The Arm Cortex M1 is a 32-bit reduced instruction set computer (RISC) processor designed by Arm for use in embedded and Internet of Things (IoT) applications. As the first processor in Arm’s Cortex-M series, the Cortex M1 established the design principles and features for Arm’s energy-efficient microcontroller family.
The key features of the Cortex M1 architecture include:
- 32-bit RISC instruction set architecture
- 3-stage pipeline
- Thumb 16-bit compressed instruction set
- NVIC nested vectored interrupt controller
- Memory protection unit
- Single-cycle 32-bit multiplier
- Low power sleep modes
Instruction Set Architecture
The Cortex M1 implements the Thumb instruction set, a 16-bit compressed variant of the 32-bit Arm instruction set architecture. The Thumb ISA provides higher code density compared to the 32-bit ISA, reducing program memory requirements. The Cortex M1 supports all Thumb-2 instructions which provide additional computational and branching capabilities compared to the original Thumb ISA.
Key features of the Thumb-2 ISA include:
- 16-bit and 32-bit instruction lengths
- Up to 32 general purpose registers
- Load/store architecture with support for C intrinsics
- Conditional execution of many instructions
- Registers R13-R15 have stack pointer, link register, and program counter roles
By using Thumb-2, the Cortex M1 achieves high code density without compromising performance. The variable instruction length allows optimizing for efficiency on an instruction-by-instruction basis.
Pipeline
The Cortex M1 utilizes a 3-stage pipeline to balance performance and power efficiency. The three stages are:
- Fetch – Instructions are fetched from program memory
- Decode – Instructions are decoded and read register operands
- Execute – Instruction is executed and write back results
Pipelining allows multiple instructions to be processed simultaneously, improving instruction throughput. The short 3-stage pipeline reduces power consumption compared to deeper pipelines in more complex processors.
Some key features of the Cortex M1 pipeline include:
- Single-cycle execution for most Thumb instructions
- Load Multiple and Store Multiple execute in 2 cycles
- Branch instructions have a 2-cycle latency
- Hardware interlocks prevent data hazards between instructions
- Pipeline stalls flush on exception entries and returns
The simple pipeline allows aggressive clock gating to reduce power during stall cycles. The regular pipeline structure also simplifies instruction timing analysis and optimization by compilers.
Memory Architecture
The Cortex M1 supports Von Neumann architecture with unified instruction and data memory. Some key memory architecture features include:
- Up to 4GB physical address space
- Little endian byte ordering
- Optional MPU with 8 regions and subregions
- Single-cycle 32-bit SRAM interface
- Bus interface unit with AHB-Lite or APB peripherals
- Bit-banding aliases memory bit access
The memory protection unit (MPU) provides limited memory access control to improve software robustness. regions and subregions allow configuring fine-grained access permissions for code and data.
The bit-banding feature maps single bit addresses to word addresses so that individual bits can be set and cleared atomically. This removes the need to perform read-modify-write sequences for bit manipulation.
Nested Vectored Interrupt Controller
The Cortex M1 includes an embedded nested vectored interrupt controller (NVIC) to handle interrupts and exceptions efficiently.
Key attributes of the NVIC include:
- Up to 32 maskable external interrupt inputs
- 16 programmable priority levels for interrupts
- Vector table with optional offset register
- Wakeup interrupt controller (WIC) for low power mode
- Processor state automatically saved on exception entry
The priorities and flexible vector table allow tailoring interrupt response latency for real-time embedded applications. The NVIC provides low interrupt latency by avoiding the overhead of a software interrupt handler.
System Control
The Cortex M1 system control capabilities include managing clocking, resets, low power modes, and configuration:
- Supports chip internal and external clock sources
- Clock gating of core and peripherals to reduce power
- Implements Arm’s Amba 3 AHB-Lite and APB interface standards
- Support external reset signal for device initialization
- Sleep and deep sleep low leakage modes
- Debug access port (DAP) for software debugging
A key priority for the Cortex M1 is achieving instant-on with low energy consumption from sleep modes. The WIC allows fast wakeup from interrupts to handle real-time tasks.
Debug Support
The Cortex M1 provides real-time debug support via Arm’s embedded debug (ED) architecture. Debug features include:
- Breakpoints, watchpoints, and software profiling
- System trace macros for program flow tracing
- Cross-trigger interface coordinating trace and debugging
- Access to core and system registers during debug halt
- Debug monitor program for complete debug tool integration
These debug components facilitate software development and validation. Extensive tracing and profiling assists with analyzing and optimizing system performance.
Summary
In summary, the key strengths of the Cortex M1 architecture include:
- Highly energy efficient 32-bit RISC core
- Thumb-2 instruction set balances code density and performance
- Pipelined architecture with 3 simple stages
- Integrated memory protection and interrupt handling
- Extensive debug and real-time trace support
- Ideal for cost and power constrained embedded applications
As the first Cortex-M series processor, the Cortex M1 established the blueprint for Arm’s ongoing microcontroller architecture evolution. It demonstrates an efficient RISC core optimized for embedded control tasks.
