The Cortex-M4 is a 32-bit ARM processor core designed for embedded and IoT applications. It is part of ARM’s Cortex-M series of processor cores, which are optimized for low power consumption and high performance in microcontroller and other resource-constrained devices.
The amount of memory available on a Cortex-M4 based microcontroller can vary significantly depending on the specific chip implementation. However, there are some general guidelines for the memory capabilities of Cortex-M4 cores:
On-Chip SRAM
The Cortex-M4 core itself does not contain any internal SRAM. However, Cortex-M4 based microcontrollers include various amounts of SRAM on-chip. A typical amount of SRAM for a Cortex-M4 chip ranges from 64KB to 512KB. Higher end microcontrollers can have 1MB or more of on-chip SRAM.
This SRAM provides fast access memory for the processor and is useful for data that needs low latency like stack, heap, and global variables. Having more SRAM on-chip allows for more data to be accessed quickly without needing to go off-chip to slower flash or external memory.
Flash Memory
As with SRAM, the raw Cortex-M4 core does not include any internal flash memory. The flash memory for program storage exists on the microcontroller chip outside the core itself.
Typical flash memory size for Cortex-M4 based microcontrollers ranges from 128KB on the low end to several MB for higher end chips. For example, the STM32F407 series has up to 1MB of flash while the STM32F427 can have up to 2MB.
The program binary executable is stored in flash memory. Having more flash allows for larger and more complex firmware. The flash memory also normally contains application data stored on it.
External Memory
Some Cortex-M4 microcontrollers include the ability to access additional external memory through various peripheral interfaces like:
- Parallel external bus
- QSPI for external flash
- SDRAM controller
Using these interfaces, a Cortex-M4 based system can have external memories like NOR flash, SRAM, and SDRAM added to expand the memory capabilities. The external memory can provide code and data storage in the MBs or GBs range.
However, code execution still happens from internal flash. The external memory supplements the on-chip memory and is typically accessed for larger non-latency critical data.
Memory Management Unit
The Cortex-M4 core includes an optional MMU (memory management unit) that can support different memory access schemes:
- Flat memory map – All memory accessed as one contiguous block. This is the default out of reset.
- Protected memory regions – Up to 16 protected regions can guard against access violations.
- MPU-based memory map – More advanced control of memory access permissions.
The MMU allows creating a memory map tailored to an application’s needs. For example, different areas of memory can be made read-only or inaccessible. This improves reliability and security.
Memory Protection Unit
The Memory Protection Unit (MPU) is an optional part of the Cortex-M4 MMU. It provides setting access permissions for up to 8 memory regions. Each region can be independently set to:
- Read-only, Read/Write or No access
- Privilege Level 0 (Unprivileged) or Level 1 (Privileged) access
- Enable/Disable execution from a memory region
The MPU allows creating a robust memory access scheme to protect and partition memory. The MPU can help prevent buffer overflows and improve system reliability.
Cortex-M4 Address Space
The Cortex-M4 has a total 4GB address space for code, data, peripherals, and external memory. This address space is divided into different regions:
- Code – Up to 2GB for program instructions.
- SRAM – Up to 2GB for SRAM data.
- Peripherals – 512MB for register mapped peripherals.
- External memory – 1GB reserved for external memory.
Having a 32-bit address space allows the Cortex-M4 access up to 4GB of memory in total. External memory can be mapped into the external memory region of the processor’s address space.
Memory Bottlenecks
While the Cortex-M4 can theoretically support GBs of memory through external interfaces, actual system memory bandwidth is limited by:
- External memory interface width – limits how much data per transfer.
- Bus clock speed – limits transfers per second.
- Contention with core/peripheral accesses.
So while large external memories are possible, the interface bandwidth can become a bottleneck in practice. Careful memory architecture is needed to maximize performance.
Caching
The Cortex-M4 does not have internal data or instruction caching. All memory accesses are single cycle. Caching would increase latency and power consumption.
However, some Cortex-M4 microcontrollers include external memory caches. This helps reduce the penalty associated with accessing slower external memories. The cache improves average access time.
Determining Total Memory
To determine the total memory available in a Cortex-M4 based system, you need to consider:
- On-chip SRAM and flash size
- External memory interface bandwidth
- Any external memory size
- Memory configuration and access scheme
The maximum possible memory is the 4GB address space limit. But in practice total usable memory is reduced by bandwidth bottlenecks. Careful memory architecture optimization is needed for larger memory sizes.
Typical Memory Configuration
A typical memory setup for a moderately configured Cortex-M4 microcontroller with external memory could look like:
- 256 KB SRAM on-chip
- 1 MB Flash on-chip
- 16MB SDRAM external
Where the on-chip memories provide fast access for code and data, while the larger external SDRAM supplements this with more data storage.
Memory Usage Optimization
To optimize memory usage on a Cortex-M4 system, focus on:
- Minimizing code and data size
- Using memory pools and profiling for fragmentation
- Utilizing hardware acceleration and DMA for peripherals
- Streaming data to minimize peak RAM usage
- Using memory access schemes to protect different regions
With careful design, a significant amount of processing can be done even on a constrained Cortex-M4 platform. The Cortex-M4 excels at such embedded processing use cases.
