Map kernel to 0xc000 rather than 0xc010

2 files changed, 51 insertions, 58 deletions

diff --git a/linker.ld b/linker.ld
index 9383b44..a9c6ec8 100644
--- a/linker.ld
+++ b/linker.ld
@@ -14,12 +14,11 @@ SECTIONS
         *(.multiboot.header)
     }
 
-    /* Begin putting sections at 1 MiB */
-    /* TODO load sections at VADDR_BASE rather than at VADDR_BASE + 1M */
-    . = VADDR_BASE + 1M;
+    /* Begin putting sections at 4 MiB */
+    . = VADDR_BASE;
     _kernel_start = .;
 
-    .text ALIGN(4K) : AT(ADDR(.text) - VADDR_BASE) {
+    .text ALIGN(4K) : AT(ADDR(.text) - VADDR_BASE + 4M) {
         begin_text = .;
         *(.multiboot.text)
         *(.text*)
@@ -27,14 +26,14 @@ SECTIONS
     }
  
     /* Read-only data. */
-    .rodata ALIGN(4K) : AT(ADDR(.rodata) - VADDR_BASE) {
+    .rodata ALIGN(4K) : AT(ADDR(.rodata) - VADDR_BASE + 4M) {
         begin_rodata = .;
         *(.rodata*)
         end_rodata = .;
     }
  
     /* Read-write data (initialized) */
-    .data ALIGN(4K) : AT(ADDR(.data) - VADDR_BASE) {
+    .data ALIGN(4K) : AT(ADDR(.data) - VADDR_BASE + 4M) {
         begin_constinit = .;
         *(.constinit)
         end_constinit = .;
@@ -49,7 +48,7 @@ SECTIONS
     }
  
     /* Read-write data (uninitialized) and stack */
-    .bss ALIGN(4K) : AT(ADDR(.bss) - VADDR_BASE) {
+    .bss ALIGN(4K) : AT(ADDR(.bss) - VADDR_BASE + 4M) {
         begin_bss = .;
         *(.pages)
         *(.bss)
diff --git a/src/boot.S b/src/boot.S
index 23c1e25..6588004 100644
--- a/src/boot.S
+++ b/src/boot.S
@@ -39,90 +39,84 @@ stack_top:
 .section .pages, "aw", @nobits
 .align 4096
 boot_page_directory:
-    .skip 4096
+    .skip 1024 * 4
 boot_page_table1:
-    .skip 4096
+    .skip 1024 * 4
 
 
 /*
     The linker script specifies _start as the entry point to the kernel and the
     bootloader will jump to this position once the kernel has been loaded.
-*/
+ */
 .section .multiboot.text, "ax"
-.set VADDR_BASE, 0xc0000000
+.set VADDR_OFFSET, 0xc0000000 - 0x400000
 .global _start
 .type _start, @function
 _start:
     cli
 
     # Physical address of boot_page_table1.
-    movl $(boot_page_table1 - VADDR_BASE), %edi
-    # First address to map is address 0.
-    movl $0, %esi
+    movl $(boot_page_table1 - VADDR_OFFSET), %edi
+
+    # Only map the kernel.
+    mov $_kernel_start - VADDR_OFFSET, %esi
     # Map 1023 pages. The 1024th will be the VGA text buffer.
     movl $1023, %ecx
 
-1:
-	# Only map the kernel.
-	cmpl $_kernel_start, %esi
-	jl 2f
-	cmpl $(_kernel_end - VADDR_BASE), %esi
-	jge 3f
+l_page_init:
+	cmpl $(_kernel_end - VADDR_OFFSET), %esi
+	jge map_vga
 
-	# Map physical address as "present, writable". Note that this maps
-	# .text and .rodata as writable. Mind security and map them as non-writable.
+    /* TODO: Map physical address as "present, writable". Note that this maps .text and .rodata as writable.
+             Mind security and map them as non-writable. */
 	movl %esi, %edx
 	orl $0x003, %edx
 	movl %edx, (%edi)
 
