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Pipelining in ARM

Last Updated : 18 Jun, 2021
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ARM Pipelining :

  • A Pipelining is the mechanism used by RISC(Reduced instruction set computer) processors to execute instructions,
  • by speeding up the execution by fetching the instruction, while other instructions are being decoded and executed simultaneously.
  • Which in turn allows the memory system and processor to work continuously.
  • The pipeline design for each  ARM family is different.

Pipelining is a design technique or a process which plays an important role in increasing the efficiency of data processing in the processor of a computer and microcontroller. By keeping the processor in a continuous process of fetching, decoding and executing called (F&E cycle).   

ARM devices need pipelining because of RISC as it emphasizes on  compiler complexity. Each stage is equivalent to 1 cycle, that is n stages = n cycles.

Pipeline :

3 stage pipelining

  • Fetch loads an instruction from memory.
  • Decode identifies the instruction to be executed.
  • Execute processes the instruction and writes the result back to the register.
  • By over lapping the above stages of execution of different instructions, the speed of execution is increased.
  • The pipelining allows the core to execute an instruction every cycle, which results in increased throughput.

ARM pipeline characteristics :

  • The ARM pipeline doesn’t process an instruction until it passes completely through the execution stage.
  • In the execution stage, the PC always points to the instruction address + 8 bytes.
  • When the processor is in thumb state, PC always points to the instruction address + 4 bytes.
  • While executing branch instructions or branching by direct modification of PC causes the ARM core to flush it’s pipeline.
  • As instruction in the execution stage will complete its execution even though an interrupt has been raised.

ARM 7 –

  • It has 3 stage pipelining as shown in the figure.
  • It can complete it’s process in 3 cycles.
  • It has the basic F&E cycle leading to optimum throughput.
  • This is why the ARM 7 has the lowest throughput as compared to that of it’s other family members.
  • It processes 32bit data.

ARM 9 –

  • Pipelining in ARM 9 is similar to ARM 7 but with 5 stages.
  • It takes 5 cycles to complete the process.

5 stage pipelining

  • Fetch- It will fetch instructions from memory.
  • Decode- It decodes the  instructions that were fetched in the first cycle.
  • ALU –  It executes the instruction that has been decoded in the previous stage.
  • LS1(Memory)  Loads/Stores the data specified by load or store instructions.
  • LS2(Write) Extracts (zero or sign) extends the data loaded by  byte or half word load instruction.
  • Because of an increase in stages and efficiency, the throughput is 10%-13% higher than ARM 7.
  • Core frequency of ARM 9 is slightly higher than that of ARM 7.

ARM 10 –

  • It is a six stage pipeline. Which in turn takes 6 cycles to complete the process.
  • Same as that of ARM 9 but with an issue stage which checks whether the instruction is ready to get decoded in the current stage or not.
  • It nearly doubles the throughput than that of ARM 7.
  • The core frequency is higher than that of ARM 9.

6 stage pipelining

 The stages of pipelining may increase or decrease on the basis of the instruction sets processed per cycle (In maximum situations, stages tend to increase to increase efficiency).

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