2 files changed, 36 insertions, 1 deletions
diff --git a/lib/arch/x86_64/asm.h b/lib/arch/x86_64/api.h
index 962ff1b..b2babc9 100644
--- a/lib/arch/x86_64/asm.h
+++ b/lib/arch/x86_64/api.h
@@ -1,4 +1,6 @@
 /* Copyright (c) 2020 Johannes Stoelp */
 
-extern "C" void thread_create();
+#pragma once
+
 extern "C" void yield(const void* new_stack, void* const* old_stack);
+void* init_stack(void* stack_ptr, void (*entry)(void*), void* ctx);
diff --git a/lib/arch/x86_64/init_stack.cc b/lib/arch/x86_64/init_stack.cc
new file mode 100644
index 0000000..748fb56
--- /dev/null
+++ b/lib/arch/x86_64/init_stack.cc
@@ -0,0 +1,33 @@
+#include <cassert>
+#include <cstdint>  // uintN_t
+
+extern "C" void thread_create();
+
+void* init_stack(void* stack_ptr, void (*entry)(void*), void* ctx) {
+    static_assert(sizeof(uint64_t) == sizeof(std::uintptr_t), "Pointer must be 64bit!");
+
+    // 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.
+
+    uint64_t* stack = static_cast<uint64_t*>(stack_ptr);
+    // Arguments for `thread_create`.
+    *(--stack) = reinterpret_cast<uint64_t>(ctx);
+    *(--stack) = reinterpret_cast<uint64_t>(entry);
+
+    // Yield epilogue.
+    *(--stack) = reinterpret_cast<uint64_t>(thread_create);  // Return address
+    *(--stack) = static_cast<uint64_t>(0);                   // rbp
+
+    // Callee saved registers.
+    *(--stack) = static_cast<uint64_t>(0);                                           // rbx
+    *(--stack) = reinterpret_cast<uint64_t>(static_cast<uint64_t*>(stack_ptr) - 4);  // rbp
+    *(--stack) = static_cast<uint64_t>(0);                                           // r12
+    *(--stack) = static_cast<uint64_t>(0);                                           // r13
+    *(--stack) = static_cast<uint64_t>(0);                                           // r14
+    *(--stack) = static_cast<uint64_t>(0);                                           // r15
+
+    assert((reinterpret_cast<std::uintptr_t>(stack) & 0xf) == 0);  // 16byte aligned
+    return static_cast<void*>(stack);
+}