-2:
 	# Size of page is 4096 bytes.
 	addl $4096, %esi
 	# Size of entries in boot_page_table1 is 4 bytes.
 	addl $4, %edi
 	# Loop to the next entry if we haven't finished.
-	loop 1b
-
-3:
-	# Map VGA video memory to 0xC03FF000 as "present, writable".
-	movl $(0x000B8000 | 0x003), boot_page_table1 - VADDR_BASE + 1023 * 4
-
-	# The page table is used at both page directory entry 0 (virtually from 0x0
-	# to 0x3FFFFF) (thus identity mapping the kernel) and page directory entry
-	# 768 (virtually from VADDR_BASE to 0xC03FFFFF) (thus mapping it in the
-	# higher half). The kernel is identity mapped because enabling paging does
-	# not change the next instruction, which continues to be physical. The CPU
-	# would instead page fault if there was no identity mapping.
-
-	# Map the page table to both virtual addresses 0x00000000 and VADDR_BASE.
-	movl $(boot_page_table1 - VADDR_BASE + 0x003), boot_page_directory - VADDR_BASE + 0
-	movl $(boot_page_table1 - VADDR_BASE + 0x003), boot_page_directory - VADDR_BASE + 768 * 4
+	loop l_page_init
 
-	# Set cr3 to the address of the boot_page_directory.
-	movl $(boot_page_directory - VADDR_BASE), %ecx
-	movl %ecx, %cr3
+map_vga:
+    # Map VGA video memory to 0xC03FF000 as "present, writable".
+    movl $(0x000B8000 | 0x003), boot_page_table1 - VADDR_OFFSET + 1023 * 4
 
-	# Enable paging and the write-protect bit.
-	movl %cr0, %ecx
-	orl $0x80010000, %ecx
-	movl %ecx, %cr0
-
-	# Jump to higher half with an absolute jump. 
-	lea (kinit), %ecx
-	jmp *%ecx
+/*
+    Enabling paging does not change the next instruction, which continues to be physical. Therefore, map the kernel
+    to both its physical address and to the higher half.
+    Use the page table at:
+      - entry   1       starts 0x0040 0000      ends 0x007f ffff
+      - entry 768       starts 0xc000 0000      ends 0xc03f ffff
+ */
+    movl $(boot_page_table1 - VADDR_OFFSET + 0x003), boot_page_directory - VADDR_OFFSET + 1 * 4
+    movl $(boot_page_table1 - VADDR_OFFSET + 0x003), boot_page_directory - VADDR_OFFSET + 768 * 4
+
+    # Set cr3 to the address of the boot_page_directory.
+    movl $(boot_page_directory - VADDR_OFFSET), %ecx
+    movl %ecx, %cr3
+
+    # Enable paging and the write-protect bit.
+    movl %cr0, %ecx
+    orl $0x80010000, %ecx
+    movl %ecx, %cr0
+
+    # Jump to higher half with an absolute jump.
+    lea (kinit), %ecx
+    jmp *%ecx
 
 .section .text
 .extern kernel_constructors
 .extern kernel_main
 kinit:
-	# At this point, paging is fully set up and enabled.
-
-	# Unmap the identity mapping as it is now unnecessary. 
-	movl $0, boot_page_directory + 0
-
-	# Reload crc3 to force a TLB flush so the changes to take effect.
-	movl %cr3, %ecx
-	movl %ecx, %cr3
+    # At this point, paging is fully set up and enabled.
+    # Unmap the identity mapping as it is now unnecessary.
+    movl $0, boot_page_directory + 1 * 4
+    # Reload crc3 to force a TLB flush so the changes to take effect.
+    movl %cr3, %ecx
+    movl %ecx, %cr3
 
         #mov $stack_bottom, %ebp
 	mov $stack_top, %esp    # point the stack pointer to the stack