The Arm Cortex-M4 processor utilizes a Harvard architecture, which means it has separate instruction and data memories. The data memory in the Cortex-M4 acts as the data storage for both data variables and the stack. This data memory is typically made up of SRAM or embedded flash memory.
Overview of the Arm Cortex-M4 Architecture
The Arm Cortex-M4 processor is based on the Arm v7-M architecture, which employs a Von Neumann or Harvard architecture design. In the Harvard architecture, the instruction and data memories are physically separated, allowing simultaneous access to both memories in a single clock cycle. This improves overall performance compared to Von Neumann where the same physical memory is accessed for both instructions and data.
The Cortex-M4 specifically utilizes a Harvard architecture with separate instruction and data buses. This enables it to fetch both an instruction and data in a single cycle for greater throughput. The processor includes three main components:
- The instruction memory, which stores program instructions
- The data memory, which holds data variables
- The register bank, which contains general purpose registers for storing temporary data
Instructions are fetched from the instruction memory into the instruction pipeline. Data can be read from and written to the physically separate data memory in parallel each cycle via the Advanced Microcontroller Bus Architecture (AMBA) interface. This parallel access is a key advantage of the Harvard design.
The Cortex-M4 Data Memory
The data memory in the Cortex-M4 acts as the storage for all data variables used in a program. It also holds the stack which stores return addresses and local variables for function calls. The exact memory used depends on the specific microcontroller implementation, but is generally composed of fast SRAM or flash:
- SRAM – Static RAM provides fast read/write times for data access. It requires power to maintain stored values, however, so it is more power-hungry. SRAM is often used for smaller, low-power Cortex-M4 designs.
- Flash – Flash memory can store data without power and has greater density than SRAM. It has slower write times, however. Flash may be used in larger Cortex-M4 implementations where more data memory is required.
The data memory size can range from 16 KB up to 1 MB or more, depending on the microcontroller. It may also be configured as single-cycle access RAM for the fastest possible data reads and writes. The Cortex-M4 supports byte, halfword, and word access types to this memory region.
Data Memory Usage
The data memory region stores both global and static variables declared in C/C++ code. Their compiled memory addresses are assigned by the linker script. The region also holds the program stack, which grows down from the top address of memory. Some key data stored in the Cortex-M4 data memory includes:
- Global & Static Variables – Variable data that needs to persist across function calls. Allocated to fixed addresses in data memory.
- Heap – Dynamic memory region used for malloc() and new operator allocation requests in C/C++.
- Stack – Holds function return addresses, parameters, local variables. Grows downward from end of memory.
The Cortex-M4 stack utilizes a full descending stack, which grows down from the highest data memory address. This allows the stack to grow downwards without overlapping global variables at lower addresses. The processor automatically manages the stack pointer during pushes/pops in hardware.
In addition to data variables and the stack, peripherals with memory-mapped registers are also mapped into the data memory address space. This allows them to be accessed using normal load/store instructions.
Data Memory Access
The Cortex-M4 core accesses data memory via the AMBA AHB-lite bus interface. This provides a 32-bit data bus and allows for high-performance single-cycle data transactions. External masters can also access data memory via this interface.
Load and store instructions are used to access data memory locations in Cortex-M4 assembly code or compiled C/C++. These include:
- LDR – Load Word
- STR – Store Word
- LDM – Load Multiple
- STM – Store Multiple
These transfer word-aligned data between registers and memory. There are also byte, halfword, dual word, and multiple access versions for more flexibility.
The Cortex-M4 also supports unaligned accesses for data not aligned to word addresses. Additional bus cycles are required for unaligned loads and stores. Bit-banding provides atomic bit manipulation by mapping each bit in the data memory region to a bit-band alias location.
Optimizing Data Memory Usage
To optimize usage of the Cortex-M4 data memory region, programmers should:
- Minimize global and static variables to conserve memory.
- Allocate frequently accessed variables to lower memory addresses to improve performance.
- Organize data to utilize word and halfword access types instead of bytes.
- Declare variables as const when possible to allow allocation to flash instead of RAM.
- Use stack efficiently by minimizing local variables and function call depth.
The linker script can place code and read-only data into flash memory, reserving the SRAM region for read-write data. Flash data accesses are slower, however, so performance-critical data should remain in SRAM.
Data Memory Protection
To protect sensitive on-chip data like return addresses and function parameters on the stack, the Cortex-M4 includes optional Memory Protection Unit (MPU) support. This allows configurable, application-based protection of memory regions from unauthorized access.
The MPU divides the memory map into a number of programmable regions. Access permissions including execution, read, and write can be set for each region to implement sandboxing and privilege separation. This helps prevent malicious or erroneous code from corrupting sensitive data in memory.
In summary, the data memory region in the Arm Cortex-M4 microcontroller architecture provides efficient storage and access of global and local data variables. Stored in fast SRAM or high-density flash memory, it operates in parallel with the instruction memory via Harvard architecture for maximum performance. Optimizations like utilizing word accesses and conserving variables can help reduce data memory usage. Optional MPU support adds configurable access control for enhanced data security.