diff --git a/examples/collatz/collatz.q b/examples/collatz/collatz.q
index f7d9037..5f71d2c 100644
--- a/examples/collatz/collatz.q
+++ b/examples/collatz/collatz.q
@@ -1,5 +1,5 @@
+import io
 import log
-import sys
 
 main() {
 	collatz(12)
@@ -19,6 +19,6 @@ collatz(x Int) {
 			return
 		}
 
-		sys.write(1, " ", 1)
+		io.out(" ")
 	}
 }
\ No newline at end of file
diff --git a/examples/fizzbuzz/fizzbuzz.q b/examples/fizzbuzz/fizzbuzz.q
index 5cca2d5..7823731 100644
--- a/examples/fizzbuzz/fizzbuzz.q
+++ b/examples/fizzbuzz/fizzbuzz.q
@@ -1,5 +1,5 @@
+import io
 import log
-import sys
 
 main() {
 	fizzbuzz(15)
@@ -10,9 +10,9 @@ fizzbuzz(n Int) {
 
 	loop {
 		switch {
-			x % 15 == 0 { print("FizzBuzz", 8) }
-			x % 5 == 0  { print("Buzz", 4) }
-			x % 3 == 0  { print("Fizz", 4) }
+			x % 15 == 0 { io.out("FizzBuzz") }
+			x % 5 == 0  { io.out("Buzz") }
+			x % 3 == 0  { io.out("Fizz") }
 			_           { log.number(x) }
 		}
 
@@ -22,10 +22,6 @@ fizzbuzz(n Int) {
 			return
 		}
 
-		print(" ", 1)
+		io.out(" ")
 	}
-}
-
-print(address Pointer, length Int) {
-	sys.write(1, address, length)
 }
\ No newline at end of file
diff --git a/examples/hello/hello.q b/examples/hello/hello.q
index 1d9220d..0475212 100644
--- a/examples/hello/hello.q
+++ b/examples/hello/hello.q
@@ -1,9 +1,5 @@
-import sys
+import io
 
 main() {
-	print("Hello\n", 6)
-}
-
-print(address Pointer, length Int) {
-	sys.write(1, address, length)
+	io.out("Hello\n")
 }
\ No newline at end of file
diff --git a/examples/prime/prime.q b/examples/prime/prime.q
index 836c831..76e3d77 100644
--- a/examples/prime/prime.q
+++ b/examples/prime/prime.q
@@ -1,5 +1,5 @@
+import io
 import log
-import sys
 
 main() {
 	n := 100
@@ -12,7 +12,7 @@ main() {
 
 		if isPrime(i) == 1 {
 			if i != 2 {
-				sys.write(1, " ", 1)
+				io.out(" ")
 			}
 
 			log.number(i)
diff --git a/examples/server/server.q b/examples/server/server.q
index a729032..6ac4ee3 100644
--- a/examples/server/server.q
+++ b/examples/server/server.q
@@ -1,6 +1,7 @@
 // Open server and client in 2 terminals:
 // [1] q run examples/server
 // [2] curl http://127.0.0.1:8080
+import io
 import net
 import sys
 
@@ -8,30 +9,30 @@ main() {
 	socket := sys.socket(2, 1, 0)
 
 	if socket < 0 {
-		sys.write(1, "socket error\n", 13)
+		io.error("socket error\n")
 		sys.exit(1)
 	}
 
 	if net.bind(socket, 8080) != 0 {
-		sys.write(1, "bind error\n", 11)
+		io.error("bind error\n")
 		sys.exit(1)
 	}
 
 	if sys.listen(socket, 128) != 0 {
-		sys.write(1, "listen error\n", 13)
+		io.error("listen error\n")
 		sys.exit(1)
 	}
 
