From d7889d3a847d1d692febdce78a28bf7a984fd25a Mon Sep 17 00:00:00 2001
From: Kat Richey <thedarkqueenaeris@zoho.com>
Date: Sat, 29 Oct 2022 10:03:56 -0500
Subject: [PATCH] Higher half, baby!!!

---
 arch/i386/boot.S    | 79 +++++++++++++++++++++++++++++++++++++++++----
 arch/i386/linker.ld | 28 +++++++++++-----
 2 files changed, 93 insertions(+), 14 deletions(-)

diff --git a/arch/i386/boot.S b/arch/i386/boot.S
index c28621b..13d29fd 100755
--- a/arch/i386/boot.S
+++ b/arch/i386/boot.S
@@ -6,25 +6,93 @@
 	.set CHECKSUM,	-(MAGIC + FLAGS) # checksum to prove we're multiboot
 
 	# Declare header for multiboot
-	.section .multiboot
+	.section .multiboot.data, "aw"
 	.align 4
 	.long MAGIC
 	.long FLAGS
 	.long CHECKSUM
 
 	# Reserve stack for initial thread
-	.section .bss
-	.align 16
+	.section .bootstrap_stack, "aw", @nobits
 stack_bottom:
 	.skip 16384 # 16K
 stack_top:
 
+	# Preallocate pages for paging
+	.section .bss, "aw", @nobits
+	.align 4096
+boot_page_directory:
+	.skip 4096
+boot_page_table1:
+  .skip 4096
+  # May need additional page for kernel > 3 MiB
+
 	# Kernel entry
-	.section .text
+	.section .multiboot.text, "a"
 	.global _start
 	.type _start, @function
 _start:
-	movl $stack_top, %esp
+	# Physical address of boot_page_table 1
+	movl $(boot_page_table1 - 0xC0000000), %edi
+	# Map address 0
+	movl $0, %esi
+	# Map first 1023 pages
+	movl $1023, %ecx
+
+1:
+	# Only map the kernel
+	cmpl $_kernel_start, %esi
+	jl 2f
+	cmpl $(_kernel_end - 0xC0000000), %esi
+	jge 3f
+
+	# Map physical address as "present, writable"
+	# TODO: map .text and .rodata as non-writable
+	movl %esi, %edx
+	orl $0x003, %edx
+	movl %edx, (%edi)
+
+2:
+	# Size of page is 4KiB
+	addl $4096, %esi
+	# Size of entries in boot_page_table1 is 4 bytes
+	addl $4, %edi
+	# Loop to next if not done
+	loop 1b
+
+3:
+	# Map VGA memory to 0xC03FF000 as "present, writable"
+	movl $(0x000B8000 | 0x003), boot_page_table1 - 0xC0000000 + 1023 * 4
+
+	# Map page table to 0x0 and 0xC0000000
+	movl $(boot_page_table1 - 0xC0000000 + 0x003), boot_page_directory - 0xC0000000 + 0
+	movl $(boot_page_table1 - 0xC0000000 + 0x003), boot_page_directory - 0xC0000000 + 768 * 4
+
+	# Set cr3 to address of boot page directory
+	movl $(boot_page_directory - 0xC0000000), %ecx
+	movl %ecx, %cr3
+
+	# Enable paging and write-protect
+	movl %cr0, %ecx
+	orl $0x80010000, %ecx
+	movl %ecx, %cr0
+
+	# Jump to higher half
+	lea 4f, %ecx
+	jmp *%ecx
+
+	.section .text
+
+4:
+	# Unmap identity mapping
+	movl $0, boot_page_directory + 0
+
+	# Reload cr3 to force TLB flush
+	movl %cr3, %ecx
+	movl %ecx, %cr3
+
+	# Set up stack
+	mov $stack_top, %esp
 
 	# Call global constructors
 	call _init
@@ -36,4 +104,3 @@ _start:
 	cli
 1:	hlt
 	jmp 1b
-	.size _start, . - _start
diff --git a/arch/i386/linker.ld b/arch/i386/linker.ld
index d766d45..9a020c5 100755
--- a/arch/i386/linker.ld
+++ b/arch/i386/linker.ld
@@ -4,21 +4,33 @@ SECTIONS
 {
 	. = 1M;
 
-	.text BLOCK (4K) : ALIGN (4K) {
-	        *(.multiboot)
-	        *(.text)
+	_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)
+	.data ALIGN (4K) : AT (ADDR(.data) - 0xC0000000) {
+	  *(.data)
 	}
 	
-	.bss BLOCK (4K) : ALIGN (4K) {
-	     	*(COMMON)
+	.bss ALIGN (4K) : AT (ADDR (.bss) - 0xC0000000) {
+	  *(COMMON)
 		*(.bss)
+		*(.bootstrap_stack)
 	}
+	_kernel_end = .;
 }