Sequencing of Control Signals


  • To execute the instructions, the CPU must generate the control signals in the proper sequence.
  • Two techniques to generate the control signals are
  1. Hardwired control
  2. Micro programmed control

HARDWIRED CONTROL

  • A decoder-encoder circuit generates the required control signals depending on the state of all inputs
  • Consider the sequence of control signals given in Table 2. In this seven non-overlapping time slots required to execute the instructions represented by this sequence. Each time slot must be at least long enough for the function specified in a particular step to be completed
  • Let us assume that all the time slots are equal in duration. So the control unit is designed based on the use of counter driven by a clock signal, CLK. Each state or count of this counter corresponds to one of the steps given in Table 2. Hence the required control signals are determined based on the following information

    • Contents of the Control counter

    Contents of the Instruction Register

    Contents of the control codes and other status flags

  • The status flag represents the state of various sections of the CPU and various control units connected to it. For example MFC

SEPERATION OF ENCODING AND DECODING UNITS

  • The step encoder provides a separate signal for each time slot in the control sequence. Similarly the output of instruction decoder consists of separate lines for each instruction.
  • When the Instruction is loaded into IR, one of the output lines INS1 to INSm is set to 1 and all other lines is set to 0.
  • All the input signals to encoder are combined to generate control signals such as Yin, PCout, ADD, AND etc.
  • The control signal Zin is generated by the logic function

    • Zin = T1 + T6.ADD +T5.BR+…

Z in is turned on during the time slot T1 for all instructions; during T6 for Add instruction and so on. Time T1 is common to all instructions because it occurs during the fetch phase 



Branching<< Previous
Next >>Practical Considerations


Support us generously: contact@lessons2all.com

Our aim is to provide information to the knowledge seekers. 


comments powered by Disqus


Footer1