Ayke Maaike van Laethem

she/her 🏳️‍⚧️

Bare metal programming from the ground up

@ayke@hachyderm.io
@aykevl
@aykevl

Who am I

MicroPython:
worked on nrf5x support
TinyGo:
creator and maintainer
Maker of blinky jewelry

Topics for this talk

Blinking LED on Arduino Uno

Blinking LED on BBC micro:bit

...without SDK!

"Hello world"

Blink LED

Turn on LED!

Arduino Uno

ATmega328
20MHz
32kB flash, 2kB RAM
AVR instruction set
Arduino IDE

I/O registers!

#include <stdint.h>

// Memory mapped I/O (values from the datasheet).
#define DDRB  *(volatile uint8_t*)(0x24)
#define PORTB *(volatile uint8_t*)(0x25)

void _start() {
    DDRB  |= 1 << 5; // set pin PB5 as an output
    PORTB |= 1 << 5; // set pin PB5 high

    // wait forever
    while (1) {}
}

sbi 0x04, 0x5   ; set bit 5 in DDRB  (0x24)
sbi 0x05, 0x5   ; set bit 5 in PORTB (0x25)
rjmp .-2        ; jump back to the same instruction (loop)

$ avr-gcc -c -o firmware.o -Os -mmcu=atmega328p arduino-1.c

$ avr-ld -o firmware.elf firmware.o

$ avrdude -c arduino -p atmega328p -P /dev/ttyACM0 \
          -U flash:w:firmware.elf:e
Reading 6 bytes for flash from input file firmware.elf
Writing 6 bytes to flash
Writing | ################################################## | 100% 0.04 s
Reading | ################################################## | 100% 0.02 s
6 bytes of flash verified

Avrdude done.  Thank you.

#include <stdint.h>

// Memory mapped I/O (values from the datasheet).
#define DDRB  *(volatile uint8_t*)(0x24)
#define PORTB *(volatile uint8_t*)(0x25)

void _start() {
    DDRB |= (1<<5); // set pin PB5 as an output

    // main loop
    while (1) {
        for (long i=1000000; i != 0; i--) {
            PORTB |= (1<<5); // set pin PB5 high (set bit)
        }
        for (long i=1000000; i != 0; i--) {
            PORTB &= ~(1<<5); // set pin PB5 low (clear bit)
        }
    }
}

MEMORY {
    FLASH (r) : ORIGIN = 0, LENGTH = 32k
}

SECTIONS
{
    .text : {
        *(.text)
    } > FLASH
}

$ avr-gcc -c -o firmware.o -Os -mmcu=atmega328p arduino-2.c

$ avr-ld -o firmware.elf firmware.o -T linkerscript-1.ld

$ avrdude -c arduino -p atmega328p -P /dev/ttyACM0 \
          -U flash:w:firmware.elf:e
Reading 40 bytes for flash from input file firmware.elf
Writing 40 bytes to flash
Writing | ################################################## | 100% 0.04 s
Reading | ################################################## | 100% 0.02 s
40 bytes of flash verified

Avrdude done.  Thank you.

.section .vectors, "a", %progbits
jmp __vector_RESET         ; address 0
jmp __vector_INT0          ; address 4
jmp __vector_INT1          ; address 8
jmp __vector_PCINT0        ; address 12
jmp __vector_PCINT1        ; address 16
jmp __vector_PCINT2        ; address 20
jmp __vector_WDT           ; address 24
jmp __vector_TIMER2_COMPA  ; address 28
jmp __vector_TIMER2_COMPB  ; address 32
jmp __vector_TIMER2_OVF    ; address 36
; etc

$ avr-gcc -c -o firmware.o -Os -mmcu=atmega328p arduino-2.c

$ avr-gcc -c -o vectors-1.o -mmcu=atmega328p vectors-1.s

$ avr-gcc -c -o vectors.o -mmcu=atmega328p vectors-1.s

$ avr-ld -o firmware.elf firmware.o vectors.o -T linkerscript-1.ld
avr-ld: vectors.o:(.vectors+0x0): undefined reference to `__vector_RESET'
avr-ld: vectors.o:(.vectors+0x4): undefined reference to `__vector_INT0'
avr-ld: vectors.o:(.vectors+0x8): undefined reference to `__vector_INT1'
avr-ld: vectors.o:(.vectors+0xc): undefined reference to `__vector_PCINT0'
avr-ld: vectors.o:(.vectors+0x10): undefined reference to `__vector_PCINT1'
avr-ld: vectors.o:(.vectors+0x14): undefined reference to `__vector_PCINT2'
avr-ld: vectors.o:(.vectors+0x18): undefined reference to `__vector_WDT'
avr-ld: vectors.o:(.vectors+0x1c): undefined reference to `__vector_TIMER2_COMPA'
avr-ld: vectors.o:(.vectors+0x20): undefined reference to `__vector_TIMER2_COMPB'
avr-ld: vectors.o:(.vectors+0x24): undefined reference to `__vector_TIMER2_OVF'
[etc]

void __vector_RESET() {
    DDRB |= (1<<5); // set pin PB5 as an output

    // main loop
    while (1) {
        for (long i=1000000; i != 0; i--) {
            PORTB |= (1<<5); // set pin PB5 high (set bit)
        }
        for (long i=1000000; i != 0; i--) {
            PORTB &= ~(1<<5); // set pin PB5 low (clear bit)
        }
    }
}

