280 Computer Architecture and Organization
9.5 PROGRAMMED CONTROL
In case of programmed control, the complexity of a hardware-control unit is not present. In this case, all
necessary actions to generate the required control signals are performed through software.
9.5.1 Working of Programmed Control
In case of programmed control, a tiny processor along with a memory of suitable size is incorporated
within the circuit, as shown in Figure 9.7 .
Just like all other processors, this processor is also driven primarily by the clock. However, instead of
a program memory with instructions to be fetched, decoded and executed, it is equipped with a memory
for control word storage. Every control word is a set of signal-states, which would implement a particu-
lar action like enabling the accumulator content to a speci c bus or latching data in a speci ed register.
If required, then two or more actions may also be implemented by one control word.
Moreover, unlike a normal processor, as we have discussed in Chapter 5, this processor does not
fetch any instruction or control word from its memory. Rather, it just sends the relevant addresses in
sequence and the data for those addresses (bunch of control signals) that come out from the control word
storage area, as shown in Figure 9.7 .
Figure 9.7 Basic structure of software control
The start address for any instruction is generated by the start address generator after decoding the
instruction, available within the IR. Note that within the control word storage area, sequential codes of
necessary control words for every instruction are available.
9.5.2 Advantages of Programmed Control
In case of micro-programmed control, any minor change in format or sequencing of control signals may
easily be implemented by modifying the control word storage. This is extremely bene cial not only
during the development stage but also at a later stage for improving the performance of any processor.
9.5.3 Disadvantages of Programmed Control
The major disadvantage of micro-programming is that its instruction execution speed is slower than the
hardwired controlled processors as multiple clock cycles are necessary to execute the micro-steps of any
instruction, which is almost instantaneous for processors with hardwired control unit. For example, the 8051
microcontroller needs a minimum of 12 clock cycles to execute an instruction as it is micro-programmed. On
the other hand, the AVR microcontroller is hardware controlled and executes instructions faster than 8051,
with the same clock frequency. Microprocessors like 8085 or 8086 are also micro-programmed.
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