diff --git a/lib/sys/linux.q b/lib/sys/linux.q
index e6d84c2..e70ffee 100644
--- a/lib/sys/linux.q
+++ b/lib/sys/linux.q
@@ -14,7 +14,7 @@ close(fd Int) -> Int {
 	return syscall(3, fd)
 }
 
-mmap(address Int, length Int, protection Int, flags Int) -> Int {
+mmap(address Int, length Int, protection Int, flags Int) -> Pointer {
 	return syscall(9, address, length, protection, flags)
 }
 
diff --git a/src/build/arch/x64/Registers.go b/src/build/arch/x64/Registers.go
index 2d9bd1c..e8ecbc8 100644
--- a/src/build/arch/x64/Registers.go
+++ b/src/build/arch/x64/Registers.go
@@ -22,9 +22,10 @@ const (
 )
 
 var (
-	AllRegisters         = []cpu.Register{RAX, RCX, RDX, RBX, RSP, RBP, RSI, RDI, R8, R9, R10, R11, R12, R13, R14, R15}
-	SyscallRegisters     = []cpu.Register{RAX, RDI, RSI, RDX, R10, R8, R9}
-	GeneralRegisters     = []cpu.Register{RCX, RBX, RBP, R11, R12, R13, R14, R15}
-	CallRegisters        = SyscallRegisters
-	ReturnValueRegisters = SyscallRegisters
+	AllRegisters           = []cpu.Register{RAX, RCX, RDX, RBX, RSP, RBP, RSI, RDI, R8, R9, R10, R11, R12, R13, R14, R15}
+	SyscallInputRegisters  = []cpu.Register{RAX, RDI, RSI, RDX, R10, R8, R9}
+	SyscallOutputRegisters = []cpu.Register{RAX, RCX, R11}
+	GeneralRegisters       = []cpu.Register{RCX, RBX, RBP, R11, R12, R13, R14, R15}
+	InputRegisters         = SyscallInputRegisters
+	OutputRegisters        = SyscallInputRegisters
 )
diff --git a/src/build/compiler/Result.go b/src/build/compiler/Result.go
index db1f814..d9e699a 100644
--- a/src/build/compiler/Result.go
+++ b/src/build/compiler/Result.go
@@ -32,13 +32,13 @@ func (r *Result) finalize() ([]byte, []byte) {
 	}
 
 	final.Call("main.main")
-	final.RegisterNumber(asm.MOVE, x64.SyscallRegisters[0], linux.Exit)
-	final.RegisterNumber(asm.MOVE, x64.SyscallRegisters[1], 0)
+	final.RegisterNumber(asm.MOVE, x64.SyscallInputRegisters[0], linux.Exit)
+	final.RegisterNumber(asm.MOVE, x64.SyscallInputRegisters[1], 0)
 	final.Syscall()
 
 	final.Label(asm.LABEL, "_crash")
-	final.RegisterNumber(asm.MOVE, x64.SyscallRegisters[0], linux.Exit)
-	final.RegisterNumber(asm.MOVE, x64.SyscallRegisters[1], 1)
+	final.RegisterNumber(asm.MOVE, x64.SyscallInputRegisters[0], linux.Exit)
+	final.RegisterNumber(asm.MOVE, x64.SyscallInputRegisters[1], 1)
 	final.Syscall()
 
 	// This will place the main function immediately after the entry point
diff --git a/src/build/core/CompileCall.go b/src/build/core/CompileCall.go
index 25466db..16c14f2 100644
--- a/src/build/core/CompileCall.go
+++ b/src/build/core/CompileCall.go
@@ -15,80 +15,80 @@ import (
 func (f *Function) CompileCall(root *expression.Expression) (*Function, error) {
 	var (
 		pkg      = f.Package
-		nameRoot = root.Children[0]
+		nameNode = root.Children[0]
 		fn       *Function
 		name     string
 		fullName string
 		exists   bool
 	)
 
