Bare metal code refers to programs that run directly on the hardware, without the support of an operating system. It is considered a type of low-level programming, as it requires directly interfacing with and controlling the underlying hardware components. However, the term “low level” itself can have different meanings in software development.
In general, low-level code is code that is close to the hardware and further away from human language. It deals with lower abstractions and provides access to more direct hardware control. The lower the level of code, the closer it is to machine code that the processor executes. High-level code, on the other hand, uses abstractions and concepts closer to human thinking and further away from hardware details.
Key Characteristics of Bare Metal Code
Here are some of the main characteristics of bare metal code:
- Runs directly on the hardware, without an operating system
- Provides direct access to registers, memory, peripherals etc.
- Requires handling interrupts and resource management manually
- Often written in Assembly, C or C++ languages
- Requires detailed knowledge of the target processor architecture
- Involves bit manipulation, pointer arithmetic, and other low-level constructs
- Used for system bootloaders, embedded systems, real-time applications
The lack of an operating system and access to raw hardware resources is what makes bare metal code truly low-level. The developer needs to be aware of the processor internals and has to directly manage tasks like memory allocation, interrupt handling and peripheral access. This results in a highly optimized end product but at the cost of portability.
Comparison to Other Types of Low-Level Code
While bare metal programming is low-level, there can be varying levels of abstraction even within low-level code:
Assembly language provides a thin layer above machine code and uses mnemonics for easier development. But it still requires managing registers, flags, memory and instructions. Bare metal code in Assembly is still extremely hardware focused with no OS assistance.
C language allows higher control constructs like loops, functions etc. but still provides low-level memory access. Bare metal C code would need to handle memory, devices and interrupts manually. C requires less hardware knowledge than Assembly but lower abstraction than high-level languages.
Device drivers run in kernel mode within an OS but interact with hardware directly. They are modular and portable, unlike bare metal code tied to a specific hardware target. Drivers still qualify as low-level code but have help from the OS for non-hardware specific tasks.
Hypervisors or virtual machine monitors run between the hardware and guest operating systems. They behave like a small OS themselves but provide virtual hardware access to higher level VMs. Hypervisor code is lower level than application and OS code but not as low as bare metal code.
BIOS and UEFI firmware initialize hardware during boot before the OS loads. Firmware acts as a middleware between full-fledged OS software and the underlying hardware. It is lower level than OS code but may not always run “bare metal” without any abstraction layer.
Advantages of Bare Metal Code
Here are some benefits provided by bare metal code:
- Performance: No OS overhead results in highly optimized code and the best possible performance.
- Control: Direct hardware access allows precise low-level control not possible via an OS.
- Real Time: Deterministic response times make bare metal suitable for real-time applications.
- Size: No bulky OS saves memory footprint and storage requirements.
- Customization: Can be tailored specifically for the target hardware without generality.
These factors make bare metal code useful in embedded systems, medical devices, industrial automation, robotics and other specialized domains.
Challenges of Bare Metal Code
There are also certain disadvantages associated with bare metal programming:
- Complexity: Requires expertise with low-level programming and hardware details.
- Portability: Tightly coupled with the underlying processor architecture.
- Reusability: Code cannot be reused across different hardware.
- Reliability: No hardware abstraction leads to platform dependent bugs.
- Development Time: Direct hardware coding is more complex and time consuming.
These factors make bare metal programming challenging, especially with complex modern hardware. Abstraction layers like OS, drivers, and virtualization help tackle hardware complexity for mainstream software.
Use Cases for Bare Metal Code
Here are some common use cases where bare metal code is the preferred solution:
- Bootloaders – System boot code runs bare metal to load the OS kernel
- Firmware – Low-level firmware such as BIOS, UEFI, device firmware etc.
- Embedded Systems – Resource constrained devices like IoT sensors, wearables etc.
- Real-Time Systems – Applications requiring deterministic response like industrial control
- System Programming – Kernel development, device drivers, hypervisors etc.
For these specialized use cases, the advantages of bare metal code outweigh the disadvantages compared to OS based development.
The Future of Bare Metal Code
While bare metal programming requires specialized expertise, certain trends are changing its use:
- Newer languages like Rust allow system programming without unsafe coding.
- Embedded OS like Zephyr and FreeRTOS provide a middle ground with some abstraction.
- Platform specific developer kits, simulated environments and debugging tools are making bare metal coding easier.
- Higher level languages are being compiled to bare metal code for embedded devices.
So while typical application development is moving towards portability and abstraction, bare metal code is still holding its own in niche domains. The constraints of embedded systems will continue to drive bare metal programming in the future.
To summarize, bare metal code runs directly on hardware without an OS and provides the lowest level control and access possible. This determinism makes it useful for specific applications like embedded devices. But it requires specialized skills and lacks portability across hardware platforms. While bare metal code is unarguably the lowest level, different languages and abstraction models provide various levels of tradeoff. For general-purpose and user-focused software, higher level languages on top of OS services provide portability and programmer productivity. However, bare metal code will continue to be relevant for particular use cases where real-time response and hardware control is a requirement.