$ avr-gcc -c -o firmware.o -Os -mmcu=atmega328p arduino-2.c

$ avr-gcc -c -o vectors-1.o -mmcu=atmega328p vectors-1.s

$ avr-gcc -c -o vectors.o -mmcu=atmega328p vectors-1.s

$ avr-ld -o firmware.elf firmware.o vectors.o -T linkerscript-1.ld
avr-ld: vectors.o:(.vectors+0x4): undefined reference to `__vector_INT0'
avr-ld: vectors.o:(.vectors+0x8): undefined reference to `__vector_INT1'
avr-ld: vectors.o:(.vectors+0xc): undefined reference to `__vector_PCINT0'
avr-ld: vectors.o:(.vectors+0x10): undefined reference to `__vector_PCINT1'
avr-ld: vectors.o:(.vectors+0x14): undefined reference to `__vector_PCINT2'
avr-ld: vectors.o:(.vectors+0x18): undefined reference to `__vector_WDT'
avr-ld: vectors.o:(.vectors+0x1c): undefined reference to `__vector_TIMER2_COMPA'
avr-ld: vectors.o:(.vectors+0x20): undefined reference to `__vector_TIMER2_COMPB'
avr-ld: vectors.o:(.vectors+0x24): undefined reference to `__vector_TIMER2_OVF'
avr-ld: vectors.o:(.vectors+0x28): undefined reference to `__vector_TIMER1_CAPT'
[etc]

.section .text
.global  __vector_default
__vector_default:
    rjmp __vector_default

.weak __vector_INT0
.set  __vector_INT0,         __vector_default

.weak __vector_INT1
.set  __vector_INT1,         __vector_default

.weak __vector_PCINT0
.set  __vector_PCINT0,       __vector_default

.weak __vector_PCINT1
.set  __vector_PCINT1,       __vector_default

; [etc]

.section .text
.global  __vector_default
__vector_default:
    rjmp __vector_default

.macro IRQ handler
    .weak  \handler
    .set   \handler, __vector_default
.endm

IRQ __vector_INT0
IRQ __vector_INT1
IRQ __vector_PCINT0
IRQ __vector_PCINT1
IRQ __vector_PCINT2
IRQ __vector_WDT
; [etc]

$ avr-gcc -c -o firmware.o -Os -mmcu=atmega328p arduino-3.c

$ avr-gcc -c -o vectors.o -mmcu=atmega328p vectors-2.s

$ avr-ld -o firmware.elf firmware.o vectors.o -T linkerscript-1.ld

$ avrdude -c arduino -p atmega328p -P /dev/ttyACM0 -U flash:w:firmware.elf:e
Reading 146 bytes for flash from input file firmware.elf
Writing 146 bytes to flash
Writing | ################################################## | 100% 0.07 s
Reading | ################################################## | 100% 0.03 s
146 bytes of flash verified

Avrdude done.  Thank you.

BBC micro:bit v2

nRF52833
64MHz
512kB flash, 128kB RAM
ARM instruction set
(Cortex-M4)

MEMORY {
    FLASH (r) : ORIGIN = 0, LENGTH = 512k
}

ENTRY(Reset_Handler)

SECTIONS
{
    .text : {
        KEEP(*(.vectors))
        *(.text)
    } > FLASH
}

__attribute__((section(".vectors")))
const uintptr_t __vector_table[16] = {
  (uintptr_t)(&__stack_pointer),   // Initial stack pointer
  (uintptr_t)Reset_Handler,        // Reset Handler
  (uintptr_t)NMI_Handler,          // NMI Handler
  (uintptr_t)HardFault_Handler,    // Hard Fault Handler
  (uintptr_t)MemManage_Handler,    // MPU Fault Handler
  (uintptr_t)BusFault_Handler,     // Bus Fault Handler
  (uintptr_t)UsageFault_Handler,   // Usage Fault Handler
  (uintptr_t)SecureFault_Handler,  // Secure Fault Handler
  0,                               // Reserved
  0,                               // Reserved
  0,                               // Reserved
  (uintptr_t)SVC_Handler,          // SVC Handler
  (uintptr_t)DebugMon_Handler,     // Debug Monitor Handler
  0,                               // Reserved
  (uintptr_t)PendSV_Handler,       // PendSV Handler
  (uintptr_t)SysTick_Handler,      // SysTick Handler

