SPI Umsetzung - Verständnisfrage

int tda7348_write_byte(uint8_t saddr, uint8_t buf) {

  uint8_t n = 0;

restart:

  if (n++ >= MAX_TRIES)

    return -1;

begin:

  // start

  TWCR = (1 << TWINT) | (1 << TWSTA) | (1 << TWEN);

  while (!(TWCR & (1 << TWINT)));

  if (TW_STATUS == TW_MT_ARB_LOST) goto begin;

  if ( (TW_STATUS != TW_REP_START) && (TW_STATUS != TW_START)) {

    return -1;

  }

  // send 0x440

  // 0x440 = 1000100 0

  // 7 bits:   <chip address>

  // last bit: <R/W> 0W? 1R?

  TWDR = 0x440;

  TWCR = (1 << TWINT) | (1 << TWEN);

  while (!(TWCR & (1 << TWINT)));

  if (TW_STATUS == TW_MT_SLA_NACK) goto restart;

  if (TW_STATUS == TW_MT_ARB_LOST) goto begin;

  if (TW_STATUS != TW_MT_SLA_ACK) goto error;

  // send 4 bits subaddress

  TWDR = saddr >> 4;

  TWCR = (1 << TWINT) | (1 << TWEN);

  while (!(TWCR & (1 << TWINT)));

  if (TW_STATUS != TW_MT_DATA_ACK) goto error;

  // send 8 bits Data

  TWDR = buf;

  TWCR = (1 << TWINT) | (1 << TWEN);

  while (!(TWCR & (1 << TWINT)));

  if (TW_STATUS != TW_MT_DATA_ACK) goto error;

quit:

  // send stop condition

  TWCR = (1 << TWINT) | (1 << TWEN) | (1 << TWSTO);

  return 1;

error:

  // send stop condition

  TWCR = (1 << TWINT) | (1 << TWEN) | (1 << TWSTO);

  return -1;

}