Error Serial1.available() Argon

Troubleshooting
4

This is my test code which renders the expected (=correct) results on a Photon but not on the Argon

#define BUF_LIMIT    80

uint8_t buf[BUF_LIMIT];
int     c;
#define txWAIT Serial1.flush()
//#define txWAIT delay(500)

void setup() {
  Serial.begin();
  Serial1.begin(115200);
  memset(buf, 0, sizeof(buf));
  for (char c = '0'; c < '0'+sizeof(buf)-2; c++)
    buf[c-'0'] = c;
}

void loop() {
  Serial.println();
  while(Serial1.read() >= 0);           // flush the RX buffer
  Serial.printlnf("Should be 0 is %d", Serial1.available());  

  Serial1.write(buf, sizeof(buf)-1);    // pre-fill RX buffer again
  txWAIT;
  
  Serial.printlnf("Content of RX buffer should be\r\n%s\r\nbut is", buf);
  while((c = Serial1.read()) >= 0)      // flush the RX buffer
    Serial.write(c);
  Serial.println();

  Serial.printlnf("Trying to fill buffer with %d characters", sizeof(buf)-5);
  Serial1.write(buf, sizeof(buf)-5);    // pre-fill RX buffer 
  txWAIT;

  Serial.printlnf("Should be %d is %d", min(sizeof(buf)-5, 63), Serial1.available());  
  
  for (int i = 0; i < 10; i++)
    Serial.write(Serial1.read());
  Serial.println();
  Serial.printlnf("Should be %d is %d", min(sizeof(buf)-15, 63-10), Serial1.available());  

  Serial1.write(buf, 10);               // re-fill RX buffer 
  txWAIT;

  Serial.printlnf("Should be %d is %d", min(sizeof(buf)-5, 63), Serial1.available());  

  delay(1000);
}

It seems the RX buffer (which is supposed to be a circular or ring buffer) is not treated the same way as on Gen1&2 devices. Also the distance between head and tail of the buffer content is not calculated correctly - hence the erronous Serial1.available() values.
It also seems you can fill the buffer beyond the expected 64 byte limit (on Gen1&2 it’s 64-1 byte to have a gap between head and tail).