-	sys.write(1, "listening...\n", 13)
+	io.out("listening...\n")
 
 	loop {
 		conn := sys.accept(socket, 0, 0)
 
 		if conn >= 0 {
-			sys.write(conn, "HTTP/1.0 200 OK\r\nContent-Length: 6\r\n\r\nHello\n", 44)
+			io.write(conn, "HTTP/1.0 200 OK\r\nContent-Length: 6\r\n\r\nHello\n")
 			sys.close(conn)
 		} else {
-			sys.write(1, "error\n", 6)
+			io.error("accept error\n")
 		}
 	}
 }
\ No newline at end of file
diff --git a/examples/shell/shell.q b/examples/shell/shell.q
index 0b094f6..3353391 100644
--- a/examples/shell/shell.q
+++ b/examples/shell/shell.q
@@ -1,3 +1,4 @@
+import io
 import mem
 import sys
 
@@ -9,8 +10,8 @@ main() {
 	info := mem.alloc(24)
 
 	loop {
-		sys.write(1, "$ ", 2)
-		n := sys.read(0, command, length)
+		io.out("$ ")
+		n := io.in(command)
 
 		if n <= 0 {
 			return
diff --git a/examples/thread/thread.q b/examples/thread/thread.q
index 929ebbb..22c9d05 100644
--- a/examples/thread/thread.q
+++ b/examples/thread/thread.q
@@ -1,3 +1,4 @@
+import io
 import sys
 import thread
 import time
@@ -10,8 +11,8 @@ main() {
 }
 
 work() {
-	sys.write(1, "[ ] start\n", 10)
+	io.out("[ ] start\n")
 	time.sleep(10 * 1000 * 1000)
-	sys.write(1, "[x] end\n", 8)
+	io.out("[x] end\n")
 	sys.exit(0)
 }
\ No newline at end of file
diff --git a/lib/io/io.q b/lib/io/io.q
new file mode 100644
index 0000000..4349f94
--- /dev/null
+++ b/lib/io/io.q
@@ -0,0 +1,21 @@
+import sys
+
+in(buffer Pointer) -> Int {
+	return sys.read(0, buffer, len(buffer))
+}
+
+out(message Pointer) -> Int {
+	return sys.write(1, message, len(message))
+}
+
+error(message Pointer) -> Int {
+	return sys.write(2, message, len(message))
+}
+
+read(fd Int, buffer Pointer) -> Int {
+	return sys.read(fd, buffer, len(buffer))
+}
+
+write(fd Int, message Pointer) -> Int {
+	return sys.write(fd, message, len(message))
+}
\ No newline at end of file
diff --git a/lib/mem/alloc_linux.q b/lib/mem/alloc_linux.q
index 70929fe..df8fe5b 100644
--- a/lib/mem/alloc_linux.q
+++ b/lib/mem/alloc_linux.q
@@ -1,5 +1,12 @@
 import sys
 
 alloc(length Int) -> Pointer {
-	return sys.mmap(0, length, 0x1|0x2, 0x02|0x20)
+	x := sys.mmap(0, length+8, 0x1|0x2, 0x02|0x20)
+
+	if x <= 0 {
+		return x
+	}
+
+	store(x, 8, length)
+	return x + 8
 }
\ No newline at end of file
diff --git a/lib/mem/alloc_mac.q b/lib/mem/alloc_mac.q
index bdeb356..42da24b 100644
--- a/lib/mem/alloc_mac.q
+++ b/lib/mem/alloc_mac.q
@@ -1,5 +1,12 @@
 import sys
 
 alloc(length Int) -> Pointer {
-	return sys.mmap(0, length, 0x1|0x2, 0x02|0x1000)
+	x := sys.mmap(0, length, 0x1|0x2, 0x02|0x1000)
+
+	if x <= 0 {
+		return x
+	}
+
+	store(x, 8, length)
+	return x + 8
 }
\ No newline at end of file
diff --git a/lib/mem/alloc_windows.q b/lib/mem/alloc_windows.q
index a6fafde..616fb84 100644
--- a/lib/mem/alloc_windows.q
+++ b/lib/mem/alloc_windows.q
@@ -1,5 +1,12 @@
 import sys
 
