今天總算把bootloader研究完了, 做些重點整理
#include <inc/mmu.h>
# Start the CPU: switch to 32-bit protected mode, jump into C.
# The BIOS loads this code from the first sector of the hard disk into
# memory at physical address 0x7c00 and starts executing in real mode
# with %cs=0 %ip=7c00.
.set PROT_MODE_CSEG, 0x8 # kernel code segment selector
.set PROT_MODE_DSEG, 0x10 # kernel data segment selector
.set CR0_PE_ON, 0x1 # protected mode enable flag
.globl start
start:
.code16 # Assemble for 16-bit 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
# Enable A20:
# For backwards compatibility with the earliest PCs, physical
# address line 20 is tied low, so that addresses higher than
# 1MB wrap around to zero by default. This code undoes this.
# LL: Study book P.84
# 為了和舊PC相容, 需要下列動做讓address大於bit20部分可以運作
seta20.1:
inb $0x64,%al # Wait for not busy
testb $0x2,%al
jnz seta20.1
movb $0xd1,%al # 0xd1 -> port 0x64
outb %al,$0x64
seta20.2:
inb $0x64,%al # Wait for not busy
testb $0x2,%al
jnz seta20.2
movb $0xdf,%al # 0xdf -> port 0x60
outb %al,$0x60
# Switch from real to protected mode, using a bootstrap GDT
# and segment translation that makes virtual addresses
# identical to their physical addresses, so that the
# effective memory map does not change during the switch.
lgdt gdtdesc # LL: I can not understand!!!
movl %cr0, %eax
orl $CR0_PE_ON, %eax
movl %eax, %cr0
# Jump to next instruction, but in 32-bit code segment.
# Switches processor into 32-bit mode.
# 透過這行才會讓 CS register讀入正確的gdt offset
# $PROT_MODE_CSEG為 0X8, 因為 0X0為SEG_NULL,
# 偏移8才為 SEG_CODE
ljmp $PROT_MODE_CSEG, $protcseg
.code32 # Assemble for 32-bit mode
protcseg:
# Set up the protected-mode data segment registers
movw $PROT_MODE_DSEG, %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
# Set up the stack pointer and call into C.
# Stack register設為 $start(0x7c00 code開始位置),
# 因為stack是往低位址跑, 所以不會和code相撞 movl $start, %esp
call bootmain
# If bootmain returns (it shouldn't), loop.
spin:
jmp spin
# Bootstrap GDT
.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
# 這寫法指 gdtdesc為一個struct有兩個member, 一個為word, 一個為long
# word是指gdt的size(0代表1byte, 所以才要減一)
# long是指由哪個位址讀取 gdt table gdtdesc:
.word 0x17 # sizeof(gdt) - 1
.long gdt # address gdt
C code部分比較簡單, 先讀4K的 Kernel(由Sector1開始, sector0是boot loader)放到
0x10000.
再來根據ELF的格式, 把各個segmentload對應的
physical address. 即可跳入Kernel.
