From d4003f1d6daf6883acb83a398e3fdba5336aefef Mon Sep 17 00:00:00 2001
From: Danny Holman <dholman@gymli.xyz>
Date: Tue, 20 Jul 2021 20:49:03 -0500
Subject: arch: i386: move the kernel to high memory

Move the start of the kernel image to the "higher half" of memory. This
also enables paging.
---
 arch/i386/boot/boot.s | 55 +++++++++++++++++++++++++++++++++++++++++++++------
 arch/i386/boot/tty.c  |  2 +-
 arch/i386/linker.ld   | 22 +++++++++++++++------
 3 files changed, 66 insertions(+), 13 deletions(-)

(limited to 'arch/i386')

diff --git a/arch/i386/boot/boot.s b/arch/i386/boot/boot.s
index 56ac227..792a790 100644
--- a/arch/i386/boot/boot.s
+++ b/arch/i386/boot/boot.s
@@ -4,29 +4,72 @@
 .set MAGIC,     0x1BADB002
 .set CHECKSUM,  -(MAGIC + FLAGS)
 
-.section .multiboot
+.section .multiboot.data, "aw"
 .align 4
 .long MAGIC
 .long FLAGS
 .long CHECKSUM
 
-.section .bss
-.align 16
+.section .bootstrap_stack, "aw", @nobits
 stack_bottom:
 .skip 16384
 stack_top:
 
-.section .text
+.section .bss, "aw", @nobits
+        .align 4096
+boot_page_directory:
+        .skip 4096
+boot_page_table0:
+        .skip 4096
+
+.section .multiboot.text, "a"
 .global _start
 .type _start, @function
 
 _start:
+        movl $(boot_page_table0 - 0xC0000000), %edi
+        movl $0, %esi
+        movl $1023, %ecx
+
+1:      cmpl $_kernel_start, %esi
+        jl 2f
+        cmpl $(_kernel_end - 0xC0000000), %esi
+        jge 3f
+
+        movl %esi, %edx
+        orl $0x003, %edx
+        movl %edx, (%edi)
+
+2:      addl $4096, %esi
+        addl $4, %edi
+        loop 1b
+
+3:      movl $(0x000B8000 | 0x003), boot_page_table0 - 0xC0000000 + 1023 * 4
+        
+        movl $(boot_page_table0 - 0xC0000000 + 0x003), boot_page_directory - 0xC0000000
+        movl $(boot_page_table0 - 0xC0000000 + 0x003), boot_page_directory - 0xC0000000 + 768 * 4
+
+        movl $(boot_page_directory - 0xC0000000), %ecx
+        movl %ecx, %cr3
+
+        movl %cr0, %ecx
+        orl $0x80010000, %ecx
+        movl %ecx, %cr0
+
+        lea 4f, %ecx
+        jmp *%ecx
+
+.section .text
+
+4:      movl $0, boot_page_directory + 0
+
+        movl %cr3, %ecx
+        movl %ecx, %cr3
+
         movl $stack_top, %esp
 
-        call _init
         call kernel_main
 
         cli
 1:      hlt
         jmp 1b
-.size _start, . - _start
diff --git a/arch/i386/boot/tty.c b/arch/i386/boot/tty.c
index 9244415..2c75760 100644
--- a/arch/i386/boot/tty.c
+++ b/arch/i386/boot/tty.c
@@ -6,7 +6,7 @@
 
 static const size_t VGA_WIDTH = 80;
 static const size_t VGA_HEIGHT = 25;
-static uint16_t *const VGA_MEMORY = (uint16_t*)0xB8000;
+static uint16_t *const VGA_MEMORY = (uint16_t*)0xC03FF000;
 
 static size_t terminal_row;
 static size_t terminal_column;
diff --git a/arch/i386/linker.ld b/arch/i386/linker.ld
index a864c34..5d06e79 100644
--- a/arch/i386/linker.ld
+++ b/arch/i386/linker.ld
@@ -1,19 +1,29 @@
 ENTRY(_start)
 
 SECTIONS {
-        . = 1M;
-        .text BLOCK(4K) : ALIGN(4K) {
-                *(.multiboot)
+        . = 0x00100000;
+        _kernel_start = .;
+        .multiboot.data : {
+                *(.multiboot.data)
+        }
+        .multiboot.text : {
+                *(.multiboot.text)
+        }
+
+        . += 0xC0000000;
+        .text ALIGN(4K) : AT(ADDR(.text) - 0xC0000000) {
                 *(.text)
         }
-        .rodata BLOCK(4K) : ALIGN(4K) {
+        .rodata ALIGN(4K) : AT(ADDR(.rodata) - 0xC0000000) {
                 *(.rodata)
         }
-        .data BLOCK(4K) : ALIGN(4K) {
+        .data ALIGN(4K) : AT(ADDR(.data) - 0xC0000000) {
                 *(.data)
         }
-        .bss BLOCK(4K) : ALIGN(4K) {
+        .bss ALIGN(4K) : AT(ADDR(.bss) - 0xC0000000) {
                 *(COMMON)
                 *(.bss)
+                *(.bootstrap_stack)
         }
+        _kernel_end = .;
 }
-- 
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