Baremetal Startup

Bare-Metal Startup

Purpose

Guide agents through bare-metal startup from reset to main(): reset and exception vectors, .data/.bss initialization, stack and heap setup, C runtime integration with linker scripts, and common Cortex-M and RISC-V bring-up bugs.

When to Use

• Firmware hangs before reaching main()
• Porting to a new MCU without vendor HAL
• Writing custom startup.s or Reset_Handler
• Integrating with skills/embedded/linker-scripts
• Debugging uninitialized globals or stack overflows at boot

Workflow

1. Boot sequence mental model

Power-on / reset
├── CPU loads SP from vector[0]
├── CPU branches to Reset_Handler from vector[1]
├── Reset_Handler: init clocks (optional), copy .data, zero .bss
├── Configure stack pointer (if not already from vector)
├── Call SystemInit() / platform_init()
└── Call main() — must not return (loop or WFI)

2. Cortex-M vector table

On Cortex-M, the vector table starts at address 0 or a relocatable base (VTOR):

| Index | Entry | |-------|-------| | 0 | Initial stack pointer (_estack) | | 1 | Reset_Handler | | 2 | NMI_Handler | | 3 | HardFault_Handler | | 4+ | IRQ handlers per NVIC |

/* startup.s — minimal Cortex-M */
.syntax unified
.thumb

.section .isr_vector,"a",%progbits
.global g_pfnVectors
g_pfnVectors:
    .word _estack
    .word Reset_Handler
    .word NMI_Handler
    .word HardFault_Handler
    /* ... device IRQ vectors from vendor CMSIS ... */

.section .text.Reset_Handler
.thumb_func
.global Reset_Handler
Reset_Handler:
    ldr r0, =_sidata    /* .data load address (LMA) */
    ldr r1, =_sdata     /* .data VMA start */
    ldr r2, =_edata
    b LoopCopyDataInit
CopyDataInit:
    ldr r3, [r0], #4
    str r3, [r1], #4
LoopCopyDataInit:
    cmp r1, r2
    blt CopyDataInit
    ldr r2, =_sbss
    ldr r4, =_ebss
    movs r3, #0
    b LoopFillZerobss
FillZerobss:
    str r3, [r2], #4
LoopFillZerobss:
    cmp r2, r4
    blt FillZerobss
    bl SystemInit
    bl main
    b .

/* Linker symbols — declared extern in startup */
extern uint32_t _estack, _sidata, _sdata, _edata, _sbss, _ebss;

3. VTOR relocation (Cortex-M)

#include "core_cm4.h"  /* CMSIS */

void relocate_vector_table(uint32_t base) {
    SCB->VTOR = base & 0xFFFFFF80U;  /* must be aligned per ARM ref manual */
    __DSB();
    __ISB();
}

Bootloader jumping to app often requires setting VTOR to the application's vector table address (e.g., 0x08020000 on STM32 flash).

4. RISC-V startup (overview)

/* entry.S — simplified */
.section .text.entry
.global _start
_start:
    la sp, _stack_top
    la t0, _bss_start
    la t1, _bss_end
clear_bss:
    beq t0, t1, bss_done
    sw zero, 0(t0)
    addi t0, t0, 4
    j clear_bss
bss_done:
    call main
    j .

Trap vector: mtvec for M-mode; see skills/platform/riscv-privileged.

5. crt0 responsibilities

| Task | Who does it | |------|-------------| | Copy .data LMA→VMA | Reset_Handler / crt0 | | Zero .bss | Reset_Handler | | Init stack | Vector[0] or explicit sp | | Heap (_sbrk) | Optional; libc or custom | | C++ constructors | __libc_init_array() if using full libc | | FPU enable | SystemInit on Cortex-M4F/M7 |

6. Linker script integration

_estack = ORIGIN(RAM) + LENGTH(RAM);
_sidata = LOADADDR(.data);

Cross-link skills/embedded/linker-scripts for MEMORY/SECTIONS layout.

7. Agent usage examples

/baremetal-startup My STM32 never reaches main — what should Reset_Handler do?
/baremetal-startup How do I relocate the vector table after bootloader jump?

Common Problems

| Symptom | Cause | Fix | |---------|-------|-----| | HardFault immediately | Stack pointer invalid | Set _estack to valid RAM top | | Globals wrong at boot | .data not copied | Copy LMA→VMA in Reset_Handler | | BSS non-zero | .bss not zeroed | Add bss clear loop | | IRQs don't fire | VTOR points to wrong table | SCB->VTOR = app_vector_addr | | main returns into garbage | No loop after main | while(1) or WFI loop | | C++ static init crash | Constructors before clocks | Call SystemInit first |

Related Skills

• skills/embedded/linker-scripts — _estack, LMA/VMA, section placement
• skills/baremetal/interrupts-and-exceptions-baremetal — vector table IRQ entries
• skills/baremetal/bootloaders-embedded — VTOR relocation on app jump
• skills/embedded/openocd-jtag — halt at Reset_Handler with GDB
• skills/low-level-programming/assembly-arm — Thumb-2 syntax
• skills/platform/riscv-privileged — RISC-V trap setup

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: mohitmishra786
• Source: mohitmishra786/low-level-dev-skills
• License: MIT
• Homepage: https://www.lowleveldevskills.com

Install and usage instructions live in the source repository linked above.