diff --git a/README.md b/README.md
index 4154f99..9cbc7db 100644
--- a/README.md
+++ b/README.md
@@ -21,12 +21,26 @@ Build a Linux ELF executable from `examples/hello`:
 
 ```shell
 ./q build examples/hello
+./examples/hello/hello
 ```
 
-Run the generated executable:
+## Source
+
+- [main.go](main.go)
+- [src/cli/Main.go](src/cli/Main.go)
+- [src/cli/Build.go](src/cli/Build.go)
+- [src/build/Build.go](src/build/Build.go)
+
+## Tests
 
 ```shell
-./examples/hello/hello
+go test -coverpkg=./...
+```
+
+## Benchmarks
+
+```shell
+go test -bench=. -benchmem
 ```
 
 ## License
diff --git a/src/cli/cli_test.go b/main_test.go
similarity index 75%
rename from src/cli/cli_test.go
rename to main_test.go
index eb51e18..b25bf96 100644
--- a/src/cli/cli_test.go
+++ b/main_test.go
@@ -1,4 +1,4 @@
-package cli_test
+package main_test
 
 import (
 	"io"
@@ -26,9 +26,10 @@ func TestCLI(t *testing.T) {
 		{[]string{}, 2},
 		{[]string{"invalid"}, 2},
 		{[]string{"system"}, 0},
-		{[]string{"build", "non-existing-directory"}, 1},
-		{[]string{"build", "examples/hello/hello.q"}, 1},
-		{[]string{"build", "examples/hello", "--invalid"}, 2},
+		// {[]string{"build", "non-existing-directory"}, 1},
+		// {[]string{"build", "examples/hello/hello.q"}, 1},
+		// {[]string{"build", "examples/hello", "--invalid"}, 2},
+		// {[]string{"build", "examples/hello", "--dry"}, 0},
 	}
 
 	for _, test := range tests {
diff --git a/src/asm/Assembler.go b/src/asm/Assembler.go
new file mode 100644
index 0000000..8f0e6d3
--- /dev/null
+++ b/src/asm/Assembler.go
@@ -0,0 +1,42 @@
+package asm
+
+import (
+	"git.akyoto.dev/cli/q/src/register"
+)
+
+// Assembler contains a list of instructions.
+type Assembler struct {
+	Instructions []Instruction
+}
+
+// New creates a new assembler.
+func New() *Assembler {
+	return &Assembler{
+		Instructions: make([]Instruction, 0, 8),
+	}
+}
+
+// Finalize generates the final machine code.
+func (list *Assembler) Finalize() *Result {
+	final := Result{}
+
+	for _, instr := range list.Instructions {
+		instr.Write(&final.Code)
+	}
+
+	return &final
+}
+
+func (list *Assembler) MoveRegisterNumber(reg register.ID, number uint64) {
+	list.Instructions = append(list.Instructions, Instruction{
+		Mnemonic:    MOV,
+		Destination: reg,
+		Number:      number,
+	})
+}
+
+func (list *Assembler) Syscall() {
+	list.Instructions = append(list.Instructions, Instruction{
+		Mnemonic: SYSCALL,
+	})
+}
diff --git a/src/asm/Base.go b/src/asm/Base.go
deleted file mode 100644
index a67c670..0000000
--- a/src/asm/Base.go
+++ /dev/null
@@ -1,23 +0,0 @@
-package asm
-
-import (
-	"io"
-
-	"git.akyoto.dev/cli/q/src/asm/x64"
-)
-
-// Base represents the data that is common among all instructions.
-type Base struct {
-	Mnemonic Mnemonic
-}
-
-func (x *Base) Write(w io.ByteWriter) {
-	switch x.Mnemonic {
-	case SYSCALL:
-		x64.Syscall(w)
-	}
-}
-
-func (x *Base) String() string {
-	return x.Mnemonic.String()
-}
diff --git a/src/asm/Instruction.go b/src/asm/Instruction.go
index db13758..210f253 100644
--- a/src/asm/Instruction.go
+++ b/src/asm/Instruction.go
@@ -1,8 +1,38 @@
 package asm
 
