provided code

src/threads/start.S

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+	#include "threads/loader.h"
+
+#### Kernel startup code.
+
+#### The loader (in loader.S) loads the kernel at physical address
+#### 0x20000 (128 kB) and jumps to "start", defined here.  This code
+#### switches from real mode to 32-bit protected mode and calls
+#### main().
+
+/* Flags in control register 0. */
+#define CR0_PE 0x00000001      /* Protection Enable. */
+#define CR0_EM 0x00000004      /* (Floating-point) Emulation. */
+#define CR0_PG 0x80000000      /* Paging. */
+#define CR0_WP 0x00010000      /* Write-Protect enable in kernel mode. */
+
+	.section .start
+
+# The following code runs in real mode, which is a 16-bit code segment.
+	.code16
+
+.func start
+.globl start
+start:
+
+# The loader called into us with CS = 0x2000, SS = 0x0000, ESP = 0xf000,
+# but we should initialize the other segment registers.
+
+	mov $0x2000, %ax
+	mov %ax, %ds
+	mov %ax, %es
+
+# Set string instructions to go upward.
+	cld
+
+#### Get memory size, via interrupt 15h function 88h (see [IntrList]),
+#### which returns AX = (kB of physical memory) - 1024.  This only
+#### works for memory sizes <= 65 MB, which should be fine for our
+#### purposes.  We cap memory at 64 MB because that's all we prepare
+#### page tables for, below.
+
+	movb $0x88, %ah
+	int $0x15
+	addl $1024, %eax	# Total kB memory
+	cmp $0x10000, %eax	# Cap at 64 MB
+	jbe 1f
+	mov $0x10000, %eax
+1:	shrl $2, %eax		# Total 4 kB pages
+	addr32 movl %eax, init_ram_pages - LOADER_PHYS_BASE - 0x20000
+
+#### Enable A20.  Address line 20 is tied low when the machine boots,
+#### which prevents addressing memory about 1 MB.  This code fixes it.
+
+# Poll status register while busy.
+
+1:	inb $0x64, %al
+	testb $0x2, %al
+	jnz 1b
+
+# Send command for writing output port.
+
+	movb $0xd1, %al
+	outb %al, $0x64
+
+# Poll status register while busy.
+
+1:	inb $0x64, %al
+	testb $0x2, %al
+	jnz 1b
+
+# Enable A20 line.
+
+	movb $0xdf, %al
+	outb %al, $0x60
+
+# Poll status register while busy.
+
+1:	inb $0x64, %al
+	testb $0x2, %al
+	jnz 1b
+
+#### Create temporary page directory and page table and set page
+#### directory base register.
+
+# Create page directory at 0xf000 (60 kB) and fill with zeroes.
+	mov $0xf00, %ax
+	mov %ax, %es
+	subl %eax, %eax
+	subl %edi, %edi
+	movl $0x400, %ecx
+	rep stosl
+
+# Add PDEs to point to page tables for the first 64 MB of RAM.
+# Also add identical PDEs starting at LOADER_PHYS_BASE.
+# See [IA32-v3a] section 3.7.6 "Page-Directory and Page-Table Entries"
+# for a description of the bits in %eax.
+
+	movl $0x10007, %eax
+	movl $0x11, %ecx
+	subl %edi, %edi
+1:	movl %eax, %es:(%di)
+	movl %eax, %es:LOADER_PHYS_BASE >> 20(%di)
+	addw $4, %di
+	addl $0x1000, %eax
+	loop 1b
+
+# Set up page tables for one-to-map linear to physical map for the
+# first 64 MB of RAM.
+# See [IA32-v3a] section 3.7.6 "Page-Directory and Page-Table Entries"
+# for a description of the bits in %eax.
+
+	movw $0x1000, %ax
+	movw %ax, %es
+	movl $0x7, %eax
+	movl $0x4000, %ecx
+	subl %edi, %edi
+1:	movl %eax, %es:(%di)
+	addw $4, %di
+	addl $0x1000, %eax
+	loop 1b
+
+# Set page directory base register.
+
+	movl $0xf000, %eax
+	movl %eax, %cr3
+
+#### Switch to protected mode.
+
+# First, disable interrupts.  We won't set up the IDT until we get
+# into C code, so any interrupt would blow us away.
+
+	cli
+
+# Protected mode requires a GDT, so point the GDTR to our GDT.
+# We need a data32 prefix to ensure that all 32 bits of the GDT
+# descriptor are loaded (default is to load only 24 bits).
+# The CPU doesn't need an addr32 prefix but ELF doesn't do 16-bit
+# relocations.
+
+	data32 addr32 lgdt gdtdesc - LOADER_PHYS_BASE - 0x20000
+
+# Then we turn on the following bits in CR0:
+#    PE (Protect Enable): this turns on protected mode.
+#    PG (Paging): turns on paging.
+#    WP (Write Protect): if unset, ring 0 code ignores
+#       write-protect bits in page tables (!).
+#    EM (Emulation): forces floating-point instructions to trap.
+#       We don't support floating point.
+
+	movl %cr0, %eax
+	orl $CR0_PE | CR0_PG | CR0_WP | CR0_EM, %eax
+	movl %eax, %cr0
+
+# We're now in protected mode in a 16-bit segment.  The CPU still has
+# the real-mode code segment cached in %cs's segment descriptor.  We
+# need to reload %cs, and the easiest way is to use a far jump.
+# Because we're not running in a 32-bit segment the data32 prefix is
+# needed to jump to a 32-bit offset in the target segment.
+
+	data32 ljmp $SEL_KCSEG, $1f
+
+# We're now in protected mode in a 32-bit segment.
+# Let the assembler know.
+
+	.code32
+
+# Reload all the other segment registers and the stack pointer to
+# point into our new GDT.
+
+1:	mov $SEL_KDSEG, %ax
+	mov %ax, %ds
+	mov %ax, %es
+	mov %ax, %fs
+	mov %ax, %gs
+	mov %ax, %ss
+	addl $LOADER_PHYS_BASE, %esp
+	movl $0, %ebp			# Null-terminate main()'s backtrace
+
+#### Call main().
+
+	call main
+
+# main() shouldn't ever return.  If it does, spin.
+
+1:	jmp 1b
+.endfunc
+
+#### GDT
+
+	.align 8
+gdt:
+	.quad 0x0000000000000000	# Null segment.  Not used by CPU.
+	.quad 0x00cf9a000000ffff	# System code, base 0, limit 4 GB.
+	.quad 0x00cf92000000ffff        # System data, base 0, limit 4 GB.
+
+gdtdesc:
+	.word	gdtdesc - gdt - 1	# Size of the GDT, minus 1 byte.
+	.long	gdt			# Address of the GDT.
+
+#### Physical memory size in 4 kB pages.  This is exported to the rest
+#### of the kernel.
+.globl init_ram_pages
+init_ram_pages:
+	.long 0
+