Bare metal programming refers to writing software directly for hardware, without the use of an operating system or libraries. It involves interacting with hardware like processors, memory and peripherals at a low level. Bare metal programming can be challenging, but offers advantages like performance, full hardware control, and small memory footprint. Ultimately, whether bare metal programming is “good” depends on the specific project requirements and developer skills.
What is bare metal programming?
Bare metal programming means writing software directly for hardware, without any intervening operating system or software libraries. The “bare metal” term comes from the exposed metal surface of microchips inside electronic devices. In bare metal programming, the developer works directly with hardware components like:
- Memory (RAM, ROM)
- Input/output peripherals
- Communication interfaces (USB, Ethernet, etc.)
The programmer is responsible for controlling and coordinating the hardware without any abstraction layer like an OS. This includes tasks like:
- Initializing hardware
- Managing memory
- Scheduling tasks
- Handling interrupts
- Interacting with peripherals
- Implementing communication protocols
Bare metal programming requires an in-depth understanding of the target hardware architecture. The code is typically written in C or assembly language for optimal performance and hardware control.
Use cases for bare metal programming
Bare metal programming is commonly used in embedded systems which have limited resources. Some examples include:
- IoT devices – Like home appliances, sensors, wearables, etc. that need to be low cost and power efficient.
- Real-time systems – Industrial controllers, robotics, medical devices where timing is critical.
- Automotive systems – Engine control units, infotainment, advanced driver assistance.
- Aerospace/Defense – Guidance systems, avionics, satellites where reliability is paramount.
In these cases, bare metal programming allows for optimizing performance, reducing memory requirements, and getting precise timing and hardware control. The lack of an OS reduces size, cost and overhead.
Advantages of bare metal programming
Here are some of the main benefits of using bare metal programming:
- Performance – No OS allows the application to make full use of the processor and hardware resources for better speed and efficiency.
- Determinism – Precise control over instruction execution important for real-time responsiveness.
- Small footprint – No OS overhead results in a smaller memory footprint ideal for embedded devices.
- Low power – Tight hardware control enables optimizing for low power consumption.
- Flexibility – Direct hardware access allows for highly customized implementations.
- Reliability – No dependencies on complex OS features.
In devices like engine control units and industrial robots, bare metal code can provide both speed and reliability by removing OS variability. The programmer has complete control over the hardware.
Challenges with bare metal programming
However, bare metal programming has some disadvantages as well:
- Complexity – Managing hardware directly requires expertise and low-level understanding of the processor architecture.
- Time consuming – Coding everything from scratch takes more development time versus using available OS/library code.
- Lack of portability – Tight coupling with hardware makes it difficult to port the software to new platforms.
- Limited tools – Less support and debugging tools compared to OS-based development.
- Difficult concurrency – Managing task scheduling, race conditions, interrupts requires significant effort.
- Testing overhead – Rigorous testing needed across use cases due to lack of OS exception handling.
Developing and maintaining bare metal code requires engineers who specialize in the target processors and hardware. The lack of reusability means bare metal programming is best suited for specialized use cases versus general-purpose computing.
Bare metal programming process
A typical bare metal programming workflow involves:
- Selecting target hardware platform – microcontroller, SoC, FPGA, etc.
- Obtaining comprehensive hardware documentation of internal architecture – memory map, peripherals, interfaces, etc.
- Designing software architecture and defining requirements like performance, memory limits, I/O, etc.
- Implementing basic startup code for the processor – configuring clocks, memory, registers, etc.
- Writing drivers for on-chip peripherals like GPIO, timers, communication buses.
- Implementing scheduler, task management and interrupt handling routines.
- Coding main application functionality, layered on the OS-like foundation.
- Rigorously testing across use cases, fixing bugs.
- Validating timing, performance, power, reliability requirements.
Having a structured software architecture is key even in bare metal systems. Abstractions should be created to hide hardware dependencies where possible. Reusable drivers, protocol stacks and RTOS-like frameworks help reduce coding effort on new projects.
Bare metal programming on ARM Cortex-M
ARM Cortex-M series of 32-bit cores are one of the most popular choices for bare metal embedded and IoT applications. Reasons include:
- Wide range of microcontrollers and SoCs for various performance levels.
- Processor architectures optimized for low power consumption.
- Integrated flash memory, RAM, peripherals reduce system cost.
- Extensive toolchain support including low-cost options.
- Large developer ecosystem and code examples.
The Cortex-M cores have a simplified architecture compared to higher end application processors from ARM. Their Thumb instruction set, NVIC interrupt controller, SysTick timer and sleep modes make these cores well suited for bare metal firmware.
Cortex-M middleware like CMSIS and vendor HALs provide some abstraction from chip specifics. But ultimately, bare metal Cortex-M programmers must fully understand the target device details for efficient utilization of SoC resources.
Should you choose bare metal over RTOS or OS?
For some embedded applications, utilizing a real-time operating system (RTOS) or embedded OS may be a better choice over bare metal:
- Code reusability – RTOS provides standardized APIs across architectures.
- Better modularity – Cleaner separation between application, OS and hardware layers.
- Advanced features – Built-in multitasking, file system, networking, etc.
- Easier development – Exists expertise in working with specific RTOSes like FreeRTOS.
- Broader tools support – Profilers, debuggers, static analysis tools.
An RTOS or embedded Linux brings overhead ranging from a few KB to a few MB. The tradeoff versus bare metal is higher software complexity but faster time to market. Hybrid designs are also possible such as running key tasks on bare metal and others on RTOS.
To summarize, bare metal programming gives developers precise control over hardware resources leading to benefits like speed, small size, reliability and real-time responsiveness. However, it also comes with challenges like increased complexity, less portability and lack of reuse. Bare metal is best suited for specialized embedded and real-time use cases where performance and determinism are critical requirements.
For Cortex-M processors used in IoT edge devices, bare metal firmware can efficiently utilize hardware capabilities. With good architecture and design practices, the complexity of bare metal can be managed. Ultimately, the choice depends on the specific tradeoffs acceptable for the application and the developer experience available.