-	if nameRoot.IsLeaf() {
-		name = nameRoot.Token.Text(f.File.Bytes)
+	if nameNode.IsLeaf() {
+		name = nameNode.Token.Text(f.File.Bytes)
+
+		if name == "syscall" {
+			return nil, f.CompileSyscall(root)
+		}
 	} else {
-		pkg = nameRoot.Children[0].Token.Text(f.File.Bytes)
-		name = nameRoot.Children[1].Token.Text(f.File.Bytes)
+		pkg = nameNode.Children[0].Token.Text(f.File.Bytes)
+		name = nameNode.Children[1].Token.Text(f.File.Bytes)
 	}
 
-	isSyscall := name == "syscall"
-
-	if !isSyscall {
-		if pkg != f.File.Package {
-			if f.File.Imports == nil {
-				return nil, errors.New(&errors.UnknownPackage{Name: pkg}, f.File, nameRoot.Token.Position)
-			}
-
-			imp, exists := f.File.Imports[pkg]
-
-			if !exists {
-				return nil, errors.New(&errors.UnknownPackage{Name: pkg}, f.File, nameRoot.Token.Position)
-			}
-
-			imp.Used = true
+	if pkg != f.File.Package {
+		if f.File.Imports == nil {
+			return nil, errors.New(&errors.UnknownPackage{Name: pkg}, f.File, nameNode.Token.Position)
 		}
 
-		tmp := strings.Builder{}
-		tmp.WriteString(pkg)
-		tmp.WriteString(".")
-		tmp.WriteString(name)
-		fullName = tmp.String()
-		fn, exists = f.Functions[fullName]
+		imp, exists := f.File.Imports[pkg]
 
 		if !exists {
-			return nil, errors.New(&errors.UnknownFunction{Name: name}, f.File, nameRoot.Token.Position)
+			return nil, errors.New(&errors.UnknownPackage{Name: pkg}, f.File, nameNode.Token.Position)
 		}
+
+		imp.Used = true
+	}
+
+	tmp := strings.Builder{}
+	tmp.WriteString(pkg)
+	tmp.WriteString(".")
+	tmp.WriteString(name)
+	fullName = tmp.String()
+	fn, exists = f.Functions[fullName]
+
+	if !exists {
+		return nil, errors.New(&errors.UnknownFunction{Name: name}, f.File, nameNode.Token.Position)
 	}
 
 	parameters := root.Children[1:]
 	registers := f.CPU.Input[:len(parameters)]
 
-	if isSyscall {
-		registers = f.CPU.Syscall[:len(parameters)]
+	for i := len(parameters) - 1; i >= 0; i-- {
+		typ, err := f.ExpressionToRegister(parameters[i], registers[i])
+
+		if err != nil {
+			return nil, err
+		}
+
+		if typ != fn.Parameters[i].Type {
+			return nil, errors.New(&errors.TypeMismatch{
+				Encountered:   string(typ),
+				Expected:      string(fn.Parameters[i].Type),
+				ParameterName: fn.Parameters[i].Name,
+			}, f.File, parameters[i].Token.Position)
+		}
 	}
 
-	err := f.ExpressionsToRegisters(parameters, registers)
-
-	if err != nil {
-		return fn, err
+	for _, register := range f.CPU.Output[:len(fn.ReturnTypes)] {
+		f.SaveRegister(register)
 	}
 
-	// TODO: Save all return value registers of the function
-	f.SaveRegister(f.CPU.Output[0])
-
-	// Push
 	for _, register := range f.CPU.General {
 		if f.RegisterIsUsed(register) {
 			f.Register(asm.PUSH, register)
 		}
 	}
 
-	// Call
-	if isSyscall {
-		f.Syscall()
-	} else {
-		f.Call(fullName)
-	}
+	f.Call(fullName)
 
-	// Free parameter registers
 	for _, register := range registers {
 		if register == f.CPU.Output[0] && root.Parent != nil {
 			continue
@@ -97,7 +97,6 @@ func (f *Function) CompileCall(root *expression.Expression) (*Function, error) {
 		f.FreeRegister(register)
 	}
 
