The instruction pipeline is a key feature of Arm Cortex-M series microcontrollers that allows them to achieve high performance despite their relatively simple in-order execution. In a nutshell, the instruction pipeline breaks down instruction execution into multiple stages, allowing multiple instructions to be in different stages of execution at the same time. This increases instruction throughput and improves overall performance.
Introduction to Instruction Pipelines
An instruction pipeline is like an assembly line in a factory – each stage completes a part of the instruction execution process before passing it along to the next stage. For example, a simple 5-stage pipeline may consist of the following stages:
- Fetch – Fetch instruction from memory
- Decode – Decode instruction opcode and operands
- Execute – Perform actual operation of instruction
- Memory – Access memory for load/store instructions
- Write Back – Write result back to register file
Instead of completing the execution of one instruction before starting the next one, the stages can work on different instructions in parallel. So while one instruction is being executed, the next one can be decoded and a third one can be fetched from memory. This allows multiple instructions to be in flight leading to greater throughput.
Instruction Pipeline in Arm Cortex-M3/M4
The Arm Cortex-M3 and Cortex-M4 processors feature a high-performance 3-stage instruction pipeline:
- Fetch – Fetch instruction and increment Program Counter
- Decode & Execute – Decode instruction opcode, read operands, execute operation
- Write Back – Write results back to register file
The pipeline operates as follows:
- While an instruction is executing in the Decode & Execute stage, the next instruction is fetched.
- While the result of an instruction is being written back, the next instruction can be decoded and executed.
- If the instructions depend on each other, stalls are inserted to preserve correct order of execution.
This 3-stage pipeline improves performance and also reduces energy consumption as compared to traditional non-pipelined architectures. The Cortex-M3 and M4 can achieve 1 cycle per instruction throughput for most instructions.
Pipeline Stages Explained
Fetch Stage
In the Fetch stage, the processor loads the instruction pointed to by the Program Counter(PC) from memory. The PC is then incremented to point to the next instruction. Any change in sequential program flow like branches or jumps are also handled in Fetch stage.
Decode & Execute Stage
In this combined stage, first the instruction opcode is decoded to determine the operation required. Based on opcode, source operands are read from register file. The Arithmetic Logic Unit(ALU) then performs the desired operation on the operands.
For load/store instructions, the memory address is also calculated in this cycle. Load data or address of store instruction is passed to the Memory stage.
Memory Stage
The Memory stage is used to access data memory for Load and Store instructions. For other instructions, this stage is idle.
- For loads, data is read from data memory and passed to Writeback stage
- For stores, the address and data calculated in Decode & Execute stage is used to update data memory
Writeback Stage
In the Writeback stage, the result of the instruction execution is written back to the register file. The result may come from ALU output for arithmetic/logical instructions or loaded data for load instructions.
The register file is only updated at the end to ensure other concurrently executing instructions have a consistent view of the registers.
Pipeline Performance and Efficiency
The performance benefit of pipelining depends on how efficiently the pipeline is utilized. The pipeline efficiency is determined by:
- Inherent Parallelism – The extent of parallelism available in the code which allows instructions to be executed independently without stalls. Code optimization and reordering helps improve parallelism.
- Hazards – Pipeline stalls due to data and control hazards prevents full utilization of the pipeline stages.
To improve efficiency, hazards must be minimized through techniques like forwarding, stalling and flushing. Also, keeping the pipeline full by prefetching instructions is key.
Instruction Pipeline in Arm Cortex-M0/M0+
The Cortex-M0 and Cortex-M0+ feature a simplified 2-stage pipeline optimized for low-power operation:
- Fetch – Fetch instruction and read operands
- Execute – Decode and execute instruction
The pipeline operates as follows:
- Prefetch of next instruction happens in parallel with current instruction execution to keep pipeline full
- Writing back of execution result happens in the first half of the Execute stage for next instruction
- Operand read and decode happens in second half of Execute stage
The 2-stage pipeline reduces power consumption by eliminating unnecessary pipeline registers between stages. But it also limits performance to half the maximum core frequency. The Cortex-M0/M0+ is focused more on power efficiency than top performance.
Comparision of Pipelines
Here is a comparision of the pipelines in different Cortex-M variants:
| Feature | Cortex-M3/M4 | Cortex-M0/M0+ |
|---|---|---|
| Stages | 3-stage | 2-stage |
| Performance | High | Low |
| Pipeline Depth | Deep | Shallow |
| Efficiency | High | Low |
| Power Consumption | Moderate | Low |
| Typical Applications | Processing Intensive | Power Constrained |
Advantages of Pipelining
Some key advantages of instruction pipelining are:
- Higher Throughput – More instructions complete per cycle
- Higher Frequency – Each stage takes less time allowing higher clocks
- Overlapped Execution – Overall execution time reduced for a set of instructions
- Simpler Control Logic – Each stage has simple dedicated logic
- Modular Design – Easy to modify pipeline depth
Challenges in Pipelining
Some key challenges faced in implementing pipelines:
- Pipeline Hazards – Data, control and structural hazards stall pipeline
- Branch Prediction – Unpredictable branches disrupt instruction flow
- Memory Access – Lack of parallelism during memory loads/stores
- Resource Conflicts – Modules like register file are accessed by multiple stages
- Complex Control Logic – Required to handle all corner cases and hazards
Extensive pipelining also increases power consumption due to more operating registers. Complex pipelines are hard to validate and verify.
Conclusion
The instruction pipeline is key to achieving high performance in Arm Cortex-M series despite their in-order execution limitation. The 3-stage pipeline in Cortex-M3/M4 enables high-throughput, low latency execution while the shorter pipeline in Cortex-M0/M0+ optimizes for power efficiency.
Pipelining improves throughput but also introduces complexities like hazards. An efficient pipeline increases speed without compromising energy efficiency or cost. The Arm Cortex-M series strikes a balanced pipeline design suitable for embedded applications.