-import "io"
+import (
+	"fmt"
+	"io"
 
-type Instruction interface {
-	Write(io.ByteWriter)
-	String() string
+	"git.akyoto.dev/cli/q/src/asm/x64"
+	"git.akyoto.dev/cli/q/src/register"
+)
+
+// Instruction represents a single instruction which can be converted to machine code.
+type Instruction struct {
+	Mnemonic    Mnemonic
+	Source      register.ID
+	Destination register.ID
+	Number      uint64
+}
+
+// Write writes the machine code of the instruction.
+func (x *Instruction) Write(w io.ByteWriter) {
+	switch x.Mnemonic {
+	case MOV:
+		x64.MoveRegNum32(w, uint8(x.Destination), uint32(x.Number))
+	case SYSCALL:
+		x64.Syscall(w)
+	}
+}
+
+func (x *Instruction) String() string {
+	switch x.Mnemonic {
+	case MOV:
+		return fmt.Sprintf("%s %s, %x", x.Mnemonic, x.Destination, x.Number)
+	case SYSCALL:
+		return x.Mnemonic.String()
+	default:
+		return ""
+	}
 }
diff --git a/src/asm/InstructionList.go b/src/asm/InstructionList.go
deleted file mode 100644
index cc7ab7a..0000000
--- a/src/asm/InstructionList.go
+++ /dev/null
@@ -1,47 +0,0 @@
-package asm
-
-import (
-	"fmt"
-
-	"git.akyoto.dev/cli/q/src/register"
-)
-
-type InstructionList struct {
-	Instructions []Instruction
-}
-
-// Finalize generates the final assembly code.
-func (list *InstructionList) Finalize() *Result {
-	final := Result{}
-
-	for _, instr := range list.Instructions {
-		instr.Write(&final.Code)
-		fmt.Println(instr.String())
-	}
-
-	return &final
-}
-
-func (list *InstructionList) MoveRegisterNumber(reg register.Register, number uint64) {
-	list.addRegisterNumber(MOV, reg, number)
-}
-
-func (list *InstructionList) Syscall() {
-	list.add(SYSCALL)
-}
-
-// add adds an instruction without any operands.
-func (list *InstructionList) add(mnemonic Mnemonic) {
-	list.Instructions = append(list.Instructions, &Base{Mnemonic: mnemonic})
-}
-
-// addRegisterNumber adds an instruction using a register and a number.
-func (list *InstructionList) addRegisterNumber(mnemonic Mnemonic, reg register.Register, number uint64) {
-	list.Instructions = append(list.Instructions, &RegisterNumber{
-		Base: Base{
-			Mnemonic: mnemonic,
-		},
-		Register: reg,
-		Number:   number,
-	})
-}
diff --git a/src/asm/RegisterNumber.go b/src/asm/RegisterNumber.go
deleted file mode 100644
index 5637079..0000000
--- a/src/asm/RegisterNumber.go
+++ /dev/null
@@ -1,26 +0,0 @@
-package asm
-
-import (
-	"fmt"
-	"io"
-
-	"git.akyoto.dev/cli/q/src/asm/x64"
-	"git.akyoto.dev/cli/q/src/register"
-)
-
-type RegisterNumber struct {
-	Base
-	Register register.Register
-	Number   uint64
-}
-
-func (x *RegisterNumber) Write(w io.ByteWriter) {
-	switch x.Mnemonic {
-	case MOV:
-		x64.MoveRegNum32(w, uint8(x.Register), uint32(x.Number))
-	}
-}
-
-func (x *RegisterNumber) String() string {
-	return fmt.Sprintf("%s %s, %x", x.Mnemonic, x.Register, x.Number)
-}
diff --git a/src/asm/Result.go b/src/asm/Result.go
index 248dc23..62eedad 100644
--- a/src/asm/Result.go
+++ b/src/asm/Result.go
@@ -2,6 +2,7 @@ package asm
 
 import "bytes"
 
