From c24a6014f7f2be1a7ec57f2abeb55303726e0db8 Mon Sep 17 00:00:00 2001
From: Thomas Oltmann <thomas.oltmann.hhg@gmail.com>
Date: Sat, 5 Jul 2025 20:41:51 +0200
Subject: [PATCH] Set up a separate long-mode GDT

---
 lboot.S | 39 +++++++++++++++++++++++++++++++--------
 1 file changed, 31 insertions(+), 8 deletions(-)

diff --git a/lboot.S b/lboot.S
index 641c4be..650a3be 100644
--- a/lboot.S
+++ b/lboot.S
@@ -43,7 +43,7 @@ _start:     cli
 
             jmp         hang
 
-.set        COM1,       0x3F8
+            .set        COM1,       0x3F8
 .macro      com1_write  offset=0, byte
             mov         $COM1+\offset, %dx
             mov         $\byte, %al
@@ -67,7 +67,7 @@ enable_a20: // TODO more thorough implementation
 
             // unreal: Enter unreal mode
 unreal:     push        %ds
-            lgdt        gdt_ptr
+            lgdt        gdt16_ptr
 
             mov         %cr0, %eax
             or          $0x01, %al
@@ -244,7 +244,7 @@ long:
             mov         %eax, %cr3
 
             // Enable long mode
-.set        IA32_EFER,  0xC0000080
+            .set        IA32_EFER,  0xC0000080
             mov         $IA32_EFER, %ecx
             rdmsr
             or          $0x100, %eax
@@ -255,14 +255,16 @@ long:
             or          $0x80000001, %eax
             mov         %eax, %cr0
 
+            lgdt        gdt64_ptr
+
             ret
 
             // hang: sleep indefinitely
 hang:       hlt
             jmp         hang
 
-            // gdt: Protected mode / Unreal mode 16-bit GDT
-gdt:        // entry 0: null descriptor
+            // gdt16: Protected mode / Unreal mode 16-bit GDT
+gdt16:      // entry 0: null descriptor
             .word       0
             .word       0
             .byte       0
@@ -283,9 +285,30 @@ gdt:        // entry 0: null descriptor
             .byte       0b10010010
             .byte       0x8F
             .byte       0
-            .set        gdt_size, .-gdt
-gdt_ptr:    .word       gdt_size-1
-            .long       gdt
+            .set        gdt16_size, .-gdt16
+gdt16_ptr:  .word       gdt16_size-1
+            .long       gdt16
+
+            // gdt64: Long mode 64-bit GDT
+gdt64:      // entry 0: null descriptor
+            .quad       0
+            // entry 1: code segment
+            .word       0
+            .word       0
+            .byte       0
+            .byte       0x98
+            .byte       0x60
+            .byte       0
+            // entry 2: data segment
+            .word       0
+            .word       0
+            .byte       0
+            .byte       0x90
+            .byte       0x00
+            .byte       0
+            .set        gdt64_size, .-gdt64
+gdt64_ptr:  .word       gdt64_size-1
+            .quad       gdt64
 
             // Messages to print
 msg_start:  .asciz      "Netboot via fernlader v1 ...\r\n"