diff --git a/debugger/main.go b/debugger/main.go
new file mode 100644
index 0000000..d07ad86
--- /dev/null
+++ b/debugger/main.go
@@ -0,0 +1,105 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+	"log"
+	"sync"
+	"time"
+
+	"go.bug.st/serial"
+)
+
+const portDev = "/dev/ttyUSB0"
+
+func main() {
+	dbg, err := Open()
+	if err != nil {
+		log.Fatalf("connecting to debugger: %v", err)
+	}
+	defer dbg.Close()
+
+	fmt.Println("Writing...")
+	if err := dbg.Write(0x42, 123); err != nil {
+		log.Fatalf("writing to memory: %v", err)
+	}
+	v, err := dbg.Read(0x42)
+	if err != nil {
+		log.Fatalf("reading from memory: %v", err)
+	}
+	fmt.Printf("addr 0: %02x\n", v)
+}
+
+type Debugger struct {
+	port serial.Port
+	mu   sync.Mutex
+}
+
+func Open() (*Debugger, error) {
+	mode := &serial.Mode{
+		BaudRate: 115_200,
+	}
+	port, err := serial.Open(portDev, mode)
+	if err != nil {
+		return nil, err
+	}
+
+	return &Debugger{port: port}, nil
+}
+
+func (d *Debugger) Close() error {
+	d.mu.Lock() // note, deliberately no unlocking, to poison.
+	return d.port.Close()
+}
+
+func encode(addr int, write bool, data byte) [4]byte {
+	writeVal := uint8(0)
+	if write {
+		writeVal = 1
+	}
+	var ret [4]byte
+	ret[3] = data
+	ret[2] = byte(addr<<1) | writeVal
+	ret[1] = byte(addr >> 7)
+	ret[0] = byte(addr >> 15)
+	return ret
+}
+
+func (d *Debugger) Read(addr int) (byte, error) {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	if addr >= 2<<17 {
+		return 0, fmt.Errorf("read %d out of bounds", addr)
+	}
+
+	packet := encode(addr, false, 0)
+	fmt.Printf("Writing: %02x %02x %02x %02x\n", packet[0], packet[1], packet[2], packet[3])
+	if _, err := d.port.Write(packet[:]); err != nil {
+		return 0, err
+	}
+
+	d.port.SetReadTimeout(2 * time.Second)
+
+	n, err := d.port.Read(packet[:1])
+	if err != nil {
+		return 0, err
+	} else if n == 0 {
+		return 0, errors.New("no read")
+	}
+	return packet[0], nil
+}
+
+func (d *Debugger) Write(addr int, val byte) error {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	if addr >= 2<<17 {
+		return fmt.Errorf("write %d out of bounds", addr)
+	}
+
+	packet := encode(addr, true, val)
+	fmt.Printf("Writing: %02x %02x %02x %02x\n", packet[0], packet[1], packet[2], packet[3])
+	if _, err := d.port.Write(packet[:]); err != nil {
+		return err
+	}
+	return nil
+}
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..222199f
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,9 @@
+module git.sentinel65x.com/dave/gary
+
+go 1.22.6
+
+require (
+	github.com/creack/goselect v0.1.2 // indirect
+	go.bug.st/serial v1.6.2 // indirect
+	golang.org/x/sys v0.0.0-20220829200755-d48e67d00261 // indirect
+)
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..2f35f6f
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,6 @@
+github.com/creack/goselect v0.1.2 h1:2DNy14+JPjRBgPzAd1thbQp4BSIihxcBf0IXhQXDRa0=
+github.com/creack/goselect v0.1.2/go.mod h1:a/NhLweNvqIYMuxcMOuWY516Cimucms3DglDzQP3hKY=
+go.bug.st/serial v1.6.2 h1:kn9LRX3sdm+WxWKufMlIRndwGfPWsH1/9lCWXQCasq8=
+go.bug.st/serial v1.6.2/go.mod h1:UABfsluHAiaNI+La2iESysd9Vetq7VRdpxvjx7CmmOE=
+golang.org/x/sys v0.0.0-20220829200755-d48e67d00261 h1:v6hYoSR9T5oet+pMXwUWkbiVqx/63mlHjefrHmxwfeY=
+golang.org/x/sys v0.0.0-20220829200755-d48e67d00261/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=