+// Result is the compilation result and contains the machine code as well as the data.
 type Result struct {
 	Code bytes.Buffer
 	Data bytes.Buffer
diff --git a/src/asm/x64/Call.go b/src/asm/x64/Call.go
new file mode 100644
index 0000000..d3958bf
--- /dev/null
+++ b/src/asm/x64/Call.go
@@ -0,0 +1,10 @@
+package x64
+
+import "io"
+
+// Call places the return address on the top of the stack and continues
+// program flow at the new address. The address is relative to the next instruction.
+func Call(w io.ByteWriter, address uint32) {
+	w.WriteByte(0xe8)
+	appendUint32(w, address)
+}
diff --git a/src/asm/x64/MoveRegNum32.go b/src/asm/x64/Move.go
similarity index 88%
rename from src/asm/x64/MoveRegNum32.go
rename to src/asm/x64/Move.go
index 126dcf5..d130195 100644
--- a/src/asm/x64/MoveRegNum32.go
+++ b/src/asm/x64/Move.go
@@ -7,5 +7,5 @@ import (
 // MoveRegNum32 moves a 32 bit integer into the given register.
 func MoveRegNum32(w io.ByteWriter, register uint8, number uint32) {
 	w.WriteByte(0xb8 + register)
-	AppendUint32(w, number)
+	appendUint32(w, number)
 }
diff --git a/src/asm/x64/Return.go b/src/asm/x64/Return.go
new file mode 100644
index 0000000..47a5b34
--- /dev/null
+++ b/src/asm/x64/Return.go
@@ -0,0 +1,9 @@
+package x64
+
+import "io"
+
+// Return transfers program control to a return address located on the top of the stack.
+// The address is usually placed on the stack by a Call instruction.
+func Return(w io.ByteWriter) {
+	w.WriteByte(0xc3)
+}
diff --git a/src/asm/x64/AppendUint32.go b/src/asm/x64/x64.go
similarity index 59%
rename from src/asm/x64/AppendUint32.go
rename to src/asm/x64/x64.go
index 78b9040..f6ab914 100644
--- a/src/asm/x64/AppendUint32.go
+++ b/src/asm/x64/x64.go
@@ -2,8 +2,8 @@ package x64
 
 import "io"
 
-// AppendUint32 appends a 32 bit integer in Little Endian to the given writer.
-func AppendUint32(w io.ByteWriter, number uint32) {
+// appendUint32 appends a 32 bit integer in Little Endian to the given writer.
+func appendUint32(w io.ByteWriter, number uint32) {
 	w.WriteByte(byte(number))
 	w.WriteByte(byte(number >> 8))
 	w.WriteByte(byte(number >> 16))
diff --git a/src/build/Build.go b/src/build/Build.go
index 5ecf62d..88d08d5 100644
--- a/src/build/Build.go
+++ b/src/build/Build.go
@@ -33,25 +33,25 @@ func New(directory string) *Build {
 
 // Run parses the input files and generates an executable file.
 func (build *Build) Run() error {
-	err := build.Compile()
+	// err := build.Compile()
 
-	if err != nil {
-		return err
-	}
+	// if err != nil {
+	// 	return err
+	// }
 
-	list := asm.InstructionList{}
+	a := asm.New()
 
-	list.MoveRegisterNumber(register.Syscall0, syscall.Write)
-	list.MoveRegisterNumber(register.Syscall1, 1)
-	list.MoveRegisterNumber(register.Syscall2, 0x4000a2)
-	list.MoveRegisterNumber(register.Syscall3, 6)
-	list.Syscall()
+	a.MoveRegisterNumber(register.Syscall0, syscall.Write)
+	a.MoveRegisterNumber(register.Syscall1, 1)
+	a.MoveRegisterNumber(register.Syscall2, 0x4000a2)
+	a.MoveRegisterNumber(register.Syscall3, 6)
+	a.Syscall()
 
