typedef struct {
// Bit 7: interrupt status, low when interrupt request pending.
// Bit 6: Parity error
// Bit 5: Receiver overrun
// Bit 4: Framing error
// Bit 3: Clear To Send
// Bit 2: Data Carrier Detected
// Bit 1: Transmit Data Register Empty (TDRE)
// Bit 0: Receive Data Register Full (RDRF)
// We care about bits 7, 1, 0, maybe 5 later.
uint8_t status;
// Bit 7: interrupt enable
// Bits 6:5: RTS + transmit interrupt enable
// Bits 4:2: parity + stop bit
// Bits 1:0: speed divider
// We don't actually care about any of those except the interrupt enable
// bits.
uint8_t control;
uint8_t rx;
uint8_t tx;
// Will be set to true the first time acia_has_irq() is called when IRQ is
// set. Then, as long as it stays true, acia_has_irq() will return false.
// When IRQ status is reset, so is in_int.
bool in_int;
} ACIA;
