diff --git a/core/sys/windows/types.odin b/core/sys/windows/types.odin
index 47c180db8..d0551c6bf 100644
--- a/core/sys/windows/types.odin
+++ b/core/sys/windows/types.odin
@@ -395,13 +395,6 @@ CS_BYTEALIGNWINDOW : UINT : 0x2000
 CS_GLOBALCLASS     : UINT : 0x4000
 CS_DROPSHADOW      : UINT : 0x0002_0000
 
-GWL_EXSTYLE    : c_int : -20
-GWLP_HINSTANCE : c_int : -6
-GWLP_ID        : c_int : -12
-GWL_STYLE      : c_int : -16
-GWLP_USERDATA  : c_int : -21
-GWLP_WNDPROC   : c_int : -4
-
 WS_BORDER           : UINT : 0x0080_0000
 WS_CAPTION          : UINT : 0x00C0_0000
 WS_CHILD            : UINT : 0x4000_0000
@@ -493,6 +486,48 @@ HWND_BOTTOM    :: HWND( uintptr(1))     //  1
 HWND_TOPMOST   :: HWND(~uintptr(0))     // -1
 HWND_NOTOPMOST :: HWND(~uintptr(0) - 1) // -2
 
+// Window field offsets for GetWindowLong()
+GWL_STYLE   :: -16
+GWL_EXSTYLE :: -20
+GWL_ID      :: -12
+
+when ODIN_ARCH == .i386 {
+	GWL_WNDPROC    :: -4
+	GWL_HINSTANCE  :: -6
+	GWL_HWNDPARENT :: -8
+	GWL_USERDATA   :: -21
+}
+
+GWLP_WNDPROC    :: -4
+GWLP_HINSTANCE  :: -6
+GWLP_HWNDPARENT :: -8
+GWLP_USERDATA   :: -21
+GWLP_ID         :: -12
+
+// Class field offsets for GetClassLong()
+GCL_CBWNDEXTRA :: -18
+GCL_CBCLSEXTRA :: -20
+GCL_STYLE      :: -26
+GCW_ATOM       :: -32
+
+when ODIN_ARCH == .i386 {
+	GCL_MENUNAME      :: -8
+	GCL_HBRBACKGROUND :: -10
+	GCL_HCURSOR       :: -12
+	GCL_HICON         :: -14
+	GCL_HMODULE       :: -16
+	GCL_WNDPROC       :: -24
+	GCL_HICONSM       :: -34
+}
+
+GCLP_MENUNAME      :: -8
+GCLP_HBRBACKGROUND :: -10
+GCLP_HCURSOR       :: -12
+GCLP_HICON         :: -14
+GCLP_HMODULE       :: -16
+GCLP_WNDPROC       :: -24
+GCLP_HICONSM       :: -34
+
 // GetSystemMetrics() codes
 SM_CXSCREEN          :: 0
 SM_CYSCREEN          :: 1
diff --git a/core/sys/windows/user32.odin b/core/sys/windows/user32.odin
index 4068ecdb2..3985c9ad6 100644
--- a/core/sys/windows/user32.odin
+++ b/core/sys/windows/user32.odin
@@ -10,19 +10,19 @@ foreign user32 {
 	GetClassInfoExA :: proc(hInsatnce: HINSTANCE, lpszClass: LPCSTR, lpwcx: ^WNDCLASSEXA) -> BOOL ---
 	GetClassInfoExW :: proc(hInsatnce: HINSTANCE, lpszClass: LPCWSTR, lpwcx: ^WNDCLASSEXW) -> BOOL ---
 
