#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