The Cortex-M0 is a 32-bit ARM processor optimized for low-power embedded applications. It has a simplified 3-stage pipeline compared to more complex Cortex-A and Cortex-R series processors. Understanding the pipeline stages helps explain the performance and timing of instructions on the Cortex-M0.
Fetch Stage
The fetch stage loads instructions from memory into the pipeline. On the Cortex-M0, all instructions are 32-bit Thumb instructions. The program counter points to the current instruction being fetched. The Cortex-M0 has a single-issue pipeline, meaning it fetches one instruction per clock cycle in normal conditions.
The fetch stage contains a 32-byte instruction cache to improve performance by avoiding waits to load instructions from slower Flash or RAM memory. However, the cache is tiny compared to multi-kilobyte caches on advanced processors. Cache hits allow back-to-back single-cycle instruction fetches. A cache miss stalls the pipeline until the instruction can be read from main memory.
Prefetching logic looks ahead up to 3 instructions and speculatively loads them into the cache. This reduces pipeline stalls from sequential instruction fetches. However, taken branches and interrupts flush the prefetch buffer to avoid loading instructions from the wrong code path.
Overall, the small cache and prefetching logic help reduce pipeline stalls from instruction fetches. But the pipeline is still sensitive to cache misses, branches, and interrupts disrupting sequential instruction flow from memory.
Decode Stage
After fetch, the decode stage interprets the 32-bit Thumb instruction. It determines the instruction type and opcode. The decoder extracts register operands and immediate constants encoded in the instruction. It also reads register values from the register file.
The Cortex-M0 uses register renaming to avoid write-after-read and write-after-write hazards. Logical registers specified in the instructions are renamed to physical registers transparently. This prevents pipeline stalls from read-after-write hazards on the same logical register.
The decoder translates the Thumb instructions into equivalent microinstructions. These microinstructions control execution in the next pipeline stage. Decoding complex instructions like branches may take more than one cycle.
The decoder is also responsible for generating the address for the next instruction fetch. This may involve incrementing the program counter or setting it to a branch target. Interrupt processing happens in the decode stage to start fetching the interrupt handler code.
Execute Stage
The execute stage performs the actual operation for the decoded instruction using the Arithmetic Logic Unit (ALU) and shifter. This may include arithmetic, logical, and move operations on register operands.
Memory load and store instructions access data memory in this stage. The load data is forwarded to the decoder to be written into the register file without waiting for writeback. This avoids pipeline stalls for read-after-write hazards.
Branch instructions are executed conditionally based on status flags. The branch target address is set in the decode stage and fetched in the next cycle without a pipeline bubble.
The Cortex-M0 can only perform one operation per clock cycle. Multi-cycle instructions like multiply and divide tie up the ALU for multiple cycles. The pipeline is stalled during these instructions.
Because the processor has only simple integer execution resources, most instructions execute in a single cycle. The pipeline usually moves one instruction per clock through the execute stage under normal conditions.
Writeback Stage
The final pipeline stage writes the results from the execute stage into the register file so they can be accessed by subsequent instructions. Values are written into the physical register assigned to the logical register destination.
Stores to data memory also happen in the writeback stage. The data is written out to the memory addressed in the execute stage.
The writeback stage completes the pipeline for most instructions. Branches, loads, and multi-cycle instructions may require additional cycles to finish execution after writeback.
Writeback is a single cycle stage and does not stall the pipeline under normal conditions. Hazard detection makes sure instructions do not try to use registers before their values are written back.
Pipeline Performance
The Cortex-M0 achieves close to 1 instruction per cycle throughput despite its simple 3-stage pipeline. This is enabled by:
- Single-issue, single-cycle execution of most integer instructions
- Tiny prefetch buffer to reduce stalls from instruction cache misses
- Register renaming to avoid data hazards between pipeline stages
- In-order execution and hazard detection to stall the pipeline when necessary
However, there are some caveats to the performance:
- Tightly coupled data and instruction memories impose access latency
- Short prefetch buffer is easily disrupted by branches and interrupts
- No speculative execution or branch prediction
- Cache misses stall the pipeline for multiple cycles
- Multi-cycle instructions like multiply create pipeline bubbles
In the end, the simplicity of the pipeline and limited execution resources are a tradeoff to achieve the goal of a low-power microcontroller capable of delivering decent performance at very low cost and power consumption.
Optimizing Code for the Pipeline
Understanding the pipeline stages can help developers write efficient code for the Cortex-M0. Some tips include:
- Minimize branches to avoid disrupting prefetching
- Optimize hot loops to fit in the prefetch buffer
- Place frequently used data and code in internal SRAM to avoid cache misses
- Minimize large stack frames that spill out of internal registers
- Use single-cycle integer instructions where possible instead of multi-cycle operations
- Employ register renaming and hazard detection rules when accessing the same register in close succession
- Take interrupt latency into account when optimizing real-time code
With awareness of the pipeline design, developers can create efficient Cortex-M0 code that maximizes performance and minimizes power consumption.
