Bare-Metal C Basics

What Bare Metal Means

Bare-metal firmware runs directly on the CPU with no operating system. You control the startup sequence, memory layout, interrupt vectors, and every peripheral register. Advantages: deterministic timing, minimal overhead, smallest footprint. The startup sequence: CPU resets → loads PC from reset vector → runs startup code → initializes .data section from flash, zeros .bss section → calls main(). If main() returns, the CPU executes whatever is next in memory — always add an infinite loop.

The Memory Map

A microcontroller's address space is divided into regions defined by the manufacturer. Typical ARM Cortex-M: 0x00000000–0x1FFFFFFF = flash (code), 0x20000000–0x3FFFFFFF = SRAM (data/stack/heap), 0x40000000–0x5FFFFFFF = peripheral registers, 0xE0000000+ = core peripherals (NVIC, SysTick, etc.). The linker script (.ld file) maps your code sections (.text, .data, .bss) to these physical addresses.

CMSIS and Peripheral Headers

CMSIS (Cortex Microcontroller Software Interface Standard) provides a common C header for ARM Cortex-M peripherals. MCU vendors provide device-specific headers that define all peripheral register addresses as C structures: GPIOA->ODR = 0x01 sets PA0 high directly via memory-mapped I/O. No HAL needed — write to the register address and the hardware responds immediately.

// Bare-metal GPIO blink on STM32F1 (no HAL)
// Directly access peripheral registers via CMSIS headers

#include "stm32f1xx.h"

// The startup file provides this symbol
extern uint32_t SystemCoreClock;

void delay_ms(uint32_t ms) {
    // Simple busy-wait using SysTick
    uint32_t ticks = (SystemCoreClock / 1000) * ms;
    volatile uint32_t i;
    for (i = 0; i < ticks; i++) __NOP();
}

int main(void) {
    // 1. Enable clock to GPIOC (bit 4 of APB2ENR)
    RCC->APB2ENR |= RCC_APB2ENR_IOPCEN;

    // 2. Configure PC13 as general-purpose push-pull output, 2MHz
    // CRH controls pins 8-15; PC13 is bits 23:20
    GPIOC->CRH &= ~(0xF << 20);   // clear bits
    GPIOC->CRH |=  (0x2 << 20);   // output, 2MHz (CNF=00, MODE=10)

    while (1) {
        // Set PC13 high (LED OFF on most Blue Pill boards — inverted)
        GPIOC->BSRR = (1 << 13);     // Bit Set Register
        delay_ms(500);

        // Set PC13 low (LED ON)
        GPIOC->BRR  = (1 << 13);     // Bit Reset Register
        delay_ms(500);
    }
    // Never return — would be undefined behavior
}