 alloc(length Int) -> Pointer {
-	return sys.mmap(0, length, 0x0004, 0x3000)
+	x := sys.mmap(0, length, 0x0004, 0x3000)
+
+	if x <= 0 {
+		return x
+	}
+
+	store(x, 8, length)
+	return x + 8
 }
\ No newline at end of file
diff --git a/lib/sys/io_windows.q b/lib/sys/io_windows.q
index 0bf7b09..a07a586 100644
--- a/lib/sys/io_windows.q
+++ b/lib/sys/io_windows.q
@@ -1,3 +1,9 @@
+read(fd Int, address Pointer, length Int) -> Int {
+	kernel32.ReadFile(fd, address, length)
+	return length
+}
+
 write(fd Int, address Pointer, length Int) -> Int {
-	return kernel32.WriteFile(fd, address, length)
+	kernel32.WriteFile(fd, address, length)
+	return length
 }
\ No newline at end of file
diff --git a/src/asm/Assembler.go b/src/asm/Assembler.go
index c5c9b8d..3f3cb88 100644
--- a/src/asm/Assembler.go
+++ b/src/asm/Assembler.go
@@ -24,5 +24,5 @@ func (a *Assembler) SetData(label string, bytes []byte) {
 		a.Data = data.Data{}
 	}
 
-	a.Data[label] = bytes
+	a.Data.Insert(label, bytes)
 }
diff --git a/src/asm/Memory.go b/src/asm/Memory.go
index 1c0cc4e..e97924e 100644
--- a/src/asm/Memory.go
+++ b/src/asm/Memory.go
@@ -4,7 +4,7 @@ import "git.akyoto.dev/cli/q/src/cpu"
 
 type Memory struct {
 	Base           cpu.Register
-	Offset         byte
+	Offset         int8
 	OffsetRegister cpu.Register
 	Length         byte
 }
diff --git a/src/asmc/resolvePointers.go b/src/asmc/resolvePointers.go
index 748af18..0dc879e 100644
--- a/src/asmc/resolvePointers.go
+++ b/src/asmc/resolvePointers.go
@@ -80,7 +80,7 @@ restart:
 	dataStart, _ := fs.Align(c.codeStart+Address(len(c.code)), config.Align)
 
 	for _, pointer := range c.dataPointers {
-		address := config.BaseAddress + dataStart + pointer.Resolve()
+		address := config.BaseAddress + dataStart + pointer.Resolve() + 8
 		slice := c.code[pointer.Position : pointer.Position+4]
 		binary.LittleEndian.PutUint32(slice, uint32(address))
 	}
diff --git a/src/core/CompileAssignArray.go b/src/core/CompileAssignArray.go
index 08f73e6..1cab08a 100644
--- a/src/core/CompileAssignArray.go
+++ b/src/core/CompileAssignArray.go
@@ -40,7 +40,7 @@ func (f *Function) CompileAssignArray(node *ast.Assign) error {
 			return err
 		}
 
-		memory.Offset = byte(offset)
+		memory.Offset = int8(offset)
 	} else {
 		_, indexRegister, isTemporary, err := f.Evaluate(index)
 
diff --git a/src/core/CompileAssignField.go b/src/core/CompileAssignField.go
index 852ae2d..412be15 100644
--- a/src/core/CompileAssignField.go
+++ b/src/core/CompileAssignField.go
@@ -32,7 +32,7 @@ func (f *Function) CompileAssignField(node *ast.Assign) error {
 
 	memory := asm.Memory{
 		Base:           variable.Register,
-		Offset:         byte(field.Offset),
+		Offset:         int8(field.Offset),
 		OffsetRegister: math.MaxUint8,
 		Length:         byte(field.Type.Size()),
 	}
diff --git a/src/core/CompileCall.go b/src/core/CompileCall.go
index 46b687c..69db515 100644
--- a/src/core/CompileCall.go
+++ b/src/core/CompileCall.go
@@ -27,6 +27,14 @@ func (f *Function) CompileCall(root *expression.Expression) (*Function, error) {
 		name = nameNode.Token.Text(f.File.Bytes)
 