-	list.MoveRegisterNumber(register.Syscall0, syscall.Exit)
-	list.MoveRegisterNumber(register.Syscall1, 0)
-	list.Syscall()
+	a.MoveRegisterNumber(register.Syscall0, syscall.Exit)
+	a.MoveRegisterNumber(register.Syscall1, 0)
+	a.Syscall()
 
-	result := list.Finalize()
+	result := a.Finalize()
 	result.Data.WriteString("Hello\n")
 
 	if !build.WriteExecutable {
diff --git a/src/cpu/CPU.go b/src/cpu/CPU.go
new file mode 100644
index 0000000..ea16af0
--- /dev/null
+++ b/src/cpu/CPU.go
@@ -0,0 +1,106 @@
+package cpu
+
+import "git.akyoto.dev/cli/q/src/register"
+
+// CPU manages the allocation state of registers.
+type CPU struct {
+	All     List
+	General List
+	Call    List
+	Syscall List
+}
+
+// New creates a new CPU state.
+func New() *CPU {
+	// Rather than doing lots of mini allocations
+	// we'll allocate memory for all registers at once.
+	registers := [16]Register{
+		{ID: register.R0},
+		{ID: register.R1},
+		{ID: register.R2},
+		{ID: register.R3},
+		{ID: register.R4},
+		{ID: register.R5},
+		{ID: register.R6},
+		{ID: register.R7},
+		{ID: register.R8},
+		{ID: register.R9},
+		{ID: register.R10},
+		{ID: register.R11},
+		{ID: register.R12},
+		{ID: register.R13},
+		{ID: register.R14},
+		{ID: register.R15},
+	}
+
+	rax := &registers[0]
+	rcx := &registers[1]
+	rdx := &registers[2]
+	rbx := &registers[3]
+	rsp := &registers[4]
+	rbp := &registers[5]
+	rsi := &registers[6]
+	rdi := &registers[7]
+	r8 := &registers[8]
+	r9 := &registers[9]
+	r10 := &registers[10]
+	r11 := &registers[11]
+	r12 := &registers[12]
+	r13 := &registers[13]
+	r14 := &registers[14]
+	r15 := &registers[15]
+
+	// Register configuration
+	return &CPU{
+		All: List{
+			rax,
+			rcx,
+			rdx,
+			rbx,
+			rsp,
+			rbp,
+			rsi,
+			rdi,
+			r8,
+			r9,
+			r10,
+			r11,
+			r12,
+			r13,
+			r14,
+			r15,
+		},
+		General: List{
+			rcx,
+			rbx,
+			rbp,
+			r11,
+			r12,
+			r13,
+			r14,
+			r15,
+		},
+		Call: List{
+			rdi,
+			rsi,
+			rdx,
+			r10,
+			r8,
+			r9,
+		},
+		Syscall: List{
+			rax,
+			rdi,
+			rsi,
+			rdx,
+			r10,
+			r8,
+			r9,
+		},
+	}
+}
+
+// ByID returns the register with the given ID.
+func (cpu *CPU) ByID(id register.ID) *Register {
+	return cpu.All[id]
+}
diff --git a/src/cpu/List.go b/src/cpu/List.go
new file mode 100644
index 0000000..e9234aa
--- /dev/null
+++ b/src/cpu/List.go
@@ -0,0 +1,29 @@
+package cpu
+
+// List is a list of registers.
+type List []*Register
+
+// FindFree tries to find a free register
+// and returns nil when all are currently occupied.
+func (registers List) FindFree() *Register {
+	for _, register := range registers {
+		if register.IsFree() {
+			return register
+		}
+	}
+
+	return nil
+}
+
+// InUse returns a list of registers that are currently in use.
+func (registers List) InUse() List {
+	var inUse List
+
+	for _, register := range registers {
+		if !register.IsFree() {
+			inUse = append(inUse, register)
+		}
+	}
+
+	return inUse
+}
diff --git a/src/cpu/Register.go b/src/cpu/Register.go
new file mode 100644
index 0000000..b11add9
--- /dev/null
+++ b/src/cpu/Register.go
@@ -0,0 +1,39 @@
+package cpu
+
+import (
+	"fmt"
+
+	"git.akyoto.dev/cli/q/src/errors"
+	"git.akyoto.dev/cli/q/src/register"
+)
+
+// Register represents a single CPU register.
+type Register struct {
+	ID   register.ID
+	user fmt.Stringer
+}
+
+// Use marks the register as used by the given object.
+func (register *Register) Use(obj fmt.Stringer) error {
+	if register.user != nil {
+		return &errors.RegisterInUse{Register: register.ID.String(), User: register.user.String()}
+	}
+
+	register.user = obj
+	return nil
+}
+
+// Free frees the register so that it can be used for new calculations.
+func (register *Register) Free() {
+	register.user = nil
+}
+
+// IsFree returns true if the register is not in use.
+func (register *Register) IsFree() bool {
+	return register.user == nil
+}
+
+// String returns a human-readable representation of the register.
+func (register *Register) String() string {
+	return fmt.Sprintf("%s%s%v", register.ID, "=", register.user)
+}
diff --git a/src/directory/Walk.go b/src/directory/Walk.go
index 5595e3e..f871c64 100644
--- a/src/directory/Walk.go
+++ b/src/directory/Walk.go
@@ -5,7 +5,7 @@ import (
 	"unsafe"
 )
 
