tasking: add simple base for running things in ring 3

src/tasking/switch.asm

@@ -94,3 +94,24 @@ switchProcess:
      ; |------------------------|
 
      ret
+
+global jump_usermode
+extern test_user_function
+jump_usermode:
+	mov ax, (4 * 8) | 3 ; ring 3 data with bottom 2 bits set for ring 3
+	mov ds, ax
+	mov es, ax 
+	mov fs, ax 
+	mov gs, ax ; SS is handled by iret
+
+	; and eax, ~0x80000000
+	; mov cr0, eax
+
+	; set up the stack frame iret expects
+	mov eax, esp
+	push (4 * 8) | 3 ; data selector
+	push eax ; current esp
+	pushf ; eflags
+	push (3 * 8) | 3 ; code selector (ring 3 code with bottom 2 bits set for ring 3)
+	push test_user_function ; instruction address to return to
+	iret