 		switch name {
+		case "len":
+			return &Function{
+				Output: []*Output{
+					{
+						Type: types.Int,
+					},
+				},
+			}, f.CompileLen(root)
 		case "syscall":
 			return nil, f.CompileSyscall(root)
 		case "new":
diff --git a/src/core/CompileLen.go b/src/core/CompileLen.go
new file mode 100644
index 0000000..cb7cb44
--- /dev/null
+++ b/src/core/CompileLen.go
@@ -0,0 +1,26 @@
+package core
+
+import (
+	"math"
+
+	"git.akyoto.dev/cli/q/src/asm"
+	"git.akyoto.dev/cli/q/src/expression"
+)
+
+// CompileLen returns the length of a slice.
+func (f *Function) CompileLen(root *expression.Expression) error {
+	_, register, isTemporary, err := f.Evaluate(root.Children[1])
+
+	if err != nil {
+		return err
+	}
+
+	f.SaveRegister(f.CPU.Output[0])
+	f.MemoryRegister(asm.LOAD, asm.Memory{Base: register, Offset: -8, OffsetRegister: math.MaxUint8, Length: 8}, f.CPU.Output[0])
+
+	if isTemporary {
+		f.FreeRegister(register)
+	}
+
+	return nil
+}
diff --git a/src/core/ExpressionToRegister.go b/src/core/ExpressionToRegister.go
index 17f3feb..a72b381 100644
--- a/src/core/ExpressionToRegister.go
+++ b/src/core/ExpressionToRegister.go
@@ -40,7 +40,7 @@ func (f *Function) ExpressionToRegister(node *expression.Expression, register cp
 	if node.Token.Kind == token.Array {
 		array := f.VariableByName(node.Children[0].Token.Text(f.File.Bytes))
 		offset, err := f.Number(node.Children[1].Token)
-		f.MemoryRegister(asm.LOAD, asm.Memory{Base: array.Register, Offset: byte(offset), Length: 1}, register)
+		f.MemoryRegister(asm.LOAD, asm.Memory{Base: array.Register, Offset: int8(offset), Length: 1}, register)
 		return types.Int, err
 	}
 
@@ -51,7 +51,7 @@ func (f *Function) ExpressionToRegister(node *expression.Expression, register cp
 		right := node.Children[1]
 		name = right.Token.Text(f.File.Bytes)
 		field := variable.Type.(*types.Pointer).To.(*types.Struct).FieldByName(name)
-		f.MemoryRegister(asm.LOAD, asm.Memory{Base: variable.Register, Offset: byte(field.Offset), Length: byte(field.Type.Size())}, register)
+		f.MemoryRegister(asm.LOAD, asm.Memory{Base: variable.Register, Offset: int8(field.Offset), Length: byte(field.Type.Size())}, register)
 		return field.Type, nil
 	}
 
diff --git a/src/core/TokenToRegister.go b/src/core/TokenToRegister.go
index af65813..d2daab6 100644
--- a/src/core/TokenToRegister.go
+++ b/src/core/TokenToRegister.go
@@ -1,6 +1,8 @@
 package core
 
 import (
+	"encoding/binary"
+
 	"git.akyoto.dev/cli/q/src/asm"
 	"git.akyoto.dev/cli/q/src/cpu"
 	"git.akyoto.dev/cli/q/src/errors"
@@ -45,7 +47,12 @@ func (f *Function) TokenToRegister(t token.Token, register cpu.Register) (types.
 	case token.String:
 		data := t.Bytes(f.File.Bytes)
 		data = String(data)
-		label := f.AddBytes(data)
+
+		slice := make([]byte, len(data)+8)
+		binary.LittleEndian.PutUint64(slice, uint64(len(data)))
+		copy(slice[8:], data)
+
+		label := f.AddBytes(slice)
 		f.SaveRegister(register)
 		f.RegisterLabel(asm.MOVE, register, label)
 		return types.PointerAny, nil
diff --git a/src/register/SaveRegister.go b/src/register/SaveRegister.go
index c5a56c9..c0aa17b 100644
--- a/src/register/SaveRegister.go
+++ b/src/register/SaveRegister.go
@@ -26,4 +26,5 @@ func (f *Machine) SaveRegister(register cpu.Register) {
 	newRegister := f.NewRegister()
 	f.RegisterRegister(asm.MOVE, newRegister, register)
 	variable.Register = newRegister
+	f.FreeRegister(register)
 }
diff --git a/src/x86/Load.go b/src/x86/Load.go
index ae32daa..1a56236 100644
--- a/src/x86/Load.go
+++ b/src/x86/Load.go
@@ -3,6 +3,6 @@ package x86
 import "git.akyoto.dev/cli/q/src/cpu"
 
