The Arm Cortex-M4 processor is a 32-bit RISC CPU that is widely used in embedded and IoT applications. While it has some CISC-like features, the Cortex-M4 is fundamentally a reduced instruction set computer (RISC) architecture.
What is RISC?
RISC (Reduced Instruction Set Computer) processors utilize a smaller, simplified set of instructions that can be executed very quickly in one clock cycle. This allows RISC CPUs to achieve high performance despite their simplicity.
Key characteristics of RISC architectures include:
- Small, highly optimized set of instructions
- Most instructions execute in a single clock cycle
- Uniform instruction format and fixed length
- Register-based with lots of general purpose registers
- Limited instruction modes and addressing modes
- Hardwired control unit for simple execution
- Pipelining for high throughput
- Few data types supported
- No microcode needed
By having simple instructions that take just one cycle, RISC processors can execute more instructions per second at a given clock speed. The tradeoff is that some complex operations require multiple RISC instructions. But by optimizing the common case, RISC CPUs achieve high performance for most code.
What is CISC?
CISC (Complex Instruction Set Computer) processors have a large set of multi-cycle instructions, allowing complex operations to be done with a single instruction. But this comes at the cost of reduced performance.
Typical attributes of CISC architectures include:
- Very large set of instructions
- Instructions vary widely in format and length
- Many instructions take multiple cycles
- Lots of specialized addressing modes
- Instructions can operate directly on memory
- Hardware to decode complex instructions
- Microcode often used
- Many data types supported
By having instructions that can do complex functions like string copy or search, CISC chips need fewer instructions overall. But multi-cycle instructions reduce the average instructions per cycle. CISC designs trade simplicity for code density.
Key Features of Arm Cortex-M4
The Arm Cortex-M4 CPU utilizes a 32-bit RISC architecture optimized for embedded applications. Some of its key features include:
- 32-bit instruction set
- High code density thanks to Thumb-2 technology
- Most instructions execute in a single cycle
- Pipelined to achieve 1 cycle per instruction throughput
- Hardware multiply and divide instructions
- Dedicated barrel shifter unit
- 16 registers in register bank
- Low power consumption
- Memory Protection Unit (MPU) for real-time OS support
- Nested Vectored Interrupt Controller (NVIC)
- Hardware floating point unit (FPU) option
The combination of single cycle execution for most instructions, simple addressing modes, and a high degree of pipelining allows the Cortex-M4 to achieve excellent performance despite its RISC architecture. The inclusion of an FPU and MPU also make it suitable for more advanced applications compared to smaller Cortex-M cores.
RISC Characteristics of Cortex-M4
At its core, the Arm Cortex-M4 possesses all the major traits of a RISC processor. Let’s examine some of the key RISC-like aspects of the Cortex-M4 architecture:
- Simplified instructions – Has just 56 base instructions that are easy to pipeline and execute in a single cycle.
- Fixed instruction length – All instructions are 32 bits long. Simplifies fetch and decode.
- Few addressing modes – Only 4 addressing modes are supported to reduce complexity.
- Load/store architecture – Data processing only works on registers. Memory access is only through load/store instructions.
- Lots of registers – 16 registers available in main register bank for fast access.
- Pipelined execution – 3-stage pipeline allows fetching, decoding, and executing instructions concurrently.
- Branch delay slots – Most branch instructions have a one cycle delay for efficient pipelining.
- No microcode – All instructions execute directly in hardware. Simpler design.
By adhering to these RISC principles, the Cortex-M4 achieves exceptional performance and efficiency for embedded applications despite using simpler instructions than CISC architectures.
CISC-like Features of Cortex-M4
While largely RISC, the Cortex-M4 does incorporate some attributes commonly associated with CISC architectures:
- Medium sized instruction set – Has over 150 instructions, larger than a pure RISC ISA.
- Denser code – Thumb-2 provides higher code density than traditional 32-bit ARM code.
- Complex instructions – Some instructions like nested IT blocks are complex and multi-cycle.
- Hardware multiply/divide – Dedicated circuitry for fast MUL and DIV operations unlike pure RISC.
- Barrel shifter – Hardware shifter allows shifts/rotates in a single cycle unlike general RISC.
- Lots of registers – More registers than most CISC processors for fast access.
- Memory Protection Unit – Hardware MPU provides memory protection and segmentation like in CISC CPUs.
By incorporating these CISC-like capabilities, the Cortex-M4 achieves better code density, faster multicycle instructions, and support for memory protection compared to stripped down RISC cores. But it retains the single-cycle execution and pipelining of RISC for most instructions.
Is Cortex-M4 RISC or CISC?
In summary, while the Arm Cortex-M4 processor has some attributes of CISC architectures, it is fundamentally a pipelined 32-bit RISC CPU designed for embedded applications. Features like its simplified instructions, fixed length encoding, load/store architecture, heavy pipelining, and lack of microcode make the Cortex-M4 unmistakably RISC at its core.
CISC aspects like the medium instruction set size, higher code density, and hardware multiply/divide units give the Cortex-M4 some of the code density benefits and fast multi-cycle instructions of CISC. But single cycle execution for most instructions and a hardwired control path maintain the Cortex-M4’s RISC lineage and efficiency advantages.
So while not a pure RISC design, the Arm Cortex-M4 is overwhelmingly biased towards the RISC approach. This blending of RISC and CISC principles is what enables the Cortex-M4 to deliver outstanding performance for 32-bit embedded applications.
