From 350aedae0e50749f9821ac7dc6b8316cf35f24bb Mon Sep 17 00:00:00 2001
From: Fabian Imhof <fabian.imhof@ost.ch>
Date: Sun, 6 Apr 2025 07:47:27 +0000
Subject: wip context switch in asm

---
 arch/x86_64/src/boot/boot.s | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

(limited to 'arch/x86_64/src/boot/boot.s')

diff --git a/arch/x86_64/src/boot/boot.s b/arch/x86_64/src/boot/boot.s
index 7932045..85ae1a1 100644
--- a/arch/x86_64/src/boot/boot.s
+++ b/arch/x86_64/src/boot/boot.s
@@ -352,6 +352,35 @@ prepare_page_maps:
 .section .boot_text, "ax", @progbits
 .code64
 
+.global context_switch
+context_switch:
+    // ring 3 data with bottom 2 bits set for ring 3
+	mov $((4 * 16) | 3), %rax
+    mov %rax, %ds
+    mov %rax, %es
+    mov %rax, %fs
+    mov %rax, %gs
+    // SS is handled by iret https://wiki.osdev.org/Getting_to_Ring_3
+
+	// set up the stack frame iret expects
+	mov %rsp, %rax
+    // user data selector
+	push $((4 * 16) | 3)
+    // current exp
+	push %rax
+    // push eflags
+	pushf
+    // push code selector (ring 3 code with bottom 2 bits set for ring 3)
+	push $((3 * 16) | 3)
+    // instruction address to return to
+	push test_function
+
+	iret
+
+test_function:
+    cli
+    ret
+
 _transition_to_long_mode:
     xor %rax, %rax
     mov %rax, %ss
-- 
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