diff --git a/chip8/chip8.go b/chip8/chip8.go
index 3ef4e8b..bfec8cc 100644
--- a/chip8/chip8.go
+++ b/chip8/chip8.go
@@ -1,13 +1,23 @@
 package chip8
 
+const (
+	MemorySize      = 4096 // 4KB of memory
+	StackSize       = 16
+	InstructionSize = 2
+)
+
+const (
+	ProgramAddress     = uint16(0x0200)
+	StackAddress       = uint16(0x0EA0)
+	VideoBufferAddress = uint16(0x0F00)
+)
+
 type Emulator struct {
-	V      [16]uint8   // general registers
-	I      uint16      // address register
-	SP     uint16      // stack pointer
-	PC     uint16      // program counter
-	Memory [4096]uint8 // 4KB of system RAM
-	Stack  []uint8     // 32 bytes of stack. starting at 0x0EA0
-	Screen []uint8     // 256 bytes of display buffer. starting at 0x0F00
+	V      [16]uint8         // general registers
+	I      uint16            // address register
+	SP     uint16            // stack pointer
+	PC     uint16            // program counter
+	Memory [MemorySize]uint8 // 4KB of system RAM
 	Timer  struct {
 		Delay uint8 // delay timer
 		Sound uint8 // sound timer
@@ -23,13 +33,11 @@ func NewEmulator() *Emulator {
 
 func (emulator *Emulator) Reset() {
 	emulator.V = [16]uint8{}
-	emulator.I = 0x00
-	emulator.SP = 0x00
-	emulator.PC = 0x0200
-	emulator.Memory = [4096]uint8{}
-	emulator.Stack = emulator.Memory[0x0EA0:]
-	emulator.Screen = emulator.Memory[0x0F00:]
+	emulator.I = 0
+	emulator.SP = StackAddress
+	emulator.PC = ProgramAddress
+	emulator.Memory = [MemorySize]uint8{}
 	emulator.Timer.Delay = 0
 	emulator.Timer.Sound = 0
-	copy(emulator.Memory[0x0200:], emulator.ROM)
+	copy(emulator.Memory[ProgramAddress:], emulator.ROM)
 }
diff --git a/chip8/instructions.go b/chip8/instructions.go
new file mode 100644
index 0000000..89dff7b
--- /dev/null
+++ b/chip8/instructions.go
@@ -0,0 +1,43 @@
+package chip8
+
+func (emulator *Emulator) ClearScreen() {
+	// TODO: clear the screen buffer
+	emulator.PC += InstructionSize
+}
+
+func (emulator *Emulator) Return() {
+	emulator.StackPop()
+}
+
+func (emulator *Emulator) Jump(addr uint16) {
+	emulator.PC = addr
+}
+
+func (emulator *Emulator) Call(addr uint16) {
+	emulator.StackPush()
+	emulator.PC = addr
+}
+
+func (emulator *Emulator) SkipEqual(x uint8, value uint8) {
+	if emulator.V[x] == value {
+		emulator.PC += InstructionSize * 2
+	} else {
+		emulator.PC += InstructionSize
+	}
+}
+
+func (emulator *Emulator) SkipNotEqual(x uint8, value uint8) {
+	if emulator.V[x] != value {
+		emulator.PC += InstructionSize * 2
+	} else {
+		emulator.PC += InstructionSize
+	}
+}
+
+func (emulator *Emulator) SkipRegistersEqual(x uint8, y uint8) {
+	if emulator.V[x] == emulator.V[y] {
+		emulator.PC += InstructionSize * 2
+	} else {
+		emulator.PC += InstructionSize
+	}
+}
diff --git a/chip8/instructions_test.go b/chip8/instructions_test.go
new file mode 100644
index 0000000..3a3f634
--- /dev/null
+++ b/chip8/instructions_test.go
@@ -0,0 +1 @@
+package chip8_test
diff --git a/chip8/stack.go b/chip8/stack.go
new file mode 100644
index 0000000..4767ffd
--- /dev/null
+++ b/chip8/stack.go
@@ -0,0 +1,20 @@
+package chip8
+
+import "encoding/binary"
+
+func (emulator *Emulator) StackPush() {
+	if emulator.SP == StackAddress+StackSize*2 {
+		panic("chip8: stack overflow")
+	}
+	binary.BigEndian.PutUint16(emulator.Memory[emulator.SP:], emulator.PC)
+	emulator.SP += 2
+}
+
+func (emulator *Emulator) StackPop() {
+	if emulator.SP == StackAddress {
+		panic("chip8: nothing to pop from stack")
+	}
+	emulator.SP -= 2
+	emulator.PC = binary.BigEndian.Uint16(emulator.Memory[emulator.SP:])
+	binary.BigEndian.PutUint16(emulator.Memory[emulator.SP:], 0x00) // clean the stack position
+}
diff --git a/chip8/stack_test.go b/chip8/stack_test.go
new file mode 100644
index 0000000..f0fd33b
--- /dev/null
+++ b/chip8/stack_test.go
@@ -0,0 +1,31 @@
+package chip8_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/tangzero/chip8-emulator/chip8"
+)
+
+func TestEmulator_StackPush(t *testing.T) {
+	emulator := chip8.NewEmulator()
+
+	emulator.PC = 0xABCD
+	emulator.StackPush()
+
+	assert.Equal(t, uint8(0xAB), emulator.Memory[chip8.StackAddress])
+	assert.Equal(t, uint8(0xCD), emulator.Memory[chip8.StackAddress+1])
+	assert.Equal(t, chip8.StackAddress+2, emulator.SP)
+}
+
+func TestEmulator_StackPop(t *testing.T) {
+	emulator := chip8.NewEmulator()
+
+	emulator.SP = chip8.StackAddress + 32
+	emulator.Memory[chip8.StackAddress+30] = 0xEE
+	emulator.Memory[chip8.StackAddress+31] = 0xFF
+	emulator.StackPop()
+
+	assert.Equal(t, uint16(0xEEFF), emulator.PC)
+	assert.Equal(t, chip8.StackAddress+30, emulator.SP)
+}