressing mode.

2. Register-Reference Instruction

opcode=111 , I=0
Here memory operand is not needed.
Operation is perform with AC register.

2. Input-Output Instruction

opcode=111 , I=1
Memory operand is not needed
Operation is performed with AC register.

Timing and Control

The timing for all register in the basic computer is controlled by a master clock generator.The CLK pluses are applied to all flip flop and registers in the system.The clock pulse do not change the state of register unless register is enabled by control signal. The control signal are generated in the control unit and provide control unit for the multiplexer in the common bus.

Know More>>>

• Introduction of Computer Organization
• Addressing Mode
• Common Bus System
• Control Unit
• Type of Instructions

Content is Reference form Compute System Architecture, MORRIS M. MANO