The ARM Cortex-M3 processor supports both little endian and big endian data storage formats. The endianness can be configured through the CPU’s control registers during start up. Choosing the right endianness format is important for performance and interoperability.
Introduction to Endianness
Endianness refers to the byte ordering used to store data in memory. There are two formats:
- Little endian – The least significant byte is stored at the lowest address. This is used by Intel x86 processors.
- Big endian – The most significant byte is stored at the lowest address. This is used by ARM and PowerPC processors.
For example, the 32-bit hex value 0x12345678 would be stored in memory as:
| Address | Little Endian | Big Endian |
|---|---|---|
| 0x100 | 0x78 | 0x12 |
| 0x101 | 0x56 | 0x34 |
| 0x102 | 0x34 | 0x56 |
| 0x103 | 0x12 | 0x78 |
The ARM Cortex-M3 supports both endian formats. The choice depends on the specific requirements of the application and industry standards.
Configuring Endianness in the Cortex-M3
The Cortex-M3 endianness is configured through the System Control Register (SCTLR) which is located in the System Control Space. Specifically, bit 25 of the SCTLR controls the endianness:
- 0 = Little endian
- 1 = Big endian
To configure the SCTLR, the steps are:
- Set CONTROL.MPP to 0 to enter privileged mode
- Write the desired value (0 or 1) to SCTLR[25]
- Execute the ISB instruction to synchronize the pipeline
Here is example code to set little endian mode on start up: // Set privileged mode CONTROL.MPP = 0; // Configure SCTLR SCTLR.Endian = 0; // Little endian // Synchronize pipeline ISB;
This configures the Cortex-M3 to use little endian data storage in memory. The endianness applies to all load/store instructions like LDR, STR, LDM, STM. The setting cannot be changed dynamically at run-time.
Endianness Considerations
The main factors to consider when choosing endian format are:
- Industry conventions – Many standards mandate big vs little endian. Eg. USB uses little endian.
- Interoperability – Data exchanged with other systems should use the same endian format.
- Performance – Little endian is better when the processor is little endian. The Cortex-M3 has a little endian datapath.
Additionally, the endianness applies to the entire system – the processor, peripherals, memories, etc. It is not possible to configure different endian formats for different components.
Software also needs to handle endian conversions when interfacing with multi-endian systems. This usually requires byte-swapping algorithms implemented in the device drivers.
Dealing with Endianness Mismatches
If there is an endianness mismatch between the processor and an external interface, byte swapping needs to be performed in software. This typically occurs in the device driver code when transferring data.
For example, if the Cortex-M3 uses little endian but needs to read a big endian value from a sensor, the 4 bytes would need to be re-arranged after reading the sensor: uint32_t read_big_endian_value(uint32_t big_endian_val) { uint8_t val[4]; // Read raw bytes from sensor read_sensor(val); // Byte swap uint32_t little_endian_val; little_endian_val |= val[3] << 24; little_endian_val |= val[2] << 16; little_endian_val |= val[1] << 8; little_endian_val |= val[0]; return little_endian_val; }
The same concept applies when transmitting little endian data to an external big endian system. The bytes need to be re-arranged into big endian format before transmission.
Hardware accelerators like DMA controllers may also perform automatic byte swapping during transfers. This avoids the software overhead.
Impacts on Data Structures
The CPU’s endianness also impacts the layout of data structures in memory. When accessing multi-byte data types like 32-bit ints, the compiler will arrange the bytes based on the configured endianness.
For example, take a simple struct: struct Data { uint8_t b1; uint32_t w1; uint16_t h1; };
On a little endian Cortex-M3, this would be laid out in memory as: Address Values 0x100 -> b1 0x101 -> (LSB) w1 0x102 -> 0x103 -> 0x104 -> (MSB) w1 0x105 -> (LSB) h1 0x106 -> (MSB) h1
The 4 bytes of w1 are arranged from LSB to MSB in ascending memory. The 2 bytes of h1 are similar.
On a big endian system, the addresses would be reversed: Address Values 0x100 -> (MSB) w1 0x101 -> 0x102 -> 0x103 -> (LSB) w1 0x104 -> b1 0x105 -> (MSB) h1 0x106 -> (LSB) h1
LSB and MSB are swapped. Therefore endianness impacts how the compiler maps data structures.
Effect on Code Portability
Byte order differences reduce code portability between ARM Cortex-M3 and other architectures. Any data structures shared in memory must be converted to the native endianness.
One solution is to use compiler pragmas to enforce a consistent endian mapping across platforms. For example: #pragma pack(push, 1) // Exact fit – no padding struct Data { uint8_t b1; uint32_t w1; uint16_t h1; }; #pragma pack(pop) // Restore default packing
This ensures the memory layout of the struct is identical on all compilers, regardless of endianness. However, byte-swapping may still be required when transferring the actual data.
Effects on Performance
Using the native endianness of the Cortex-M3 generally provides the best performance. Since the processor has an ARM architecture with a little endian datapath, little endian data can be processed faster.
When loading a naturally-aligned 32-bit value, the Cortex-M3 performs an LDR instruction which efficiently reads the register in a single cycle. No byte re-arrangement is needed.
However, a big endian value requires additional instructions to swap the bytes into the correct order. This adds cycles to the read operation. The same applies for writes.
Unaligned data also requires more processing for either endian format. The processor may need to execute multiple read cycles and shifts to access all 4 bytes. Unaligned data should be avoided for performance reasons.
Effects on Power Consumption
In general, little endian data saves power on the Cortex-M3. The main reasons are:
- More efficient data transfers and memory access. The processor can read/write data using fewer instructions and bus cycles.
- Requires less software overhead for byte swapping. The CPU spends less time re-arranging data.
- Cleaner data alignment. Big endian values are more likely to be unaligned in a little endian system. Unaligned data wastes power.
For power-sensitive apps, using the native little endian format is recommended if possible.
Testing Endianness Code
Here are some tips for testing endianness handling in firmware:
- Write test cases that initialize data in both big and little endian formats. Verify the values are handled correctly.
- Introduce misaligned data and verify alignment exceptions occur as expected.
- Test endian conversion code with randomized datasets.
- Transmit data between systems with different endianness and validate conversions.
- Use run-time detection code to verify configured endianness matches hardware.
- Run on both endian architectures and check for consistent behavior.
Unit tests should aim to trigger any edge cases in the endianness handling logic. Automated testing helps catch bugs early.
Conclusion
In summary, the ARM Cortex-M3 supports configurable endianness through the SCTLR register. This should be set during start up based on the specific system requirements and industry conventions.
Little endian is generally recommended for the Cortex-M3 to maximize performance and minimize power consumption. However, endian mismatches must be handled in software through byte swapping routines.
Proper configuration and handling of endianness is needed to build robust and portable applications on the Cortex-M3 platform.