-	// Pop
 	for i := len(f.CPU.General) - 1; i >= 0; i-- {
 		register := f.CPU.General[i]
 
diff --git a/src/build/core/CompileDefinition.go b/src/build/core/CompileDefinition.go
index aeed43d..81ea095 100644
--- a/src/build/core/CompileDefinition.go
+++ b/src/build/core/CompileDefinition.go
@@ -41,7 +41,7 @@ func (f *Function) CompileDefinition(node *ast.Define) error {
 	}
 
 	count := 0
-	_, err := f.CompileCall(right)
+	called, err := f.CompileCall(right)
 
 	if err != nil {
 		return err
@@ -54,6 +54,10 @@ func (f *Function) CompileDefinition(node *ast.Define) error {
 			return err
 		}
 
+		if called != nil {
+			variable.Type = called.ReturnTypes[count]
+		}
+
 		f.RegisterRegister(asm.MOVE, variable.Register, f.CPU.Output[count])
 		f.AddVariable(variable)
 		count++
diff --git a/src/build/core/CompileReturn.go b/src/build/core/CompileReturn.go
index ca59d56..e15bc22 100644
--- a/src/build/core/CompileReturn.go
+++ b/src/build/core/CompileReturn.go
@@ -2,6 +2,8 @@ package core
 
 import (
 	"git.akyoto.dev/cli/q/src/build/ast"
+	"git.akyoto.dev/cli/q/src/build/errors"
+	"git.akyoto.dev/cli/q/src/build/types"
 )
 
 // CompileReturn compiles a return instruction.
@@ -12,5 +14,22 @@ func (f *Function) CompileReturn(node *ast.Return) error {
 		return nil
 	}
 
-	return f.ExpressionsToRegisters(node.Values, f.CPU.Output)
+	for i := len(node.Values) - 1; i >= 0; i-- {
+		typ, err := f.ExpressionToRegister(node.Values[i], f.CPU.Output[i])
+
+		if err != nil {
+			return err
+		}
+
+		if typ != types.Any && typ != f.ReturnTypes[i] {
+			return errors.New(&errors.TypeMismatch{
+				Encountered:   string(typ),
+				Expected:      string(f.ReturnTypes[i]),
+				ParameterName: "",
+				IsReturn:      true,
+			}, f.File, node.Values[i].Token.Position)
+		}
+	}
+
+	return nil
 }
diff --git a/src/build/core/CompileSyscall.go b/src/build/core/CompileSyscall.go
new file mode 100644
index 0000000..63b3a23
--- /dev/null
+++ b/src/build/core/CompileSyscall.go
@@ -0,0 +1,32 @@
+package core
+
+import (
+	"git.akyoto.dev/cli/q/src/build/expression"
+)
+
+// CompileSyscall executes a kernel syscall.
+func (f *Function) CompileSyscall(root *expression.Expression) error {
+	parameters := root.Children[1:]
+	registers := f.CPU.SyscallInput[:len(parameters)]
+	err := f.ExpressionsToRegisters(parameters, registers)
+
+	if err != nil {
+		return err
+	}
+
+	for _, register := range f.CPU.SyscallOutput {
+		f.SaveRegister(register)
+	}
+
+	f.Syscall()
+
+	for _, register := range registers {
+		if register == f.CPU.SyscallOutput[0] && root.Parent != nil {
+			continue
+		}
+
+		f.FreeRegister(register)
+	}
+
+	return nil
+}
diff --git a/src/build/core/ExpressionToRegister.go b/src/build/core/ExpressionToRegister.go
index c084351..7d7d834 100644
--- a/src/build/core/ExpressionToRegister.go
+++ b/src/build/core/ExpressionToRegister.go
@@ -31,7 +31,7 @@ func (f *Function) ExpressionToRegister(node *expression.Expression, register cp
 		}
 
 		if fn == nil || len(fn.ReturnTypes) == 0 {
-			return types.Invalid, err
+			return types.Any, err
 		}
 
 		return fn.ReturnTypes[0], err
@@ -72,6 +72,10 @@ func (f *Function) ExpressionToRegister(node *expression.Expression, register cp
 		return types.Invalid, err
 	}
 
