The short answer is yes, the ARM Cortex-M4 processor does contain a cache, specifically an instruction cache. The Cortex-M4 is part of ARM’s Cortex-M series of embedded microcontroller cores, designed for low-power and low-cost applications. While earlier Cortex-M cores like the Cortex-M0 and M3 did not include any caches, the M4 added a small instruction cache to improve performance without significantly increasing power consumption or cost.
Overview of the ARM Cortex-M4
The Cortex-M4 is a 32-bit RISC processor core designed for deeply embedded applications. It includes features like a memory protection unit, floating point unit, DSP instructions, low latency interrupts, and optional data watchpoint tracing. The M4 strikes a balance between performance, power efficiency, and cost.
Some key features of the Cortex-M4 core:
- ARMv7-M Thumb-2 instruction set architecture
- Up to 225 DMIPS at 225 MHz clock speed
- Single-cycle fast multiplier
- Low-latency interrupt handling
- Memory Protection Unit (MPU)
- Optional Embedded Trace Macrocell (ETM)
- Digital Signal Processing (DSP) instructions
- Single precision floating point unit (FPU)
- 2-stage pipeline
The M4 is designed to offer better performance than earlier Cortex-M cores without significantly increasing power consumption. The inclusion of the instruction cache is one way ARM achieved this goal.
The Cortex-M4 Instruction Cache
The Cortex-M4 contains a partial instruction cache of up to 32KB. This small cache improves performance by reducing accesses to slower flash memory. The cache stores pre-fetched instructions, allowing the processor to access them quickly without needing to refetch from flash on each access.
Some key characteristics of the Cortex-M4 instruction cache:
- 2-way set associative cache structure
- Up to 32 KB cache size
- Pseudo-random replacement policy
- Optional lockdown regions to guarantee caching of critical code
- Coherency support for multiprocessing
- Cache can be disabled for debug or low-power operation
The cache is organized as 64 sets of 4-byte instructions in each way. Pseudo-random replacement policy avoids deterministic worst-case scenario misses compared to LRU replacement. Lockdown regions allow critical code like interrupt handlers to be permanently cached.
Performance Benefits
The addition of the small instruction cache in the Cortex-M4 provides noticeable performance improvements for most workloads. Benchmarks by ARM show average performance increases of 15% compared to the cache-less Cortex-M3. Individual application speedups depend on factors like the branchiness and size of code.
The cache significantly improves performance when executing loops and branches. By caching loop and branch targets, it avoids having to refetch this data from flash on each loop iteration or branch. This increases speed and reduces power consumption.
Applications with a small active working set of code that fits in the cache see the largest gains. The gains diminish for large code bases that exceed the 32KB cache size. Still, even larger programs benefit from having the most active code cached.
Implementation in Cortex-M4 SoCs
The Cortex-M4 is a CPU core IP that gets integrated into full System-on-Chip (SoC) designs by various semiconductor vendors. Vendors can customize configuration options like cache size based on their chip.
For example, Microchip’s SAM4L processors contain a Cortex-M4 with 8KB instruction cache. NXP’s Kinetis K series has Cortex-M4 chips with 32KB cache. STMicroelectronics’ STM32F4 series scales cache from 0KB up to 32KB across models. Cacheless options exist for ultra low-power or cost-sensitive applications.
On some SoCs the Cortex-M4 cache is 4-way associative rather than 2-way. So cache configuration can vary across implementations, but presence of instruction caching is standard.
Interaction with Compilers and Programming
The Cortex-M4 instruction cache is transparent to developers and compilers. ARM designed it to not require any cache-related code changes compared to earlier cacheless Cortex-Ms. Standard compiler optimizations like Basic Block reordering are compatible and help improve cache hit rates.
However, developers can further optimize for the M4 cache if desired. Techniques like loop unrolling, function inlining, and reducing code size can improve performance. But in many cases no manual optimization is needed to benefit from caching.
The cache operates on entire functions. Code should be arranged so performance-critical functions fit within the cache size. Placing frequently called functions in separate flash pages also helps avoid cache conflicts.
In hard real-time systems, the pseudo-random replacement policy could introduce undesirable variability. This can be mitigated by using cache lockdown regions for time-critical code segments.
Overall the presence of the cache rarely requires any code-level consideration. Compilers combined with the hardware prefetcher transparently leverage the cache to speed up most applications compiled for Cortex-M4.
Power and Cost Considerations
While the Cortex-M4 instruction cache improves performance, it also increases power consumption and chip area relative to earlier cacheless Cortex-M cores. However ARM designed it to have minimal impact suitable for embedded applications.
The cache adds about 10,000 gates of logic to the Cortex-M4. The cache RAM itself consumes some additional memory footprint and static leakage power. Dynamic power from cache hits is mostly offset by reduced external memory accesses.
ARM estimates the Cortex-M4 cache adds less than 5% to power consumption for a typical workload compared to the M3. So embedded applications can utilize it without major power cost. The benefits outweigh the small overheads.
For the most power-sensitive microcontroller applications, implementers can disable the cache entirely. But it’s valuable for IoT-class designs balancing performance and power.
The Cortex-M4 hits a sweet spot maintaining the energy efficiency of an embedded MCU while utilizing caching to improve real-world speed. This combination of efficiency and higher performance opened up new markets like automotive control systems for ARM’s Cortex-M series.
Alternatives for Higher Performance
While the addition of instruction caching boosted Cortex-M4 performance, there are ways to achieve even greater speed:
- Larger Cache Sizes – Increasing max cache beyond 32KB would allow caching more code
- Full MMU – Protected memory management unit for OS support and virtualization
- Data Caching – The M4 only has instruction cache
- Superscalar Pipelines – Allow simultaneous execution of multiple instructions
- Out-of-Order Execution – Reorder instruction execution for optimal speed
- Speculative Execution – Predictively execute branches and rollback on mispredicts
These techniques require substantially greater transistor budgets and power consumption unsuitable for embedded applications. They are employed by ARM’s Cortex-A series designed for application processors.
The incremental improvement from the Cortex-M4 instruction cache hits a sweet spot for deeply embedded MCU-class processors. More advanced architectures would sacrifice the energy efficiency and deterministic real-time behavior desired in microcontrollers.
Conclusion
In summary, the ARM Cortex-M4 includes a small low-power instruction cache of up to 32KB. This provides noticeable speed improvements through caching frequently used code while only minimally impacting power and cost. The instruction cache is transparent to developers and requires no code changes. It helped boost performance at similar power levels compared to earlier cacheless Cortex-M cores. The Cortex-M4 hits a sweet spot adding just enough caching to benefit embedded microcontroller applications without the overheads of more advanced cache architectures.
