#include <stdint.h>

#define pREG32 (REG32 *)
typedef volatile uint32_t REG32;
#define UART_BASE_ADDRESS                         (0x40008000)

#define UART_U0LSR_THRE                           ((unsigned int) 0x00000020)
#define UART_U0LSR                                (*(pREG32 (0x40008014)))

/*  Receive buffer */
#define UART_U0RBR                                (*(pREG32 (0x40008000)))

#define GPIO_GPIO2_BASE                           (0x50020000)

#define GPIO_GPIO2DATA                            (*(pREG32 (0x50023FFC)))    // Port data register
#define GPIO_GPIO2DIR                             (*(pREG32 (0x50028000)))    // Data direction register

#define RAM_START  0x10000000
#define RAM_SIZE   8192

void pwm(int level, int top);

__attribute__((naked))
void start(void)
{
  GPIO_GPIO2DIR = 0xFFFF;

  int top = 100;

  for (;;)
    {
      for (int level = 0; level <= top; level++)
	{
	  pwm(level, top);
	}
      for (int level = top; level >= 0; level--)
	{
	  pwm(level, top);
	}
    }
}

void pwm(int level, int top)
{
  int runs = 100;

  for (int run = 0; run < runs; run++)
    {
      GPIO_GPIO2DATA = 0;
      
      for (volatile int j = 0; j < level; j++)
	;
      
      GPIO_GPIO2DATA = 1;
      
      for (volatile int j = level; j <= top; j++)
	;
    }
}

void startx(void)
{
  GPIO_GPIO2DIR = 0xFFFF;

  for (;;)

  for (int i = 'a'; i <= 'z'; i++)
    {
      while ((UART_U0LSR & UART_U0LSR_THRE) == 0)
	;

      UART_U0RBR = i;

      GPIO_GPIO2DATA = i & 1;

      for (volatile int j = 0; j < 100000; j++)
	{
	}
    }
}

__attribute__((section(".vectors")))
const void * vectors[] = {
  (void *)(RAM_START + RAM_SIZE - 64),
  start,
  0,
  0,
  0,
  0,
  0,
  0,
};