i6502/devices/acia6551.go

package devices

import (
	"fmt"
	"time"
)

const (
	aciaData = iota
	aciaStatus
	aciaCommand
	aciaControl
)

var baudRateSelectors = [...]int{0, 50, 75, 110, 135, 150, 300, 600, 1200, 1800, 2400, 3600, 4800, 7200, 9600, 19200}

type Acia6551 struct {
	// Registers
	rx              byte
	tx              byte
	commandRegister byte
	controlRegister byte

	// Other required bits and pieces
	lastTxWrite   int64
	lastRxRead    int64
	overrun       bool
	baudRate      int
	baudRateDelay int64
	rxFull        bool
	txEmpty       bool

	rxInterruptEnabled bool
	txInterruptEnabled bool

	InterruptChan chan bool

	TxChan chan byte
}

func NewAcia6551(interruptChan chan bool) *Acia6551 {
	acia := &Acia6551{InterruptChan: interruptChan}
	acia.Reset()

	return acia
}

func (a *Acia6551) Reset() {
	a.TxChan = make(chan byte, 4096)

	a.tx = 0
	a.txEmpty = true

	a.rx = 0
	a.rxFull = false

	a.lastTxWrite = 0
	a.lastRxRead = 0
	a.overrun = false

	a.rxInterruptEnabled = false
	a.txInterruptEnabled = false
}

func (a *Acia6551) Size() int {
	return 4
}

func (a *Acia6551) Read(address uint16) byte {
	switch address {
	case aciaData:
		return a.rxRead()
	case aciaStatus:
		return a.statusRegister()
	case aciaCommand:
		return a.commandRegister
	case aciaControl:
		return a.controlRegister
	default:
		panic(fmt.Errorf("ACIA 6551 cannot handle addressing 0x%04X", address))
	}
}

func (a *Acia6551) rxRead() byte {
	a.lastRxRead = unixTime()
	a.overrun = false
	a.rxFull = false
	return a.rx
}

func (a *Acia6551) RxWrite(data byte) {
	// Oh noes!
	if a.rxFull {
		a.overrun = true
	}

	a.rx = data
	a.rxFull = true

	if a.rxInterruptEnabled {
		a.InterruptChan <- true
	}
}

func (a *Acia6551) statusRegister() byte {
	now := unixTime()
	status := byte(0)

	if a.rxFull && (now >= (a.lastRxRead + a.baudRateDelay)) {
		status |= 0x08
	}

	if a.txEmpty && (now >= (a.lastTxWrite + a.baudRateDelay)) {
		status |= 0x10
	}

	if a.overrun {
		status |= 0x04
	}

	return status
}

func (a *Acia6551) Write(address uint16, value byte) {
	switch address {
	case aciaData:
		a.txWrite(value)
	case aciaStatus:
		a.Reset()
	case aciaCommand:
		a.setCommandRegister(value)
	case aciaControl:
		a.setControlRegister(value)
	default:
		panic(fmt.Errorf("ACIA 6551 cannot handle addressing 0x%04X", address))
	}
}

func (a *Acia6551) txWrite(value byte) {
	a.lastTxWrite = unixTime()
	a.tx = value
	a.txEmpty = false

	// Post for others
	a.debugTxOutput()
}

func (a *Acia6551) TxRead() byte {
	a.txEmpty = true

	if a.txInterruptEnabled {
		a.InterruptChan <- true
	}

	return a.tx
}

func (a *Acia6551) HasTx() bool {
	return !a.txEmpty
}

func (a *Acia6551) HasRx() bool {
	return a.rxFull
}

func (a *Acia6551) debugTxOutput() {
	if a.HasTx() {
		a.TxChan <- a.TxRead()
	}
}

func (a *Acia6551) setCommandRegister(data byte) {
	fmt.Printf("Setting Acia6551 Command Register: %02X\n", data)

	a.commandRegister = data

	a.rxInterruptEnabled = ((data >> 1) & 1) == 0
	a.txInterruptEnabled = ((data>>2)&1) == 1 && ((data>>3)&1) == 0

	fmt.Printf("RxIRQ: %t; TxIRQ: %t\n", a.rxInterruptEnabled, a.txInterruptEnabled)
}

func (a *Acia6551) setControlRegister(data byte) {
	a.controlRegister = data

	if data == 0x00 {
		a.Reset()
	} else {
		a.setBaudRate(baudRateSelectors[data&0x0f])
	}
}

func (a *Acia6551) setBaudRate(baudRate int) {
	fmt.Printf("Setting baudrate at %d\n", baudRate)

	a.baudRate = baudRate

	// Set baudRateDelay in nanoseconds. It's an approximation.
	if baudRate > 0 {
		a.baudRateDelay = int64((1.0 / float64(baudRate)) * 1000000000 * 8)
	} else {
		a.baudRateDelay = 0
	}
}

func unixTime() int64 {
	return time.Now().UnixNano()
}