The entire decoder-encoder unit of logic circuit can be implemented in the form of a single Programmable Logic array (PLA). Thus the control section of a CPU of any digital system can be organized as shown

MICROPROGRAMMED CONTROL

Control signals can be generated by another method known as micro programmed control. Definitions of some frequently used terms  

• Control Word (CW): A word whose individual bits represent the control signals. Therefore a control step in a control sequence of an instruction is a unique combinations of 1s and 0s in the control word.
• Micro-routine: A sequence of control words corresponding to the control sequence of a machine instruction constitutes the micro routine for that instruction.
• Micro-program Memory: It is a special memory in which the micro-routines corresponding to the instruction set of a computer are stored. The control unit can sequentially generate the control signals for any instruction and also the control words
• Micro-program Counter (µPC): This is used to read the control words sequentially from the micro-program memory.

Following is the control word corresponding to steps 5, 6, and 7 of table2

Basic Organization of Microprogrammed Control Unit

• When a new instruction is loaded in IR, the starting address is loaded into the µPC from the starting address generator block
• Automatically µPC is incremented by the clock, causing successive microinstructions to be read from the memory. Thus, the control signals are loaded into various parts of the CPU in the correct sequence.

Sequencing of Control Signals << Previous
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