目前我们处于 protected mode(32bit) 接下面来切换到 long mode。long mode 包含两种子模式:64-bit mode 和 compatibility mode(32bit)。这里我们想要的是 64-bit mode。在这种模式下寄存器扩展到 64 位(rax, rcx, rdx 等)，并且增加了新的 8 个通用寄存器(r8-r15)
CPUID#
CPUID 指令可以用于获取 CPU 的信息，我们可以用其来检测 long mode 是否存在。首先需要判断是否支持 CPUID 指令(代码从 OSDev WiKi) 上拷贝的)
; Check if CPUID is supported by attempting to flip the ID bit (bit 21) in
; the FLAGS register. If we can flip it, CPUID is available.
; Copy FLAGS in to EAX via stack
pushfd
pop eax
; Copy to ECX as well for comparing later on
mov ecx, eax
; Flip the ID bit
xor eax, 1 << 21
; Copy EAX to FLAGS via the stack
push eax
popfd
; Copy FLAGS back to EAX (with the flipped bit if CPUID is supported)
pushfd
pop eax
; Restore FLAGS from the old version stored in ECX (i.e. flipping the ID bit
; back if it was ever flipped).
push ecx
popfd
; Compare EAX and ECX. If they are equal then that means the bit wasn't
; flipped, and CPUID isn't supported.
xor eax, ecx
jz .no_cpuid
ret
如果不支持我们需要将这一信息打印到屏幕上,所以我们需要制作一个通用的错误处理
; boot.asm
; Prints `ERR: ` and the given error code to screen and hangs.
; parameter: error code (in ascii) in al
error:
mov WORD [0xb8000], 0x1f45
mov WORD [0xb8002], 0x1f52
mov WORD [0xb8004], 0x1f52
mov WORD [0xb8006], 0x1f3a
mov WORD [0xb8008], 0x1f20
mov BYTE [0xb800A], al
hlt
接下来我们需要一个栈空间(stack)，保存调用函数时的返回地址。
section .bss
stack_bottom:
resb 64
stack_top:
初始化 esp 寄存器
section .text
bits 32
start:
mov esp, stack_top
增加检测支持 CPUID 的
; boot.asm
check_cpuid:
; 上面的代码
.no_cpuid:
mov al, "1"
jmp error
以 . 开头的 Label 称为 Local Label，它会和前面最近的 non-local lable 进行关联
x86 or x86-64#
接下来检测是否可以进入 long mode
; boot.asm
check_long_mode:
mov eax, 0x80000000 ; Set the A-register to 0x80000000.
cpuid ; CPU identification.
cmp eax, 0x80000001 ; Compare the A-register with 0x80000001.
jb .no_long_mode ; if it's less, the CPU is too old for long mode
; use extended info to test if long mode is available
mov eax, 0x80000001 ; Set the A-register to 0x80000001.
cpuid ; CPU identification.
test edx, 1 << 29 ; Test if the LM-bit, which is bit 29, is set in the D-register.
jz .no_long_mode ; If it's not set, there is no long mode
ret
.no_long_mode:
mov al, "2"
jmp error
merge#
在 start 后应当立即进行这些检测
section .text
bits 32
start:
mov esp, stack_top
call check_cpuid
call check_long_mode
Auto Make#
关于 Makefile 的语法可以参考 跟我一起写 Makefile
arch ?= x86_64
kernel := build/kernel-$(arch).bin
iso := build/os-$(arch).iso
linker_script := src/arch/$(arch)/linker.ld
grub_cfg := src/arch/$(arch)/grub.cfg
assembly_source_files := $(wildcard src/arch/$(arch)/*.asm)
assembly_object_files := $(patsubst src/arch/$(arch)/%.asm, \
build/arch/$(arch)/%.o, $(assembly_source_files))
.PHONY: all clean run iso
all: $(kernel)
clean:
@rm -r build
run: $(iso)
@qemu-system-x86_64 -cdrom $(iso)
iso: $(iso)
$(iso): $(kernel) $(grub_cfg)
@mkdir -p build/isofiles/boot/grub
@cp $(kernel) build/isofiles/boot/kernel.bin
@cp $(grub_cfg) build/isofiles/boot/grub
@grub-mkrescue -o $(iso) build/isofiles 2> /dev/null
@rm -r build/isofiles
$(kernel): $(assembly_object_files) $(linker_script)
@ld -n -T $(linker_script) -o $(kernel) $(assembly_object_files)
# compile assembly files
build/arch/$(arch)/%.o: src/arch/$(arch)/%.asm
@mkdir -p $(shell dirname $@)
@nasm -f elf64 $< -o $@