 // LoadRegister loads from memory into a register.
-func LoadRegister(code []byte, destination cpu.Register, offset byte, length byte, source cpu.Register) []byte {
+func LoadRegister(code []byte, destination cpu.Register, offset int8, length byte, source cpu.Register) []byte {
 	return memoryAccess(code, 0x8A, 0x8B, source, offset, length, destination)
 }
diff --git a/src/x86/Load_test.go b/src/x86/Load_test.go
index 2633e09..32d4405 100644
--- a/src/x86/Load_test.go
+++ b/src/x86/Load_test.go
@@ -12,7 +12,7 @@ func TestLoadRegister(t *testing.T) {
 	usagePatterns := []struct {
 		Destination cpu.Register
 		Source      cpu.Register
-		Offset      byte
+		Offset      int8
 		Length      byte
 		Code        []byte
 	}{
diff --git a/src/x86/Store.go b/src/x86/Store.go
index 0bf06e0..c0fc055 100644
--- a/src/x86/Store.go
+++ b/src/x86/Store.go
@@ -7,7 +7,7 @@ import (
 )
 
 // StoreNumber stores a number into the memory address included in the given register.
-func StoreNumber(code []byte, register cpu.Register, offset byte, length byte, number int) []byte {
+func StoreNumber(code []byte, register cpu.Register, offset int8, length byte, number int) []byte {
 	code = memoryAccess(code, 0xC6, 0xC7, register, offset, length, 0b000)
 
 	switch length {
@@ -22,6 +22,6 @@ func StoreNumber(code []byte, register cpu.Register, offset byte, length byte, n
 }
 
 // StoreRegister stores the contents of the `source` register into the memory address included in the given register.
-func StoreRegister(code []byte, register cpu.Register, offset byte, length byte, source cpu.Register) []byte {
+func StoreRegister(code []byte, register cpu.Register, offset int8, length byte, source cpu.Register) []byte {
 	return memoryAccess(code, 0x88, 0x89, register, offset, length, source)
 }
diff --git a/src/x86/Store_test.go b/src/x86/Store_test.go
index e9e1e26..7d34503 100644
--- a/src/x86/Store_test.go
+++ b/src/x86/Store_test.go
@@ -11,7 +11,7 @@ import (
 func TestStoreNumber(t *testing.T) {
 	usagePatterns := []struct {
 		Register cpu.Register
-		Offset   byte
+		Offset   int8
 		Length   byte
 		Number   int
 		Code     []byte
@@ -159,7 +159,7 @@ func TestStoreNumber(t *testing.T) {
 func TestStoreRegister(t *testing.T) {
 	usagePatterns := []struct {
 		RegisterTo   cpu.Register
-		Offset       byte
+		Offset       int8
 		Length       byte
 		RegisterFrom cpu.Register
 		Code         []byte
diff --git a/src/x86/memoryAccess.go b/src/x86/memoryAccess.go
index b1dd605..af526a0 100644
--- a/src/x86/memoryAccess.go
+++ b/src/x86/memoryAccess.go
@@ -3,7 +3,7 @@ package x86
 import "git.akyoto.dev/cli/q/src/cpu"
 
 // memoryAccess encodes a memory access.
-func memoryAccess(code []byte, opCode8 byte, opCode32 byte, register cpu.Register, offset byte, numBytes byte, source cpu.Register) []byte {
+func memoryAccess(code []byte, opCode8 byte, opCode32 byte, register cpu.Register, offset int8, numBytes byte, source cpu.Register) []byte {
 	opCode := opCode32
 
 	if numBytes == 1 {
@@ -27,7 +27,7 @@ func memoryAccess(code []byte, opCode8 byte, opCode32 byte, register cpu.Registe
 	}
 
 	if mod == AddressMemoryOffset8 {
-		code = append(code, offset)
+		code = append(code, byte(offset))
 	}
 
 	return code