-	GetClassLongPtrA :: proc(hWnd: HWND, nIndex: c_int) -> DWORD ---
-	GetClassLongPtrW :: proc(hWnd: HWND, nIndex: c_int) -> DWORD ---
-	SetClassLongPtrA :: proc(hWnd: HWND, nIndex: c_int, dwNewLong: LONG_PTR) -> ULONG_PTR ---
-	SetClassLongPtrW :: proc(hWnd: HWND, nIndex: c_int, dwNewLong: LONG_PTR) -> ULONG_PTR ---
+	GetClassLongA :: proc(hWnd: HWND, nIndex: c_int) -> DWORD ---
+	GetClassLongW :: proc(hWnd: HWND, nIndex: c_int) -> DWORD ---
+	SetClassLongA :: proc(hWnd: HWND, nIndex: c_int, dwNewLong: LONG) -> DWORD ---
+	SetClassLongW :: proc(hWnd: HWND, nIndex: c_int, dwNewLong: LONG) -> DWORD ---
+
+	GetWindowLongA :: proc(hWnd: HWND, nIndex: c_int) -> LONG ---
+	GetWindowLongW :: proc(hWnd: HWND, nIndex: c_int) -> LONG ---
+	SetWindowLongA :: proc(hWnd: HWND, nIndex: c_int, dwNewLong: LONG) -> LONG ---
+	SetWindowLongW :: proc(hWnd: HWND, nIndex: c_int, dwNewLong: LONG) -> LONG ---
 
 	GetClassNameA :: proc(hWnd: HWND, lpClassName: LPSTR, nMaxCount: c_int) -> c_int ---
 	GetClassNameW :: proc(hWnd: HWND, lpClassName: LPWSTR, nMaxCount: c_int) -> c_int ---
 
-	GetWindowLongPtrA :: proc(hWnd: HWND, nIndex: c_int) -> LONG_PTR ---
-	GetWindowLongPtrW :: proc(hWnd: HWND, nIndex: c_int) -> LONG_PTR ---
-	SetWindowLongPtrA :: proc(hWnd: HWND, nIndex: c_int, dwNewLong: LONG_PTR) -> LONG_PTR ---
-	SetWindowLongPtrW :: proc(hWnd: HWND, nIndex: c_int, dwNewLong: LONG_PTR) -> LONG_PTR ---
-
 	RegisterClassA :: proc(lpWndClass: ^WNDCLASSA) -> ATOM ---
 	RegisterClassW :: proc(lpWndClass: ^WNDCLASSW) -> ATOM ---
 	RegisterClassExA :: proc(^WNDCLASSEXA) -> ATOM ---
@@ -198,6 +198,31 @@ CreateWindowW :: #force_inline proc "stdcall" (
 	)
 }
 
+when ODIN_ARCH == .amd64 {
+	@(default_calling_convention="stdcall")
+	foreign user32 {
+		GetClassLongPtrA :: proc(hWnd: HWND, nIndex: c_int) -> ULONG_PTR ---
+		GetClassLongPtrW :: proc(hWnd: HWND, nIndex: c_int) -> ULONG_PTR ---
+		SetClassLongPtrA :: proc(hWnd: HWND, nIndex: c_int, dwNewLong: LONG_PTR) -> ULONG_PTR ---
+		SetClassLongPtrW :: proc(hWnd: HWND, nIndex: c_int, dwNewLong: LONG_PTR) -> ULONG_PTR ---
+
+		GetWindowLongPtrA :: proc(hWnd: HWND, nIndex: c_int) -> LONG_PTR ---
+		GetWindowLongPtrW :: proc(hWnd: HWND, nIndex: c_int) -> LONG_PTR ---
+		SetWindowLongPtrA :: proc(hWnd: HWND, nIndex: c_int, dwNewLong: LONG_PTR) -> LONG_PTR ---
+		SetWindowLongPtrW :: proc(hWnd: HWND, nIndex: c_int, dwNewLong: LONG_PTR) -> LONG_PTR ---
+	}
+} else when ODIN_ARCH == .i386 {
+	GetClassLongPtrA :: GetClassLongA
+	GetClassLongPtrW :: GetClassLongW
+	SetClassLongPtrA :: SetClassLongA
+	SetClassLongPtrW :: SetClassLongW
+
+	GetWindowLongPtrA :: GetWindowLongA
+	GetWindowLongPtrW :: GetWindowLongW
+	SetWindowLongPtrA :: GetWindowLongA
+	SetWindowLongPtrW :: GetWindowLongW
+}
+
 GET_SC_WPARAM :: #force_inline proc(wparam: WPARAM) -> i32 {
 	return i32(wparam) & 0xFFF0
 }