From 5a05c48afc22e854ffd6df14ea681e80469e3681 Mon Sep 17 00:00:00 2001
From: Alexander Myltsev <a.myltsev@gmail.com>
Date: Sat, 18 Jan 2025 01:11:31 +0400
Subject: [PATCH] kstart.S: remove leftovers from the 64-bit version.

---
 kernel/kstart.S | 48 ++++++++----------------------------------------
 1 file changed, 8 insertions(+), 40 deletions(-)

diff --git a/kernel/kstart.S b/kernel/kstart.S
index 51327a1..e1e87b8 100644
--- a/kernel/kstart.S
+++ b/kernel/kstart.S
@@ -12,36 +12,22 @@
     .global _start
     .asciz "kernel start\n"
 _start:
-.code32
-    mov dword ptr [patch_addr - KERNBASE], offset gdt32_begin - KERNBASE
-    lgdt gdtr32 - KERNBASE
-
-    // zero our PD at 0x1000 and PT at 0x2000
+    // zero out PD at 0x1000
     xor eax, eax
-    mov ecx, 2048
+    mov ecx, 1024
     rep stosd
 
-    // Set up the page table for the lowest 4 MiB: PD[0] -> PT
-    mov dword ptr [0x1000], 0x2000 | 0x3
-
-    // KERNBASE = 0x8000_0000
-    // Use the same page table for the first 4 MiB after KERNBASE: PD[0x200] -> PT
-    mov dword ptr [0x1000 + ((KERNBASE >> 22) * 4)], 0x2000 | 0x3
-
     // Enable 4 MiB pages
     mov eax, cr4
     or eax, 0x10        // Set the PSE bit (bit 4)
     mov cr4, eax
 
-    // Identity map the first 2 MiB
-    mov edi, 0x2000      // Start of the PT
-    mov ebx, 0x00000003  // Present, R/W
-    mov ecx, 512         // 2 MiB / 4 KiB = 512
-SetEntry:
-    mov dword ptr [edi], ebx
-    add ebx, 0x1000
-    add edi, 4
-    loop SetEntry
+    // Identity map low 4 MiB
+    mov dword ptr [0x1000], 0 | PTE_P | PTE_W | PTE_PS
+
+    // KERNBASE = 0x8000_0000
+    // Same mapping for the first 4 MiB after KERNBASE
+    mov dword ptr [0x1000 + ((KERNBASE >> 22) * 4)], 0 | PTE_P | PTE_W | PTE_PS
 
     // Load physical address of PD into CR3
     mov edi, 0x1000
@@ -52,25 +38,7 @@ SetEntry:
     or eax, 1 << 31      // Set the PG bit
     mov cr0, eax
 
-    .att_syntax
-    jmp $0x8, $loadseg - KERNBASE
-    .intel_syntax noprefix
-
-loadseg:
     // jump to the high half
     add esp, KERNBASE
     lea eax, kmain
     jmp eax
-
-.align 16
-gdtr32:
-    .word gdt32_end - gdt32_begin - 1
-patch_addr:
-    .long gdt32_begin - KERNBASE
-
-.align 16
-gdt32_begin:
-    .long 0x00000000, 0x00000000      # Null descriptor
-    .long 0x0000ffff, 0x00cf9a00      # Code segment
-    .long 0x0000ffff, 0x00cf9200      # Data segment
-gdt32_end: