#define BUFLEN 1024
static char buffer[BUFLEN];
static char *rbp = buffer;
static volatile int need_data = 1024;
static char *wbp = buffer;

IRQ-send-to-PWM {
 send-to-PWM(*rbp++);
 need_data++;
 if ( rbp == buffer + BUFLEN )
   rbp = buffer;
}

main()
{
 while()
 {
  volatile int fnf;
  cli(); /* disable interrupts *.
  fnf = need_data;
  sei(); /* resume interrupts */
  if ( fnf > 512 ) {
    .. write 512 bytes, move wbp forward 512 bytes, roll it over
  }
  cli(); /* disable interrupts *.
  need_data -= 512;
  sei(); /* resume interrupts */
 }
}