diff --git a/README.md b/README.md
index 3f36e89..e5e860f 100644
--- a/README.md
+++ b/README.md
@@ -51,7 +51,13 @@ q build examples/hello
 q build examples/hello --dry
 ```
 
-To produce verbose output, add the `-v` flag which shows the generated assembly instructions:
+Adding the `-a` or `--assembler` flag shows the generated assembly instructions:
+
+```shell
+q build examples/hello -a
+```
+
+Adding the `-v` or `--verbose` flag shows verbose compiler information:
 
 ```shell
 q build examples/hello -v
diff --git a/src/build/Call.go b/src/build/Call.go
new file mode 100644
index 0000000..2051499
--- /dev/null
+++ b/src/build/Call.go
@@ -0,0 +1,44 @@
+package build
+
+import (
+	"git.akyoto.dev/cli/q/src/build/config"
+	"git.akyoto.dev/cli/q/src/build/expression"
+)
+
+// CompileFunctionCall executes a function call.
+func (f *Function) CompileFunctionCall(expr *expression.Expression) error {
+	funcName := expr.Children[0].Token.Text()
+
+	if funcName == "syscall" {
+		return f.CompileSyscall(expr)
+	}
+
+	function := f.functions[funcName]
+
+	if function != f {
+		function.Wait()
+	}
+
+	parameters := expr.Children[1:]
+	registers := f.cpu.Call[:len(parameters)]
+
+	err := f.ExpressionsToRegisters(parameters, registers)
+
+	if config.Verbose {
+		f.Logf("call: %s", funcName)
+	}
+
+	f.assembler.Call(funcName)
+	f.sideEffects += function.sideEffects
+	return err
+}
+
+// CompileSyscall executes a syscall.
+func (f *Function) CompileSyscall(expr *expression.Expression) error {
+	parameters := expr.Children[1:]
+	registers := f.cpu.Syscall[:len(parameters)]
+	err := f.ExpressionsToRegisters(parameters, registers)
+	f.assembler.Syscall()
+	f.sideEffects++
+	return err
+}
diff --git a/src/build/Execute.go b/src/build/Execute.go
index 8947218..bd50435 100644
--- a/src/build/Execute.go
+++ b/src/build/Execute.go
@@ -166,6 +166,10 @@ func (f *Function) ExpressionToRegister(root *expression.Expression, register cp
 
 // ExpressionsToRegisters moves multiple expressions into the specified registers.
 func (f *Function) ExpressionsToRegisters(expressions []*expression.Expression, registers []cpu.Register) error {
+	for _, register := range registers {
+		f.SaveRegister(register)
+	}
+
 	for i := len(expressions) - 1; i >= 0; i-- {
 		expression := expressions[i]
 		register := registers[i]
diff --git a/src/build/Function.go b/src/build/Function.go
index 8c271e0..8853605 100644
--- a/src/build/Function.go
+++ b/src/build/Function.go
@@ -20,6 +20,7 @@ type Function struct {
 
 // Compile turns a function into machine code.
 func (f *Function) Compile() {
+	defer close(f.finished)
 	f.assembler.Label(f.Name)
 	f.err = f.CompileTokens(f.Body)
 	f.assembler.Return()
@@ -124,6 +125,11 @@ func (f *Function) String() string {
 	return f.Name
 }
 
+// Wait will block until the compilation finishes.
+func (f *Function) Wait() {
+	<-f.finished
+}
+
 // identifierExists returns true if the identifier has been defined.
 func (f *Function) identifierExists(name string) bool {
 	_, exists := f.variables[name]
diff --git a/src/build/FunctionCall.go b/src/build/FunctionCall.go
deleted file mode 100644
index 8b62e25..0000000
--- a/src/build/FunctionCall.go
+++ /dev/null
@@ -1,43 +0,0 @@
-package build
-
-import (
-	"git.akyoto.dev/cli/q/src/build/config"
-	"git.akyoto.dev/cli/q/src/build/cpu"
-	"git.akyoto.dev/cli/q/src/build/expression"
-)
-
-// CompileFunctionCall executes a function call.
-func (f *Function) CompileFunctionCall(expr *expression.Expression) error {
-	var (
-		funcName   = expr.Children[0].Token.Text()
-		parameters = expr.Children[1:]
-		isSyscall  = funcName == "syscall"
-		registers  []cpu.Register
-	)
-
-	if isSyscall {
-		registers = f.cpu.Syscall
-	} else {
-		registers = f.cpu.Call
-	}
-
-	registers = registers[:len(parameters)]
-
-	for _, register := range registers {
-		f.SaveRegister(register)
-	}
-
-	err := f.ExpressionsToRegisters(parameters, registers)
-
-	if config.Verbose {
-		f.Logf("call: %s", funcName)
-	}
-
-	if isSyscall {
-		f.assembler.Syscall()
-	} else {
-		f.assembler.Call(funcName)
-	}
-
-	return err
-}
diff --git a/src/build/compile.go b/src/build/compile.go
index 2e1f4b3..0430873 100644
--- a/src/build/compile.go
+++ b/src/build/compile.go
@@ -27,6 +27,7 @@ func compile(functions <-chan *Function, errs <-chan error) (Result, error) {
 				continue
 			}
 
+			function.functions = allFunctions
 			allFunctions[function.Name] = function
 		}
 	}
diff --git a/src/build/compiler.go b/src/build/compiler.go
index 3e727be..19b75a2 100644
--- a/src/build/compiler.go
+++ b/src/build/compiler.go
@@ -11,12 +11,15 @@ import (
 
 // compiler is the data structure we embed in each function to preserve compilation state.
 type compiler struct {
-	assembler asm.Assembler
-	count     counter
-	cpu       cpu.CPU
-	debug     []debug
-	err       error
-	variables map[string]*Variable
+	assembler   asm.Assembler
+	count       counter
+	cpu         cpu.CPU
+	debug       []debug
+	err         error
+	variables   map[string]*Variable
+	functions   map[string]*Function
+	sideEffects int
+	finished    chan struct{}
 }
 
 // counter stores how often a certain statement appeared so we can generate a unique label from it.
diff --git a/src/build/scan.go b/src/build/scan.go
index 695a401..701a824 100644
--- a/src/build/scan.go
+++ b/src/build/scan.go
@@ -251,6 +251,7 @@ func scanFile(path string, functions chan<- *Function) error {
 					Return:  x64.ReturnValueRegisters,
 				},
 				variables: map[string]*Variable{},
+				finished:  make(chan struct{}),
 			},
 		}