+	if typ == types.Pointer && (node.Token.Kind == token.Add || node.Token.Kind == token.Sub) && right.Token.Kind == token.Identifier && f.VariableByName(right.Token.Text(f.File.Bytes)).Type == types.Pointer {
+		typ = types.Int
+	}
+
 	err = f.Execute(node.Token, register, right)
 
 	if register != final {
diff --git a/src/build/core/NewFunction.go b/src/build/core/NewFunction.go
index 2772db2..736bf56 100644
--- a/src/build/core/NewFunction.go
+++ b/src/build/core/NewFunction.go
@@ -26,11 +26,12 @@ func NewFunction(pkg string, name string, file *fs.File, body []token.Token) *Fu
 				Scopes: []*scope.Scope{{}},
 			},
 			CPU: cpu.CPU{
-				All:     x64.AllRegisters,
-				Input:   x64.CallRegisters,
-				General: x64.GeneralRegisters,
-				Syscall: x64.SyscallRegisters,
-				Output:  x64.ReturnValueRegisters,
+				All:           x64.AllRegisters,
+				General:       x64.GeneralRegisters,
+				Input:         x64.InputRegisters,
+				Output:        x64.OutputRegisters,
+				SyscallInput:  x64.SyscallInputRegisters,
+				SyscallOutput: x64.SyscallOutputRegisters,
 			},
 		},
 	}
diff --git a/src/build/core/TokenToRegister.go b/src/build/core/TokenToRegister.go
index c1d38ff..1d3df22 100644
--- a/src/build/core/TokenToRegister.go
+++ b/src/build/core/TokenToRegister.go
@@ -42,7 +42,7 @@ func (f *Function) TokenToRegister(t token.Token, register cpu.Register) (types.
 		label := f.AddBytes(data)
 		f.SaveRegister(register)
 		f.RegisterLabel(asm.MOVE, register, label)
-		return types.Int, nil
+		return types.Pointer, nil
 
 	default:
 		return types.Invalid, errors.New(errors.InvalidExpression, f.File, t.Position)
diff --git a/src/build/cpu/CPU.go b/src/build/cpu/CPU.go
index 9edbc06..4fe0571 100644
--- a/src/build/cpu/CPU.go
+++ b/src/build/cpu/CPU.go
@@ -2,9 +2,10 @@ package cpu
 
 // CPU represents the processor.
 type CPU struct {
-	All     []Register
-	General []Register
-	Syscall []Register
-	Input   []Register
-	Output  []Register
+	All           []Register
+	General       []Register
+	Input         []Register
+	Output        []Register
+	SyscallInput  []Register
+	SyscallOutput []Register
 }
diff --git a/src/build/errors/TypeMismatch.go b/src/build/errors/TypeMismatch.go
new file mode 100644
index 0000000..cb8aae4
--- /dev/null
+++ b/src/build/errors/TypeMismatch.go
@@ -0,0 +1,26 @@
+package errors
+
+import "fmt"
+
+// TypeMismatch represents an error where a type requirement was not met.
+type TypeMismatch struct {
+	Encountered   string
+	Expected      string
+	ParameterName string
+	IsReturn      bool
+}
+
+// Error generates the string representation.
+func (err *TypeMismatch) Error() string {
+	subject := "type"
+
+	if err.IsReturn {
+		subject = "return type"
+	}
+
+	if err.ParameterName != "" {
+		return fmt.Sprintf("Expected parameter '%s' of %s '%s' (encountered '%s')", err.ParameterName, subject, err.Expected, err.Encountered)
+	}
+
+	return fmt.Sprintf("Expected %s '%s' instead of '%s'", subject, err.Expected, err.Encountered)
+}
diff --git a/src/build/scanner/scanFile.go b/src/build/scanner/scanFile.go
index b7174b0..714ba72 100644
--- a/src/build/scanner/scanFile.go
+++ b/src/build/scanner/scanFile.go
@@ -229,7 +229,12 @@ func (s *Scanner) scanFile(path string, pkg string) error {
 		function := core.NewFunction(pkg, name, file, body)
 
 		if typeStart != -1 {
-			function.ReturnTypes = append(function.ReturnTypes, types.New(tokens[typeStart:typeEnd]))
+			if tokens[typeStart].Kind == token.GroupStart && tokens[typeEnd-1].Kind == token.GroupEnd {
+				typeStart++
+				typeEnd--
+			}
+
+			function.ReturnTypes = types.NewList(tokens[typeStart:typeEnd], contents)
 		}
 
