diff --git a/src/arm/Call.go b/src/arm/Call.go
index d311431..98765e9 100644
--- a/src/arm/Call.go
+++ b/src/arm/Call.go
@@ -1,8 +1,15 @@
 package arm
 
+import "git.urbach.dev/cli/q/src/cpu"
+
 // Call branches to a PC-relative offset, setting the register X30 to PC+4.
 // The offset starts from the address of this instruction and is encoded as "imm26" times 4.
 // This instruction is also known as BL (branch with link).
 func Call(offset int) uint32 {
 	return uint32(0b100101<<26) | uint32(offset&mask26)
+}
+
+// Calls a function whose address is stored in the given register.
+func CallRegister(register cpu.Register) uint32 {
+	return uint32(0b1101011000111111<<16) | uint32(register)<<5
 }
\ No newline at end of file
diff --git a/src/arm/Call_test.go b/src/arm/Call_test.go
index 206f90e..7ab06c5 100644
--- a/src/arm/Call_test.go
+++ b/src/arm/Call_test.go
@@ -4,6 +4,7 @@ import (
 	"testing"
 
 	"git.urbach.dev/cli/q/src/arm"
+	"git.urbach.dev/cli/q/src/cpu"
 	"git.urbach.dev/go/assert"
 )
 
@@ -22,4 +23,21 @@ func TestCall(t *testing.T) {
 		code := arm.Call(pattern.Offset)
 		assert.Equal(t, code, pattern.Code)
 	}
+}
+
+func TestCallRegister(t *testing.T) {
+	usagePatterns := []struct {
+		Register cpu.Register
+		Code     uint32
+	}{
+		{arm.X0, 0xD63F0000},
+		{arm.X1, 0xD63F0020},
+		{arm.X2, 0xD63F0040},
+	}
+
+	for _, pattern := range usagePatterns {
+		t.Logf("blr %d", pattern.Register)
+		code := arm.CallRegister(pattern.Register)
+		assert.Equal(t, code, pattern.Code)
+	}
 }
\ No newline at end of file
diff --git a/src/asm/compilerARM.go b/src/asm/compilerARM.go
index 297c06c..d743afd 100644
--- a/src/asm/compilerARM.go
+++ b/src/asm/compilerARM.go
@@ -2,6 +2,7 @@ package asm
 
 import (
 	"encoding/binary"
+	"fmt"
 
 	"git.urbach.dev/cli/q/src/arm"
 )
@@ -33,11 +34,24 @@ func (c *compilerARM) Compile(instr Instruction) {
 			binary.LittleEndian.PutUint32(c.code[start:start+4], arm.Call(offset))
 		})
 	case *CallExtern:
-		panic("not implemented")
+		start := len(c.code)
+		c.append(arm.LoadAddress(arm.X0, 0))
+		c.append(arm.LoadRegister(arm.X0, arm.X0, 0, 8))
+		c.append(arm.CallRegister(arm.X0))
+
+		c.Defer(func() {
+			index := c.libraries.Index(instr.Library, instr.Function)
+
+			if index == -1 {
+				panic(fmt.Sprintf("unknown extern function '%s' in library '%s'", instr.Function, instr.Library))
+			}
+
+			address := c.importsStart + index*8
+			offset := address - start
+			binary.LittleEndian.PutUint32(c.code[start:start+4], arm.LoadAddress(arm.X0, offset))
+		})
 	case *CallExternStart:
-		panic("not implemented")
 	case *CallExternEnd:
-		panic("not implemented")
 	case *Jump:
 		start := len(c.code)
 		c.append(arm.Jump(0))