1 files changed, 35 insertions, 0 deletions
diff --git a/lib/arch/arm/init_stack.cc b/lib/arch/arm/init_stack.cc
new file mode 100644
index 0000000..422be64
--- /dev/null
+++ b/lib/arch/arm/init_stack.cc
@@ -0,0 +1,35 @@
+#include <cassert>
+#include <cstdint>  // uintN_t
+
+extern "C" void thread_create();
+
+void* init_stack(void* stack_ptr, void (*entry)(void*), const void* ctx) {
+    static_assert(sizeof(uint32_t) == sizeof(std::uintptr_t), "Pointer must be 32bit!");
+
+    // Setup initial stack frame which will be popped when yielding
+    // first time into the thread.
+    // Basic idea is to yield into Thread::entry() function which will
+    // then call the user function.
+
+    uint32_t* stack = static_cast<uint32_t*>(stack_ptr);
+    // Arguments for `thread_create`.
+    *(--stack) = reinterpret_cast<uint32_t>(ctx);
+    *(--stack) = reinterpret_cast<uint32_t>(entry);
+
+    // Yield epilogue.
+    *(--stack) = reinterpret_cast<uint32_t>(thread_create);  // r15 (PC)
+    *(--stack) = static_cast<uint32_t>(0);                   // r11 (FP)
+
+    // Callee saved registers.
+    *(--stack) = static_cast<uint32_t>(0);  // r11
+    *(--stack) = static_cast<uint32_t>(0);  // r10
+    *(--stack) = static_cast<uint32_t>(0);  // r9
+    *(--stack) = static_cast<uint32_t>(0);  // r8
+    *(--stack) = static_cast<uint32_t>(0);  // r7
+    *(--stack) = static_cast<uint32_t>(0);  // r6
+    *(--stack) = static_cast<uint32_t>(0);  // r5
+    *(--stack) = static_cast<uint32_t>(0);  // r4
+
+    assert((reinterpret_cast<std::uintptr_t>(stack) & 0x7) == 0);  // 8byte aligned
+    return static_cast<void*>(stack);
+}