diff --git a/src/BUILD.gn b/src/BUILD.gn
index cad0a06..4a730b4 100644
--- a/src/BUILD.gn
+++ b/src/BUILD.gn
@@ -15,20 +15,41 @@
 # Hypervisor specific code.
 source_set("src") {
   sources = [
-    "api.c",
     "cpio.c",
-    "cpu.c",
     "load.c",
     "main.c",
+  ]
+
+  deps = [
+    ":common",
+    ":fdt",
+    ":memiter",
+    ":src_testable",
+  ]
+
+  if (is_debug) {
+    deps += [ ":dlog" ]
+  }
+}
+
+# One day, this will contain all the hypervisor's source but only once it can
+# all be built against the fake arch for unit tests. Utilities that are shared
+# e.g. with VM used in the VM tests have their own targets to facilitate
+# sharing.
+source_set("src_testable") {
+  sources = [
+    "alloc.c",
+    "api.c",
+    "cpu.c",
+    "fdt_handler.c",
+    "mm.c",
     "vm.c",
   ]
 
   deps = [
     ":common",
     ":fdt",
-    ":fdt_handler",
     ":memiter",
-    ":mm",
   ]
 
   if (is_debug) {
@@ -55,6 +76,7 @@
   ]
 }
 
+# Flattened Device Tree (FDT) utilities.
 source_set("fdt") {
   sources = [
     "fdt.c",
@@ -69,44 +91,17 @@
   }
 }
 
-source_set("fdt_handler") {
-  sources = [
-    "fdt_handler.c",
-  ]
-
-  deps = [
-    ":common",
-    ":fdt",
-  ]
-
-  if (is_debug) {
-    deps += [ ":dlog" ]
-  }
-}
-
 source_set("memiter") {
   sources = [
     "memiter.c",
   ]
 }
 
-source_set("mm") {
-  sources = [
-    "alloc.c",
-    "mm.c",
-  ]
-
-  deps = []
-
-  if (is_debug) {
-    deps += [ ":dlog" ]
-  }
-}
-
 executable("unit_tests") {
   testonly = true
   sources = [
     "abi_test.cc",
+    "api_test.cc",
     "fdt_handler_test.cc",
     "fdt_test.cc",
     "mm_test.cc",
@@ -132,9 +127,7 @@
     "-defsym=bin_end=600",
   ]
   deps = [
-    ":fdt",
-    ":fdt_handler",
-    ":mm",
+    ":src_testable",
     "//third_party:gtest_main",
   ]
 }
diff --git a/src/api_test.cc b/src/api_test.cc
new file mode 100644
index 0000000..fb6b8cb
--- /dev/null
+++ b/src/api_test.cc
@@ -0,0 +1,32 @@
+/*
+ * Copyright 2018 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+extern "C" {
+#include "hf/api.h"
+}
+
+#include <gmock/gmock.h>
+
+namespace
+{
+using ::testing::Eq;
+
+TEST(api, vm_get_count)
+{
+	EXPECT_THAT(api_vm_get_count(), Eq(0));
+}
+
+} /* namespace */
diff --git a/src/arch/aarch64/inc/hf/arch/cpu.h b/src/arch/aarch64/inc/hf/arch/cpu.h
index d2c1a12..28061a6 100644
--- a/src/arch/aarch64/inc/hf/arch/cpu.h
+++ b/src/arch/aarch64/inc/hf/arch/cpu.h
@@ -116,6 +116,11 @@
 	r->r[0] = arg;
 }
 
+static inline void arch_regs_set_vcpu_index(struct arch_regs *r, uint32_t index)
+{
+	r->lazy.vmpidr_el2 = index;
+}
+
 static inline void arch_regs_set_retval(struct arch_regs *r, size_t v)
 {
 	r->r[0] = v;
diff --git a/src/arch/fake/inc/hf/arch/cpu.h b/src/arch/fake/inc/hf/arch/cpu.h
index 501c303..421a7f7 100644
--- a/src/arch/fake/inc/hf/arch/cpu.h
+++ b/src/arch/fake/inc/hf/arch/cpu.h
@@ -18,8 +18,11 @@
 
 #include <stdint.h>
 
+#include "hf/addr.h"
+
 struct arch_regs {
-	uint32_t r[5];
+	uint64_t r[5];
+	uint16_t vcpu_index;
 };
 
 static inline struct cpu *cpu(void)
@@ -37,3 +40,25 @@
 {
 	/* TODO */
 }
+
+static inline void arch_cpu_update(bool is_primary)
+{
+	/* TODO */
+	(void)is_primary;
+}
+
+static inline void arch_regs_init(struct arch_regs *r, ipaddr_t pc, size_t arg)
+{
+	/* TODO */
+	(void)pc;
+	r->r[0] = arg;
+}
+static inline void arch_regs_set_vcpu_index(struct arch_regs *r, uint16_t index)
+{
+	r->vcpu_index = index;
+}
+
+static inline void arch_regs_set_retval(struct arch_regs *r, size_t v)
+{
+	r->r[0] = v;
+}
diff --git a/src/arch/fake/inc/hf/arch/mm.h b/src/arch/fake/inc/hf/arch/mm.h
index 9c16cb7..053eafa 100644
--- a/src/arch/fake/inc/hf/arch/mm.h
+++ b/src/arch/fake/inc/hf/arch/mm.h
@@ -204,4 +204,9 @@
 	return true;
 }
 
-void arch_mm_set_vm(uint64_t vmid, paddr_t table);
+static inline void arch_mm_set_vm(uint64_t vmid, paddr_t table)
+{
+	/* TODO */
+	(void)vmid;
+	(void)table;
+}
diff --git a/src/cpu.c b/src/cpu.c
index 03b92e1..739fdcc 100644
--- a/src/cpu.c
+++ b/src/cpu.c
@@ -133,8 +133,7 @@
 	sl_init(&vcpu->lock);
 	vcpu->vm = vm;
 	vcpu->state = vcpu_state_off;
-	/* TODO: This needs to be moved to arch-dependent code. */
-	vcpu->regs.lazy.vmpidr_el2 = vcpu - vm->vcpus;
+	arch_regs_set_vcpu_index(&vcpu->regs, vcpu - vm->vcpus);
 }
 
 void vcpu_on(struct vcpu *vcpu)