Design of Control Unit

The Control Unit is classified into two major categories:

1. Hardwired Control
2. Microprogrammed Control

Hardwired Control

The Hardwired Control organization involves the control logic to be implemented with gates, flip-flops, decoders, and other digital circuits.

The following image shows the block diagram of a Hardwired Control organization.

• A Hard-wired Control consists of two decoders, a sequence counter, and a number of logic gates.
• An instruction fetched from the memory unit is placed in the instruction register (IR).
• The component of an instruction register includes; I bit, the operation code, and bits 0 through 11.
• The operation code in bits 12 through 14 are coded with a 3 x 8 decoder.
• The outputs of the decoder are designated by the symbols D0 through D7.
• The operation code at bit 15 is transferred to a flip-flop designated by the symbol I.
• The operation codes from Bits 0 through 11 are applied to the control logic gates.
• The Sequence counter (SC) can count in binary from 0 through 15.

Micro-programmed Control

The Microprogrammed Control organization is implemented by using the programming approach.

In Microprogrammed Control, the micro-operations are performed by executing a program consisting of micro-instructions.

The following image shows the block diagram of a Microprogrammed Control organization.

• The Control memory address register specifies the address of the micro-instruction.
• The Control memory is assumed to be a ROM, within which all control information is permanently stored.
• The control register holds the microinstruction fetched from the memory.
• The micro-instruction contains a control word that specifies one or more micro-operations for the data processor.
• While the micro-operations are being executed, the next address is computed in the next address generator circuit and then transferred into the control address register to read the next microinstruction.
• The next address generator is often referred to as a micro-program sequencer, as it determines the address sequence that is read from control memory.