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bootstart.S
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bootstart.S
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###############################################################################
# BOOT ENTRY POINT
#
# After the BIOS initializes the hardware on startup or system reset,
# it loads the first 512-byte sector of the hard disk
# into physical addresses 0x7C00-0x7DFF.
# Then it jumps to address 0x7C00 -- and the OS starts running!
#
# This file contains the code loaded at that address.
# The 'start' routine switches the CPU out of compatibility mode, which
# requires some subtle assembly programming.
# Then it calls the "bootmain" routine to finish the booting
# process. This routine is defined in the C programming language, in one
# of the *-boot.c files.
#
# There is no need to understand this code in detail!
# (You may stop reading now.)
#
###############################################################################
###############################################################################
# For Your Information: COMPATIBILITY MODES
#
# The Intel x86 architecture has many compatibility modes, going back to
# the 8086, which supported only 16-bit addresses. When the BIOS calls
# into the OS, it is running in the "most compatible" mode, 16-bit real
# mode. The machine acts like addresses are only 16 bits long,
# there's no paging support, and there isn't even any support for
# user-mode applications -- the processor is always privileged, so there's
# no way to implement memory protection! We want to get into "386" mode,
# with 32-bit addresses and memory protection, as soon as possible. And
# that's what this code does.
#
# Again, there is no need to understand this code in detail!
#
###############################################################################
# Define some constants
.set SEGSEL_BOOT_CODE,0x8 # code segment selector
.set SEGSEL_BOOT_DATA,0x10 # data segment selector
.set CR0_PE_ON,0x1 # protected mode enable flag
.globl start # Entry point
start: .code16 # This runs in real mode
cli # Disable interrupts
cld # String operations increment
# Set up the important data segment registers (DS, ES, SS).
xorw %ax,%ax # Segment number zero
movw %ax,%ds # -> Data Segment
movw %ax,%es # -> Extra Segment
movw %ax,%ss # -> Stack Segment
# Set up the stack pointer, growing downward from 0x7c00.
movw $start,%sp # Stack Pointer
# Enable A20:
# For fascinating historical reasons (related to the fact that
# the earliest 8086-based PCs could only address 1MB of physical memory
# and subsequent 80286-based PCs wanted to retain maximum compatibility),
# physical address line 20 is tied to low when the machine boots.
# Obviously this a bit of a drag for us, especially when trying to
# address memory above 1MB. This code undoes this.
seta20.1: inb $0x64,%al # Get status
testb $0x2,%al # Busy?
jnz seta20.1 # Yes
movb $0xd1,%al # Command: Write
outb %al,$0x64 # output port
seta20.2: inb $0x64,%al # Get status
testb $0x2,%al # Busy?
jnz seta20.2 # Yes
movb $0xdf,%al # Enable
outb %al,$0x60 # A20
# Switch from real to protected mode:
# Up until now, there's been no protection, so we've gotten along perfectly
# well without explicitly telling the processor how to translate addresses.
# When we switch to protected mode, this is no longer true!
# We need at least to set up some "segments" that tell the processor it's
# OK to run code at any address, or write to any address.
# The 'gdt' and 'gdtdesc' tables below define these segments.
# This code loads them into the processor.
# We need this setup to ensure the transition to protected mode is smooth.
real_to_prot: cli # Don't allow interrupts: mandatory,
# since we didn't set up an interrupt
# descriptor table for handling them
lgdt gdtdesc # load GDT: mandatory in protected mode
movl %cr0, %eax # Turn on protected mode
orl $CR0_PE_ON, %eax
movl %eax, %cr0
# CPU magic: jump to relocation, flush prefetch queue, and
# reload %cs. Has the effect of just jmp to the next
# instruction, but simultaneously loads CS with
# $SEGSEL_BOOT_CODE.
ljmp $SEGSEL_BOOT_CODE, $protcseg
.code32 # run in 32-bit protected mode
# Set up the protected-mode data segment registers
protcseg: movw $SEGSEL_BOOT_DATA, %ax # Our data segment selector
movw %ax, %ds # -> DS: Data Segment
movw %ax, %es # -> ES: Extra Segment
movw %ax, %fs # -> FS
movw %ax, %gs # -> GS
movw %ax, %ss # -> SS: Stack Segment
call bootmain # finish the boot! Shouldn't return,
spinloop: jmp spinloop # ..but in case it does, spin.
# Segment descriptors
# These macros are used to define segment descriptors.
#define SEG_NULL \
.word 0, 0; \
.byte 0, 0, 0, 0
#define SEG(type,base,lim) \
.word (((lim) >> 12) & 0xffff), ((base) & 0xffff); \
.byte (((base) >> 16) & 0xff), (0x90 | (type)), \
(0xC0 | (((lim) >> 28) & 0xf)), (((base) >> 24) & 0xff)
#define STA_X 0x8 // Executable segment
#define STA_W 0x2 // Writeable (non-executable segments)
#define STA_R 0x2 // Readable (executable segments)
.p2align 2 # force 4 byte alignment
gdt: SEG_NULL # null seg
SEG(STA_X|STA_R, 0x0, 0xffffffff) # code seg
SEG(STA_W, 0x0, 0xffffffff) # data seg
gdtdesc: .word 0x17 # sizeof(gdt) - 1
.long gdt # address gdt