The Arm Cortex-R4 is a 32-bit RISC processor optimized for real-time applications. Configuring the memory and caches properly is key to achieving optimal performance. This article provides a comprehensive guide on configuring the memory and caches when working with the Cortex-R4 processor.
Overview of Cortex-R4 Memory and Caches
The Cortex-R4 contains separate instruction and data caches, along with tightly coupled memory (TCM). Key features include:
- 16KB to 64KB 2-way set associative instruction cache
- 16KB to 64KB 4-way set associative data cache
- Up to 64KB TCM with single cycle access
The caches use a pseudo-least recently used (PLRU) replacement policy. Cache line sizes are 4 words (16 bytes) for the instruction cache and 8 words (32 bytes) for the data cache. Write buffers are used to reduce stalls on store operations. By default, write allocation is enabled in the data cache.
Considerations for Cache Configuration
There are several factors to consider when configuring the Cortex-R4 caches:
- Application requirements – Real-time applications often benefit from disabling caches to ensure predictable timing. Other applications like signal processing may require caches for performance.
- Memory access patterns – Applications with a high degree of spatial locality and sequential accesses benefit more from caching.
- Memory latency – Slower memories make caching more beneficial to hide access latency.
- Power and area constraints – Larger caches improve performance but consume more power and area. The configuration must balance this tradeoff.
Understanding the application behavior and requirements is key for configuring an optimal cache configuration tailored to the use case.
Cache Configuration Options
The Cortex-R4 cache configuration options include:
- Cache size – Instruction and data cache sizes can be configured from 16KB to 64KB.
- Set associativity – The instruction cache is 2-way set associative while the data cache can be 2-way or 4-way set associative.
- Write buffer size – The write buffer depth can be configured as 2, 4, or 8 entries.
- Write allocation – Write allocation for the data cache can be enabled or disabled.
- Cache lockdown – Ways within the cache can be locked for critical code/data.
- Cache enable/disable – Caches can be enabled or disabled altogether.
These options provide flexibility to tune the cache configuration for different use cases. The optimal configuration depends on the application requirements and tradeoffs between performance, power, and area.
Guidelines for Configuration
Here are some guidelines for Cortex-R4 cache configuration:
- For real-time applications, consider disabling caches altogether and relying on TCM for low latency access.
- For applications with high spatial locality, use larger cache sizes in the 32KB-64KB range.
- When memory latency is high, 4-way set associative caches can help reduce conflicts.
- Enable cache lockdown and focus caching on critical code/data regions.
- If power consumption is critical, use smaller 16KB caches.
- For data intensive applications, a 4 entry write buffer helps reduce stalls.
- Disable write allocation if the application frequently writes to non-cached regions.
It is also helpful to start with cache enabled with default sizes and then scale configuration based on profiling of cache hit/miss rates and application performance.
Enabling and Disabling Caches
The Cortex-R4 caches are enabled by default out of reset. The registers used for cache control are:
- CP15 Cache Type Register – Specifies cache size and associativity configuration.
- CP15 System Control Register – Contains cache enable/disable bits for instruction and data caches.
For example, to disable both instruction and data caches, the following sequence can be used: MRC p15, 0, R1, c0, c0, 0 ; Read CP15 System Control Register ORR R1, R1, #(0x1 <<2) ; Set I bit to disable I-cache ORR R1, R1, #(0x1 <<0) ; Set C bit to disable D-cache MCR p15, 0, R1, c0, c0, 0 ; Write updated value back
It is important to invalidate the caches before disabling by issuing the CP15 cache invalidate all operation. Cache coherency must also be managed carefully when enabling caches.
Configuring Tightly Coupled Memory
The Cortex-R4 TCM provides single cycle access latency and deterministic timing for real-time applications. Key configuration options include:
- TCM size – Up to 64KB for both instruction and data TCM.
- TCM base addresses – Defined based on memory map requirements.
- TCM access ports – 1, 2 or 3 ports can be configured.
- TCM arbitration – Round robin or fixed priority arbitration.
TCM is integrated with the processor bus interface unit. Accesses to addresses mapped to TCM are handled via dedicated ports without going to the main system bus.
TCM provides guaranteed performance for critical code and data segments. It should be configured based on application requirements for real-time response. Typical uses include:
- Interrupt handlers
- Task switching code
- High priority task routines
- Frequently used lookup tables
- Performance critical algorithms
Profiling tools can identify the hot code/data sections to place in TCM. Auto-incrementing variables commonly used in real-time code can also benefit from TCM allocation.
Optimizing Cache Performance
Some techniques for optimizing cache performance on the Cortex-R4 include:
- Aligning code and data to cache line boundaries.
- Placing critical code and data in cache or TCM.
- Organizing data structures to maximize spatial locality.
- Using const variables to allocate read-only data in cache.
- Allocating stack buffers in TCM for real-time tasks.
- Minimizing branching to improve instruction fetch performance.
- Using cache lockdown and cache clean/invalidate operations appropriately.
Profiling cache behavior using embedded trace macros (ETM) or performance counters is key for tuning cache usage. Statistics on cache hit/miss rates, write buffer stalls, and bus accesses can identify optimization opportunities.
Conclusion
Configuring memory and caches properly is vital for leveraging the full capabilities of the Cortex-R4 processor. Key considerations include application requirements, access patterns, and tradeoffs between performance, power, and area. TCM, cache size/associativity, and cache control features like lockdown provide extensive flexibility for optimization. Profiling cache behavior and targeting critical code/data is key for maximizing real-time performance. This guide provides an overview of the key configuration options and guidelines for tuning Cortex-R4 memory and caches.
