Chapter 4.3, Problem 4.20PP

Program Plan Intro

Processing stages:

• The processing of an instruction has number of operations.
• The operations are organized into particular sequence of stages.
• It attempts to follow a uniform sequence for all instructions.
• The description of stages are shown below:
  • Fetch:
    • It uses program counter “PC” as memory address to read instruction bytes from memory.
    • The 4-bit portions “icode” and “ifun” of specifier byte is extracted from instruction.
    • It fetches “valC” that denotes an 8-byte constant.
    • It computes “valP” that denotes value of “PC” plus length of fetched instruction.
  • Decode:
    • The register file is been read with two operands.
    • It gives values “valA” and “valB” for operands.
    • It reads registers with instruction fields “rA” and “rB”.
  • Execute:
    • In this stage the ALU either performs required operation or increments and decrements stack pointer.
    • The resulting value is termed as “valE”.
    • The condition codes are evaluated and destination register is updated based on condition.
    • It determines whether branch should be taken or not in a jump instruction.
  • Memory:
    • The data is been written to memory or read from memory in this stage.
    •  The value that is read is determined as “valM”.
  • Write back:
    • The results are been written to register file.
    • It can write up to two results.
  • PC update:
    • The program counter “PC” denotes memory address to read bytes of instruction from memory.
    • It is used to set next instruction’s address.

Combinational circuits and HCL expressions:

• The computational blocks are been constructed by accumulating several logic gates into network.
• The restrictions are been shown below:
  • Each of input for logic gate should be associated to any one shown below:
    • One of system inputs, that is identified as primary inputs.
    • Output connection for some element in memory.
    • Output of some logic gate.
  • Outputs obtained from more than two logic gates could not be linked together.
    • The wire would be driven to different voltages.
    • It can cause malfunction in circuit.
  • The network should not contain cycles.
    • The loops in circuit can cause ambiguity in function computed by network.
• The “HCL” denotes a hardware control language that is used for describing control logic of different processor designs.