A bare metal operating system built from scratch on the STM32F103C8T6 (Blue Pill). Written in ARM assembly and C.
Starting from power-on, the CPU reads the vector table at the start of flash (0x08000000), loads the stack pointer, and jumps to the reset handler. The reset handler copies initialized variables from flash to RAM, zeroes the BSS section, then calls main(). GPIO and an interrupt-driven SysTick timer are implemented for precise timing. No HAL or abstraction libraries, every register is accessed directly.
- Bare metal ARM programming and direct register access
- ARM Cortex-M3 boot sequence and memory map
- Linker scripts, ELF sections, and memory placement
- Interrupt-driven programming with SysTick and NVIC
- GDB debugging over SWD
- Vivonomicon bare metal STM32 series
- RM0008 — STM32F103 reference manual
- STM32F103C8 datasheet
- csrohit — linker script tutorial
- CMSIS header files
- STM32F103C8T6 (Blue Pill) - Cortex-M3, 64K flash, 20K RAM
- ST-Link V2 — SWD flashing and debugging
- CP2102 USB-UART — serial output
arm-none-eabi-gcc/arm-none-eabi-as— compiler and assemblerarm-none-eabi-gdb— debuggerst-util— GDB server over ST-Link- STM32CubeProgrammer — flashing
.
├── device_headers/
│ ├── core_cm3.h # ARM Cortex-M3 core definitions
│ ├── stm32f103xb.h # STM32F103 peripheral register definitions
│ ├── stm32f1xx.h # STM32F1 family definitions
│ ├── system_stm32f1xx.h # system configuration
│ ├── cmsis_compiler.h # compiler abstraction
│ ├── cmsis_gcc.h # GCC specific CMSIS definitions
│ └── cmsis_version.h # CMSIS version information
├── media/ # demo GIFs and screenshots
├── core.S # reset handler — copies .data, zeroes .bss, calls main
├── vtable.S # vector table — interrupt handlers, weak aliases to default
├── interrupts.c # SysTick handler and interrupt service routines
├── interrupts.h # interrupt declarations and tick counter
├── linker.ld # memory map — FLASH/SRAM regions, section placement
├── main.c # application entry point
├── main.h # pin definitions and includes
└── Makefile # build system
make
make clean# Terminal 1
st-util
# Terminal 2
arm-none-eabi-gdb main.elf
(gdb) target remote :4242
(gdb) monitor reset halt
(gdb) load
(gdb) continueBoot sequence verified via GDB: val incrementing in real time, read directly from RAM over SWD:
Bare metal GPIO: Onboard LED blinking at ~1Hz via direct GPIO register access, no HAL
Replaced NOP delay loop with interrupt-driven SysTick timer for precise 1ms ticks. Demonstrated with SOS morse code blink pattern:
Polling-based UART TX and RX via USART1. PA9 (TX) configured as alternate function push-pull, PA10 (RX) as input with pull-up. Verified with CP2102 USB-UART adapter. Alphabet streaming over TX and bidirectional echo server over RX:
