HigherHalf

2 files changed, 117 insertions, 22 deletions

diff --git a/src/as/boot.s b/src/as/boot.s
index f65cc72..adb24b7 100644
--- a/src/as/boot.s
+++ b/src/as/boot.s
@@ -1,42 +1,125 @@
-.set ALIGN,    1<<0
-.set MEMINFO,  1<<1
-.set FLAGS,    ALIGN | MEMINFO
-.set MAGIC,    0x1BADB002
-.set CHECKSUM, -(MAGIC + FLAGS)
+# Declare constants for the multiboot header.
+.set ALIGN,    1<<0             # align loaded modules on page boundaries
+.set MEMINFO,  1<<1             # provide memory map
+.set FLAGS,    ALIGN | MEMINFO  # this is the Multiboot 'flag' field
+.set MAGIC,    0x1BADB002       # 'magic number' lets bootloader find the header
+.set CHECKSUM, -(MAGIC + FLAGS) # checksum of above, to prove we are multiboot
 
-.section .multiboot
+# Declare a multiboot header that marks the program as a kernel.
+.section .multiboot.data, "aw"
 .align 4
 .long MAGIC
 .long FLAGS
 .long CHECKSUM
 
-.set CODE_SEGMENT, 0x08
-.set DATA_SEGMENT, 0x10
-
-.section .bss
-.align 16
+# Allocate the initial stack.
+.section .bootstrap_stack, "aw", @nobits
 stack_bottom:
-.skip 16384
+.skip 16384 # 16 KiB
 stack_top:
 
-.section .text
+# Preallocate pages used for paging. Don't hard-code addresses and assume they
+# are available, as the bootloader might have loaded its multiboot structures or
+# modules there. This lets the bootloader know it must avoid the addresses.
+.section .bss, "aw", @nobits
+	.align 4096
+boot_page_directory:
+	.skip 4096
+boot_page_table1:
+	.skip 4096
+boot_page_table2:
+	.skip 4096
+# Further page tables may be required if the kernel grows beyond 3 MiB.
+
+# The kernel entry point.
+.section .multiboot.text, "a"
 .global _start
 .type _start, @function
 _start:
+	movl $(boot_page_table1 - 0xC0000000), %edi
+	movl $0, %esi
+
+1:
+	jl 2f
+	cmpl $0x00400000, %esi
+	jge 3f
+
+	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:
+	movl $(boot_page_table2 - 0xC0000000), %edi
+
+4:
+	jl 5f
+	cmpl $0x00800000, %esi
+	jge 6f
+
+	movl %esi, %edx
+	orl $0x003, %edx
+	movl %edx, (%edi)
+
+5:
+	# 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 4b
+
+6:
+	movl $(boot_page_table1 - 0xC0000000 + 0x003), boot_page_directory - 0xC0000000 + 0 * 4
+	movl $(boot_page_table2 - 0xC0000000 + 0x003), boot_page_directory - 0xC0000000 + 1 * 4
+	movl $(boot_page_table1 - 0xC0000000 + 0x003), boot_page_directory - 0xC0000000 + 768 * 4
+	movl $(boot_page_table2 - 0xC0000000 + 0x003), boot_page_directory - 0xC0000000 + 769 * 4
+
+	# Set cr3 to the address of the boot_page_directory.
+	movl $(boot_page_directory - 0xC0000000), %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 7f, %ecx
+	jmp *%ecx
+
+.section .text
+
+7:
+	# 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
+	movl $0, boot_page_directory + 4
+
+	# Reload crc3 to force a TLB flush so the changes to take effect.
+	movl %cr3, %ecx
+	movl %ecx, %cr3
+
+    movl $stack_top, %esp
     call init_gdt_table
-    ljmp $CODE_SEGMENT, $code
+    ljmp $0x08, $code
 
 code:
-    movw $DATA_SEGMENT, %ax
+    movw $0x10, %ax
     movw %ax, %ds
     movw %ax, %es
     movw %ax, %fs
     movw %ax, %gs
     movw %ax, %ss
-    movl $stack_top, %esp
     cli
     call _init
     call kernel_main
     hlt
-
-.size _start, . - _start
diff --git a/src/as/paging.s b/src/as/paging.s
index e1cf42a..c3a9a0d 100644
--- a/src/as/paging.s
+++ b/src/as/paging.s
@@ -1,14 +1,16 @@
+.text
+
 .global loadPageDirectory
 loadPageDirectory:
     push %ebp
-    mov %esp, %ebp
-    mov 8(%esp), %eax
-    mov %eax, %cr3
+    movl %esp, %ebp
+    movl 8(%esp), %eax
+    subl $0xC0000000, %eax
+    movl %eax, %cr3
     mov %ebp, %esp
     pop %ebp
     ret
 
-.text
 .global enablePaging
 enablePaging:
     push %ebp
@@ -19,3 +21,13 @@ enablePaging:
     mov %ebp, %esp
     pop %ebp
     ret
+
+.global flushPaging
+flushPaging:
+    push %ebp
+    mov %esp, %ebp
+	movl %cr3, %ecx
+	movl %ecx, %cr3
+    mov %ebp, %esp
+    pop %ebp
+    ret