Lecture 12: Hardwired Control Unit

#ControlUnit #TimingandControlUnit #ComputerArchitecture
There are two major types of control organization: hardwired control and micro-programmed control. in the hardwired organization, the control logic is implemented with gates, flip-flops, decoders, and other digital circuits. It has the advantage that it can be optimized to produce a fast mode of operation. In the microprogrammed organization, the control information is stored in a control memory. The control memory is programmed to initiate the required
sequence of microoperations. A hardwired control, as the name implies, requires changes in the wiring among the various components if the design has to be modified or changed. In the microprogrammed control, and required changes or modifications can be done by updating the microprogram in control memory.

Hardwired Control Unit:
It consists of two decoders, a sequence counter, and a number of control logic gates. An instruction read from memory is placed in the instruction register (IR), which is divided into three parts: the I. bit, the operation code, and bits 0 through 11. The operation code in bits 12 through 14 are decoded with a 3 × 8 decoder. The eight outputs of the decoder are designated by the symbols D0 through D7. Bits 15 of the instruction is transferred to a flip-flop designated by the symbol I. Bits 0 through 11 are applied to the control logic gates. The 4-bit sequence counter can count in binary from 0 through 15. The outputs of the counter are decoded into 16 timing signals T0 through T15.
The sequence counter SC can be incremented or cleared synchronously. Most of the time, the counter is incremented to provide the sequence of timing signals out of the 4 × 16 decoder. Once in a while, the counter is cleared to 0, causing the next active timing signal to be T0.

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