Bare-metal programming means writing software that runs directly on the hardware, with no operating system between the code and the chip. There is no scheduler, no process model, no system calls, and no driver layer provided for you. The program is the whole show: it owns the processor from the moment it starts, talks to the peripherals itself, and never returns. This is how the smallest microcontrollers are programmed, and it is the foundation beneath every operating system, since something has to run on bare metal before an OS can exist.
The dominant structure of a bare-metal program is the superloop: an initialization phase that configures the hardware, followed by an infinite main loop that does the work and never exits. There is nowhere to return to, because nothing called the program and nothing is waiting to take control back. The loop polls inputs, updates outputs, and cycles forever. This simple shape, set up and then loop, is the default architecture of countless embedded devices.
The main loop alone cannot respond quickly to events, so bare-metal code pairs it with interrupts. An interrupt is a hardware signal that suspends the main loop, jumps to a dedicated handler, and resumes where it left off. The avr-libc manual describes how this works in C: the AVR has a predefined vector table pointing to interrupt routines with fixed names, and “by using the appropriate name, your routine will be called when the corresponding interrupt occurs.” Writing an interrupt service routine is therefore a matter of defining a function for the right vector. The main loop handles the steady background work while interrupts handle anything that must be serviced promptly, such as a timer expiring or a byte arriving on a serial port.
Both halves depend on register-level control of the hardware. Configuring a peripheral, enabling an interrupt, or driving a pin all come down to writing specific bit patterns into the chip’s special function registers. The avr-libc documentation notes that the AVR’s “entire IO address space is made available as memory-mapped IO,” so the program reaches the hardware simply by reading and writing those addresses, with the relevant registers declared volatile so the compiler does not optimize the accesses away. The programmer is directly responsible for every peripheral, with the datasheet as the only specification.
Because nothing manages the machine for it, a bare-metal program also takes on jobs an operating system would normally handle: laying out memory, setting up the stack, configuring clocks, and deciding how to share the single CPU between competing tasks. Shared state touched by both the main loop and an interrupt must be guarded against corruption, since an interrupt can fire in the middle of an operation. These concerns are the seed from which real-time operating systems grow, when the superloop is no longer enough.
Bare-metal programming is demanding precisely because there are no abstractions to hide behind, but that is also its appeal. The behavior is fully determined by the code and the chip, with nothing else intervening, giving the predictability and tight control that real-time and safety-critical systems require.