  // more interrupt handlers follow
};

extern uintptr_t __stack_pointer[0];

void Default_Handler(void) {
    while (1) {}
}

void Reset_Handler             (void);
void NMI_Handler               (void) __attribute__ ((weak, alias("Default_Handler")));
void HardFault_Handler         (void) __attribute__ ((weak, alias("Default_Handler")));
void MemManage_Handler         (void) __attribute__ ((weak, alias("Default_Handler")));
void MemoryManagement_Handler  (void) __attribute__ ((weak, alias("Default_Handler")));
void BusFault_Handler          (void) __attribute__ ((weak, alias("Default_Handler")));
void UsageFault_Handler        (void) __attribute__ ((weak, alias("Default_Handler")));
void SecureFault_Handler       (void) __attribute__ ((weak, alias("Default_Handler")));
void SVC_Handler               (void) __attribute__ ((weak, alias("Default_Handler")));
void DebugMon_Handler          (void) __attribute__ ((weak, alias("Default_Handler")));
void PendSV_Handler            (void) __attribute__ ((weak, alias("Default_Handler")));
void SysTick_Handler           (void) __attribute__ ((weak, alias("Default_Handler")));

void Reset_Handler(void) {
    while (1) {}
}

$ clang -c --target=armv7em-none-eabi \
        -o microbit-vector.o microbit-vector.c

$ clang -c --target=armv7em-none-eabi \
        -o microbit.o microbit-led.c

$ ld.lld -o microbit.elf microbit-vector.o microbit.o \
         -T microbit.ld
ld.lld: error: undefined symbol: __stack_pointer
>>> referenced by microbit-vector.c
>>>               microbit-vector.o:(__vector_table)

MEMORY {
    FLASH (r) : ORIGIN = 0,          LENGTH = 512k
    RAM (rwx) : ORIGIN = 0x20000000, LENGTH = 128k
}

ENTRY(Reset_Handler)

SECTIONS
{
    .text : {
        KEEP(*(.vectors))
        *(.text)
    } > FLASH

    .stack : {
        . += 4096;
        __stack_pointer = .;
    } > RAM
}

$ clang -c --target=armv7em-none-eabi \
        -o microbit-vector.o microbit-vector.c

$ clang -c --target=armv7em-none-eabi \
        -o microbit.o microbit-led.c

$ ld.lld -o microbit.elf microbit-vector.o microbit.o \
         -T microbit.ld
 
$ openocd -f interface/cmsis-dap.cfg -f target/nrf52.cfg \
          -c 'transport select swd' \
          -c 'program microbit.elf reset exit'
[...lots of verbose logging...]
** Programming Finished **
** Resetting Target **
shutdown command invoked

https://tech.microbit.org/hardware/schematic/

typedef struct {
    volatile uint32_t OUT;
    volatile uint32_t OUTSET;
    volatile uint32_t OUTCLR;
    volatile uint32_t IN;
    volatile uint32_t DIR;
    volatile uint32_t DIRSET;
    volatile uint32_t DIRCLR;
    volatile uint32_t LATCH;
    volatile uint32_t DETECTMODE;
} GPIO_Type;

#define P0 ((GPIO_Type*)(0x50000504))

void Reset_Handler(void) {
    P0->DIRSET = 1 << 21; // set P0.21 (ROW1) as output
    P0->DIRSET = 1 << 28; // set P0.28 (COL1) as output
    P0->OUTSET = 1 << 21; // set P0.21 (ROW1) high

    while (1) {}
}

$ clang -c --target=armv7em-none-eabi \
        -o microbit-vector.o microbit-vector.c

$ clang -c --target=armv7em-none-eabi \
        -o microbit.o microbit-led.c

$ ld.lld -o microbit.elf microbit-vector.o microbit.o \
         -T microbit.ld
 
$ openocd -f interface/cmsis-dap.cfg -f target/nrf52.cfg \
          -c 'transport select swd' \
          -c 'program microbit.elf reset exit'
[...lots of verbose logging...]
** Programming Finished **
** Resetting Target **
shutdown command invoked

void Reset_Handler(void) {
    P0->DIRSET = 1 << 21; // set P0.21 (ROW1) as output
    P0->DIRSET = 1 << 28; // set P0.28 (COL1) as output

    while (1) {
        for (int i=0; i<1000000; i++) {
            P0->OUTSET = 1 << 21; // set P0.21 (ROW1) high
        }
        for (int i=0; i<1000000; i++) {
            P0->OUTCLR = 1 << 21; // set P0.21 (ROW1) low
        }
    }
}

$ clang -c --target=armv7em-none-eabi \
        -o microbit-vector.o microbit-vector.c

$ clang -c --target=armv7em-none-eabi \
        -o microbit.o microbit-blink.c

$ ld.lld -o microbit.elf microbit-vector.o microbit.o \
         -T microbit.ld
 
$ openocd -f interface/cmsis-dap.cfg -f target/nrf52.cfg \
          -c 'transport select swd' \
          -c 'program microbit.elf reset exit'
[...lots of verbose logging...]
** Programming Finished **
** Resetting Target **
shutdown command invoked

Questions?

Slides:
https://aykevl.nl/talks/2025-why2025-firmware/

How to find me:
@ayke@hachyderm.io
@aykevl
@aykevl