 		parameters := tokens[paramsStart:paramsEnd]
@@ -241,8 +246,8 @@ func (s *Scanner) scanFile(path string, pkg string) error {
 			}
 
 			name := tokens[0].Text(contents)
-			dataType := types.New(tokens[1:])
-			register := x64.CallRegisters[count]
+			dataType := types.New(tokens[1:].Text(contents))
+			register := x64.InputRegisters[count]
 			uses := token.Count(function.Body, contents, token.Identifier, name)
 
 			if uses == 0 {
diff --git a/src/build/scope/Stack.go b/src/build/scope/Stack.go
index 94fec99..750bc4c 100644
--- a/src/build/scope/Stack.go
+++ b/src/build/scope/Stack.go
@@ -47,6 +47,7 @@ func (stack *Stack) PushScope(body ast.AST, buffer []byte) *Scope {
 				Name:     v.Name,
 				Register: v.Register,
 				Alive:    count,
+				Type:     v.Type,
 			})
 		}
 	}
diff --git a/src/build/types/New.go b/src/build/types/New.go
index 73e7d23..ae20817 100644
--- a/src/build/types/New.go
+++ b/src/build/types/New.go
@@ -1,8 +1,6 @@
 package types
 
-import "git.akyoto.dev/cli/q/src/build/token"
-
 // New creates a new type from a list of tokens.
-func New(tokens token.List) Type {
-	return Int
+func New(name string) Type {
+	return Type(name)
 }
diff --git a/src/build/types/NewList.go b/src/build/types/NewList.go
new file mode 100644
index 0000000..91bb4ca
--- /dev/null
+++ b/src/build/types/NewList.go
@@ -0,0 +1,19 @@
+package types
+
+import (
+	"git.akyoto.dev/cli/q/src/build/expression"
+	"git.akyoto.dev/cli/q/src/build/token"
+)
+
+// NewList generates a list of types from comma separated tokens.
+func NewList(tokens token.List, source []byte) []Type {
+	var list []Type
+
+	expression.EachParameter(tokens, func(parameter token.List) error {
+		typ := New(parameter.Text(source))
+		list = append(list, typ)
+		return nil
+	})
+
+	return list
+}
diff --git a/src/build/types/Type.go b/src/build/types/Type.go
index 07340a0..76c04a9 100644
--- a/src/build/types/Type.go
+++ b/src/build/types/Type.go
@@ -1,8 +1,10 @@
 package types
 
-type Type int
+type Type string
 
 const (
-	Invalid Type = iota // Invalid is an invalid type.
-	Int
+	Invalid = ""
+	Any     = "Any"
+	Int     = "Int"
+	Pointer = "Pointer"
 )
diff --git a/tests/errors/TypeMismatch.q b/tests/errors/TypeMismatch.q
new file mode 100644
index 0000000..d86c306
--- /dev/null
+++ b/tests/errors/TypeMismatch.q
@@ -0,0 +1,7 @@
+main() {
+	writeToMemory(0)
+}
+
+writeToMemory(p Pointer) {
+	p[0] = 'A'
+}
\ No newline at end of file
diff --git a/tests/errors_test.go b/tests/errors_test.go
index 2ec440e..61eab3d 100644
--- a/tests/errors_test.go
+++ b/tests/errors_test.go
@@ -36,6 +36,7 @@ var errs = []struct {
 	{"MissingOperand.q", errors.MissingOperand},
 	{"MissingOperand2.q", errors.MissingOperand},
 	{"VariableAlreadyExists.q", &errors.VariableAlreadyExists{Name: "x"}},
+	{"TypeMismatch.q", &errors.TypeMismatch{Expected: "Pointer", Encountered: "Int", ParameterName: "p"}},
 	{"UnknownFunction.q", &errors.UnknownFunction{Name: "unknown"}},
 	{"UnknownFunction2.q", &errors.UnknownFunction{Name: "f"}},
 	{"UnknownIdentifier.q", &errors.UnknownIdentifier{Name: "x"}},