The Cortex-M4 processor from ARM is a very popular 32-bit microcontroller that is widely used in embedded systems and IoT devices. It has a simplified integer pipeline compared to high-performance application processors, optimized for low-power operation. At the core of the Cortex-M4 is the AMBA 3 AHB-Lite bus interface, nested vectored interrupt controller, and memory protection unit.
One of the key aspects of any processor is the number and type of registers it contains. Registers provide fast access storage locations that are critical for executing instructions efficiently. The Cortex-M4 contains 16 general purpose 32-bit core registers, R0-R15. These can be used for integer operations and addressing modes.
16 General Purpose Registers
The 16 general purpose registers in the Cortex-M4 are 32-bits wide and named R0, R1, up to R15. They can be used interchangeably for most instruction purposes. Having multiple registers allows values to be stored without needing slow memory access. The registers are not segmented into high and low portions like some architectures.
Eight of the general purpose registers (R0-R7) are available for use across function calls. The remaining registers (R8-R12) may be impacted by stack push/pop operations when functions are called. R13 is used as the stack pointer, R14 the link register, and R15 the program counter.
Any of the general purpose registers can be used for arithmetic, logical operations, load/store addressing, etc. Being 32-bit, they can store data types such as 32-bit integers, pointers, and floats. Having 16 registers provides flexibility for compilers to optimize register allocation, assign variable values to registers, and avoid spills to memory.
Three Special Registers
In addition to the 16 general purpose registers, the Cortex-M4 contains several special purpose registers:
- Program Counter (R15) – Holds the current program address being executed
- Link Register (R14) – Stores return address when function call is made
- Stack Pointer (R13) – Points to the last stacked address in memory
The program counter R15 stores the current instruction address. It is incremented as each instruction executes, sequencing through program memory. R15 allows the processor to fetch the next instruction.
R14 serves as the link register and holds the return address when a function call occurs. This allows the program to branch to a subroutine or function, and then return back to the main program flow. The link register is automatically pushed/popped with other registers during function calls.
The stack pointer R13 contains the last stacked memory address. It is used with push/pop operations with the stack to handle parameters, return values, and local variables in functions. Automatically adjusting R13 simplifies stack management.
The Cortex-M4 contains a 3-stage integer pipeline to improve performance when executing instructions. This includes the fetch, decode, and execute stages. Pipelining allows multiple instructions to be processed in parallel.
In the fetch stage, the processor uses the program counter to retrieve the next instruction from memory. In the decode phase, the instruction is interpreted to determine the operation type and operand registers. Finally, in the execute stage, the actual operation is carried out such as an add or load.
Having registers available avoids needing to read operands from slow memory in the execute stage. This allows the pipeline to achieve maximum throughput, improving overall performance.
The Cortex-M4 uses a load/store architecture where data must be loaded from memory into registers before arithmetic/logical operations can be applied. Once the operations are complete, the result can be stored back to memory from a register.
For example, to add two numbers X and Y in memory, X must be loaded into R1 and Y into R2 first. An add instruction would target R1 and R2 as the operands, storing the result back to memory. This differs from some CISC processors that allow direct memory access.
By using registers as operand sources, the Arithmetic Logic Unit (ALU) can work independently from slow memory accesses. This allows faster execution versus needing to wait and fetch operands from memory.
The combination of the 16 general purpose registers, pipeline, and load/store architecture allow the Cortex-M4 to achieve very good performance for an embedded microcontroller. It can typically execute about 1.25 DMIPS/MHz.
This means that at 100 MHz clock speed, the performance would be approximately 125 DMIPS. To compare, some earlier ARM architectures like the Cortex-M0 only achieve about 0.9 DMIPS/MHz. So the Cortex-M4 design is approximately 40% faster on an absolute scale.
Embedded systems do not require the raw horsepower of a desktop CPU. But the integer register file size, 3-stage pipeline, and load/store design result in excellent performance/MHz for the target environment. The Cortex-M4 achieves a good balance of speed, power efficiency, and die area.
In summary, the Cortex-M4 processor contains 16 32-bit general purpose core registers named R0 to R15. These registers provide fast access storage locations for integers, pointers, and floats during program execution. Having multiple registers allows operands to be stored without slow memory lookups.
In addition, the Cortex-M4 has three special registers: the program counter, link register, and stack pointer. These support program sequencing, function calls, and stack management. The integer pipeline and load/store architecture optimize the CPU for embedded use-cases where memory access latency is critical.
Overall, the Cortex-M4 hits a sweet spot of performance versus cost and power efficiency in deeply embedded applications. The register file size, pipeline depth, and load/store design allow it to execute approximately 1.25 DMIPS/MHz, providing excellent computational capabilities per MHz relative to other microcontroller architectures.