Basic Processing Unit

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basic processing unit


  • Instruction Set Processor (ISP)
  • Central Processing Unit (CPU)
  • A typical computing task consists of a series of steps specified by a sequence of machine instructions that constitute a program.
  • An instruction is executed by carrying out a sequence of more rudimentary operations.

Fundamental Concepts

  • The processor fetches one instruction at a time and performs the operation specified.
  • Instructions are fetched from successive memory locations until a branch or a jump instruction is encountered.
  • The processor keeps track of the address of the memory location containing the next instruction to be fetched using Program Counter (PC).
  • Instruction Register (IR)

Executing an Instruction

  •  Fetch the contents of the memory location pointed to by the PC.
  • The contents of this location are loaded into the IR (fetch phase).
    • IR ← [[PC]]
  • Assuming that the memory is byte addressable, increment the contents of the PC by 4 (fetch phase).
    • PC ← [PC] + 4
  • Carry out the actions specified by the instruction in the IR (execution phase).

Executing an Instruction
 Transfer a word of data from one processor register to another or to the ALU.

 Perform an arithmetic or a logic operation and store the result in a processor register.

 Fetch the contents of a given memory location and load them into a processor register.

 Store a word of data from a processor register into a given memory location.

Register Transfers

register transfer computer organization


All operations and data transfers are controlled by the processor clock


Performing an Arithmetic or Logic Operation

The ALU is a combinational circuit that has no internal storage.

  •  ALU gets the two operands from MUX and bus. The result is temporarily stored in register Z.
  • What is the sequence of operations to add the contents of register R1 to those of R2 and store the result in R3?
  1. R1out, Yin
  2. R2out, SelectY, Add, Zin
  3. Zout, R3in

Fetching a Word from Memory

  • Address into MAR; issue Read operation; data into MDR.
  • The response time of each memory access varies (cache miss, memory-mapped I/O,…).
  • To accommodate this, the processor waits until it receives an indication that the requested operation has been completed (Memory-Function-Completed, MFC).
    • Move (R1), R2
    • MAR ← [R1]
    • Start a Read operation on the memory bus
    • Wait for the MFC response from the memory
    • Load MDR from

Signals activated

  • R1out, MARin, Read
  • MDRinE, WMFC
  • MDRout, R2in

Basic Processing Unit

Timing of Basic Processing Unit

timing basic processing unit
The figure represents Timing of a memory Read operation.
Assume MAR is always available on the address lines of the memory bus.

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Execution of a Complete Instruction

  • Add (R3), R1
  • Fetch the instruction
  • Fetch the first operand (the contents of the memory location pointed to by R3)
  • Perform the addition
  • Load the result into R1

Architecture of Basic Processing Unit


Execution of a Complete Instruction

  • Add (R3), R1
    • M DRinE, WMFC

Control Sequencer execution of the instruction Add (R3),R1.

Execution of Branch Instructions in Basic Processing Unit

  • A branch instruction replaces the contents of PC with the branch target address, which is usually obtained by adding an offset X given in the branch instruction.
  • The offset X is usually the difference between the branch target address and the address immediately following the branch instruction.
  • Conditional branch



Multiple-Bus Organization

  • Add R4, R5, R6

multiple bus organization

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