stm32 link, bare metal examples?

/* Main program */

int main(void)

{

  int i;

  *(int *)0x40021014 |=  (0x01 <<17U);

  *(int *)0x48000000 &= ~(0xC00);

  *(int *)0x48000000 |=  (0x01 <<10U);

  while(1)

  {

      *(int *)0x48000014 ^= 0x00000020U;

      for (i=0; i<PERIOD; i++);

  }

}

// 4 Bits per Alternate Function

#define AFR_BITS 4U

#define AFR_MSK  0xFU

// 2 Bits per Function

#define DUALBIT_MSK 0b11U

#define BITSPERPIN  2U

// coding of mode Register from enum PioPinMode

#define ENUM_REGMODER_MSK 0b11000U

#define ENUM_REGMODER_POS 3U

// coding of MODE Bits

#define ENUM_BITMODER_MSK 0b11U

#define ENUM_BITMODER_POS 0U

// coding of slewrate

#define ENUM_OTYPE_MSK 0b100U

#define ENUM_OTYPE_POS 2U

// coding of pull resistors

#define ENUM_PUPDR_MSK 0b11

#define ENUM_PUPDR_POS 0

// how many pins in one AFR Register

#define AFR_PINS_PER_REG 8

// 2 part Register BSRR for SET/RESET

#define BSSR_SET_POS 0U

#define BSSR_RES_POS 16U

void gpio_init(const struct gpio_pin* pin){

  uint32_t pin_nbr = pin->nbr;

  uint32_t bitPosDualBit = pin->nbr * BITSPERPIN;

  if (pin_nbr >= 16U){

    return;

  }

  GPIO_TypeDef* gpio = pin->port;

  enum gpio_mode mode = pin->mode;

  // Mode (Input, GP Out, Alternate function, Analog)

  uint32_t regMode = (mode & ENUM_REGMODER_MSK) >> ENUM_REGMODER_POS;

  gpio->MODER &= ~(ENUM_BITMODER_MSK << (bitPosDualBit));

  gpio->MODER |= (regMode << (bitPosDualBit));

  // output type (Pushpull, Opendrain)

  uint32_t regPpod = (mode & ENUM_OTYPE_MSK) >> ENUM_OTYPE_POS;

  gpio->OTYPER &= ~(1 << pin_nbr);

  gpio->OTYPER |= (regPpod << pin_nbr);

  // Slewrate (Lowspeed, Mediumspeed, Fastspeed, Highspeed)

  gpio->OSPEEDR &= ~(DUALBIT_MSK << (bitPosDualBit));

  gpio->OSPEEDR |= (pin->speed << (bitPosDualBit));

  // Pullup, Pulldown

  uint32_t regPull = mode & ENUM_PUPDR_MSK;

  gpio->PUPDR &= ~(ENUM_PUPDR_MSK << (bitPosDualBit));

  gpio->PUPDR |= (regPull << (bitPosDualBit));

  // Altfunc 

  volatile uint32_t *afr = &(gpio->AFR[0U]);

  // search for fitting AFR Register

  if (pin_nbr >= AFR_PINS_PER_REG){

    afr = &(gpio->AFR[1U]);

    pin_nbr -= AFR_PINS_PER_REG;

  }

  *afr &= ~(AFR_MSK << (pin_nbr*AFR_BITS));

  *afr |=  (pin->alt_func << (pin_nbr*AFR_BITS));

}

#include <stdint.h>

#include "cmsis_includer.h"

//! \brief enum of possible Pin Modes

enum gpio_mode {

  GPO_PP =    0b01000, //!< out pushpull

  GPO_PP_PU =    0b01001, //!< out pushpull + pullup

  GPO_PP_PD =    0b01010, //!< out pushpull + pulldown

  GPO_OD =    0b01100, //!< out opendrain

  GPO_OD_PU =    0b01101, //!< out opendrain + pullup

  GPO_OD_PD =    0b01110, //!< out opendrain + pulldown

  AF_PP =      0b10000, //!< altfunc pushpull

  AF_PP_PU =    0b10001, //!< altfunc pushpull + pullup

  AF_PP_PD =    0b10010, //!< altfunc pushpull + pulldown

  AF_OD =      0b10100, //!< altfunc opendrain  

  AF_OD_PU =    0b10101, //!< altfunc opendrain + pullup

  AF_OD_PD =    0b10110, //!< altfunc opendrain + pulldown

  GPI_FLOAT =    0b00000, //!< in floating

  GPI_PU =    0b00001, //!< in + pullup

  GPI_PD =    0b00010, //!< in + pulldown

  GP_ANALOG =    0b11000, //!< analog -> input is switched off

};

//! \brief enum of possible Pin Speeds

enum gpio_speed {

  GPIO_LS = 0b00, //!< low speed

  GPIO_MS = 0b01, //!< medium speed

  GPIO_FS = 0b10, //!< full speed

  GPIO_HS = 0b11, //!< high speed

};

//! \brief structure for one PIO pin 

struct gpio_pin {

  GPIO_TypeDef* port;    //!< port of pin

  uint8_t nbr;      //!< pin Number (0 - 15)

  enum gpio_mode mode;  //!< pin mode @ref gpio_pin_mode

  enum gpio_speed speed;  //!< pin speed @ref PioPinSpeed

  uint8_t alt_func;    //!< alternate function, see datasheet

};