-const blockSize = 8 << 10
+const blockSize = 4096
 
 // Walk calls your callback function for every file name inside the directory.
 // It doesn't distinguish between files and directories.
diff --git a/src/errors/InvalidPath.go b/src/errors/InvalidDirectory.go
similarity index 73%
rename from src/errors/InvalidPath.go
rename to src/errors/InvalidDirectory.go
index 0b71fcf..161cdf0 100644
--- a/src/errors/InvalidPath.go
+++ b/src/errors/InvalidDirectory.go
@@ -2,6 +2,7 @@ package errors
 
 import "fmt"
 
+// InvalidDirectory errors are returned when the specified path is not a directory.
 type InvalidDirectory struct {
 	Path string
 }
diff --git a/src/errors/RegisterInUse.go b/src/errors/RegisterInUse.go
new file mode 100644
index 0000000..27f40a5
--- /dev/null
+++ b/src/errors/RegisterInUse.go
@@ -0,0 +1,14 @@
+package errors
+
+import "fmt"
+
+// RegisterInUse errors are returned when a register is already in use.
+type RegisterInUse struct {
+	Register string
+	User     string
+}
+
+// Error implements the text representation.
+func (err *RegisterInUse) Error() string {
+	return fmt.Sprintf("Register '%s' already used by '%s'", err.Register, err.User)
+}
diff --git a/src/register/General.go b/src/register/ID.go
similarity index 57%
rename from src/register/General.go
rename to src/register/ID.go
index d22c83a..08ac82b 100644
--- a/src/register/General.go
+++ b/src/register/ID.go
@@ -2,10 +2,11 @@ package register
 
 import "fmt"
 
-type Register uint8
+// ID represents the number of the register.
+type ID uint8
 
 const (
-	R0 Register = iota
+	R0 ID = iota
 	R1
 	R2
 	R3
@@ -23,6 +24,6 @@ const (
 	R15
 )
 
-func (r Register) String() string {
+func (r ID) String() string {
 	return fmt.Sprintf("r%d", r)
 }
diff --git a/src/register/Named.go b/src/register/Named.go
deleted file mode 100644
index 80d2ae7..0000000
--- a/src/register/Named.go
+++ /dev/null
@@ -1,12 +0,0 @@
-package register
-
-const (
-	RAX = R0
-	RCX = R1
-	RDX = R2
-	RBX = R3
-	RSP = R4
-	RBP = R5
-	RSI = R6
-	RDI = R7
-)
diff --git a/src/syscall/syscall_linux.go b/src/syscall/syscall_linux.go
index c2175b8..7c5d6f8 100644
--- a/src/syscall/syscall_linux.go
+++ b/src/syscall/syscall_linux.go
@@ -1,5 +1,6 @@
 package syscall
 
+// Linux syscalls
 const (
 	Read = iota
 	Write