diff --git a/.gitignore b/.gitignore
index 6309b28..30a5f23 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1 @@
-riscvemu
-riscvemu.debug
+cmd/riscvrun/riscvrun
diff --git a/cmd/riscvrun/main.go b/cmd/riscvrun/main.go
new file mode 100644
index 0000000..9c02259
--- /dev/null
+++ b/cmd/riscvrun/main.go
@@ -0,0 +1,43 @@
+package main
+
+import (
+	"flag"
+	"io/ioutil"
+	"os"
+
+	"riscvemu"
+)
+
+func main() {
+	memorySize := flag.Int("MemorySize", 4096, "Memory size (bytes)")
+	loadAddress := flag.Int("LoadAddress", 0, "Load address and initial progRam counter")
+
+	flag.Parse()
+
+	files := flag.Args()
+	if len(files) != 1 {
+		flag.Usage()
+		os.Exit(1)
+	}
+
+	sw := riscvemu.MemoryWorld{
+		Ram: make([]byte, *memorySize),
+	}
+
+	// Copy file into working memory
+	fb, err := ioutil.ReadFile(files[0])
+	if err != nil {
+		panic(err)
+	}
+	copy(sw.Ram[*loadAddress:*loadAddress+len(fb)], fb)
+
+	c := riscvemu.NewCPU(&sw)
+	c.Pc = uint32(*loadAddress)
+
+	for {
+		err := c.Step()
+		if err != nil {
+			panic(err)
+		}
+	}
+}
diff --git a/main.go b/cpu.go
similarity index 85%
rename from main.go
rename to cpu.go
index b96c34b..0ba8970 100644
--- a/main.go
+++ b/cpu.go
@@ -1,4 +1,4 @@
-package main
+package riscvemu
 
 import (
 	"fmt"
@@ -14,6 +14,10 @@ type CPUState struct {
 	w         World
 }
 
+func NewCPU(w World) CPUState {
+	return CPUState{w: w}
+}
+
 func opcode_rd(opcode uint32) uint32  { return (opcode >> 7) & 0b11111 }
 func opcode_rs1(opcode uint32) uint32 { return (opcode >> 15) & 0b11111 } // 2^5 is 32 for 32 registers
 func opcode_rs2(opcode uint32) uint32 { return (opcode >> 19) & 0b11111 }
@@ -27,7 +31,7 @@ func (c *CPUState) ReadRegister(r uint32) uint32 {
 }
 
 func (c *CPUState) Step() error {
-	opcode, err := c.w.ReadMemory(c.Pc)
+	opcode, err := c.w.Read32(c.Pc)
 	if err != nil {
 		return fmt.Errorf("ReadMemory: %w", err)
 	}
@@ -164,51 +168,3 @@ func (c *CPUState) Step() error {
 
 	return nil
 }
-
-type World interface {
-	ReadMemory(addr uint32) (uint32, error)
-	WriteMemory(addr, value uint32) error
-	Syscall()
-	Trap() (bool, error)
-}
-
-type SampleWorld struct {
-	ram [1024]uint32
-}
-
-func (sw *SampleWorld) ReadMemory(addr uint32) (uint32, error) {
-	if addr >= uint32(len(sw.ram)) {
-		panic("out of bounds")
-	}
-
-	return sw.ram[addr], nil
-}
-
-func (sw *SampleWorld) WriteMemory(addr, value uint32) error {
-	if addr >= uint32(len(sw.ram)) {
-		panic("out of bounds")
-	}
-
-	sw.ram[addr] = value
-	return nil
-}
-
-func (sw *SampleWorld) Syscall() {
-	panic("syscall")
-}
-
-func (sw *SampleWorld) Trap() (bool, error) {
-	panic("trap")
-}
-
-func main() {
-	sw := SampleWorld{}
-	sw.ram[0] = 0b00000000000100000000000001110011 // EBREAK
-
-	c := CPUState{
-		w: &sw,
-	}
-
-	err := c.Step()
-	panic(err)
-}
diff --git a/cpu_test.go b/cpu_test.go
new file mode 100644
index 0000000..cb89695
--- /dev/null
+++ b/cpu_test.go
@@ -0,0 +1,21 @@
+package riscvemu
+
+import (
+	"testing"
+)
+
+func TestCPU(t *testing.T) {
+
+	sw := MemoryWorld{
+		Ram: make([]byte, 1024),
+	}
+	_ = sw.Write32(0, 0b00000000000100000000000001110011) // EBREAK
+
+	c := CPUState{
+		w: &sw,
+	}
+
+	err := c.Step()
+	panic(err)
+
+}
diff --git a/memoryWorld.go b/memoryWorld.go
new file mode 100644
index 0000000..3684ee4
--- /dev/null
+++ b/memoryWorld.go
@@ -0,0 +1,45 @@
+package riscvemu
+
+type MemoryWorld struct {
+	Ram []byte
+}
+
+func (mw *MemoryWorld) ReadByte(addr uint32) (byte, error) {
+	if addr >= uint32(len(mw.Ram)) {
+		panic("out of bounds")
+	}
+
+	return mw.Ram[addr], nil
+}
+
+func (mw *MemoryWorld) Read32(addr uint32) (uint32, error) {
+	// n.b. will panic on overflow
+	return (uint32(mw.Ram[addr]) << 24) | (uint32(mw.Ram[addr+1]) << 16) | (uint32(mw.Ram[addr+2]) << 8) | uint32(mw.Ram[addr+3]), nil
+}
+
+func (mw *MemoryWorld) WriteByte(addr uint32, value byte) error {
+	if addr >= uint32(len(mw.Ram)) {
+		panic("out of bounds")
+	}
+
+	mw.Ram[addr] = value
+	return nil
+}
+
+func (mw *MemoryWorld) Write32(addr, value uint32) error {
+	// n.b. will panic on overflow
+	mw.Ram[addr] = byte((value >> 24) & 0b11111111)
+	mw.Ram[addr+1] = byte((value >> 16) & 0b11111111)
+	mw.Ram[addr+2] = byte((value >> 8) & 0b11111111)
+	mw.Ram[addr+3] = byte(value & 0b11111111)
+
+	return nil
+}
+
+func (mw *MemoryWorld) Syscall() {
+	panic("syscall")
+}
+
+func (mw *MemoryWorld) Trap() (bool, error) {
+	panic("trap")
+}
diff --git a/world.go b/world.go
new file mode 100644
index 0000000..4276a7b
--- /dev/null
+++ b/world.go
@@ -0,0 +1,12 @@
+package riscvemu
+
+type World interface {
+	ReadByte(addr uint32) (byte, error)
+	Read32(addr uint32) (uint32, error)
+	WriteByte(addr uint32, value byte) error
+	Write32(addr, value uint32) error
+
+	Syscall()
+
+	Trap() (bool, error)
+}