c - 全局变量在 C 内核 .rdata 部分中不起作用

Question

我不知道为什么全局变量不起作用。我认为.rdata部分的地址有问题。我不知道如何链接我的内核以使此部分在特定内存区域可见。

我使用的是 Windows 7

注意:我不想使用 GRUB 引导加载程序，而是想使用我自己的引导加载程序。

这是我的文件:

bootsect.asm

[org 0x7c00]
KERNEL_OFFSET equ 0x1000 ; The same one we used when linking the kernel
    mov [BOOT_DRIVE], dl ; Remember that the BIOS sets us the boot drive in 'dl' on boot
    mov bp, 0x9000
    mov sp, bp
    mov bx, KERNEL_OFFSET ; Read from disk and store in 0x1000
    mov dh, 2
    mov dl, [BOOT_DRIVE]
    pusha
    ; reading from disk requires setting specific values in all registers
    ; so we will overwrite our input parameters from 'dx'. Let's save it
    ; to the stack for later use.
    push dx
    mov ah, 0x02 ; ah <- int 0x13 function. 0x02 = 'read'
    mov al, dh   ; al <- number of sectors to read (0x01 .. 0x80)
    mov cl, 0x02 ; cl <- sector (0x01 .. 0x11)
                 ; 0x01 is our boot sector, 0x02 is the first 'available' sector
    mov ch, 0x00 ; ch <- cylinder (0x0 .. 0x3FF, upper 2 bits in 'cl')
    ; dl <- drive number. Our caller sets it as a parameter and gets it from BIOS
    ; (0 = floppy, 1 = floppy2, 0x80 = hdd, 0x81 = hdd2)
    mov dh, 0x00 ; dh <- head number (0x0 .. 0xF)
    ; [es:bx] <- pointer to buffer where the data will be stored
    ; caller sets it up for us, and it is actually the standard location for int 13h
    int 0x13      ; BIOS interrupt
    jc $ ; if error (stored in the carry bit)
    pop dx
    cmp al, dh    ; BIOS also sets 'al' to the # of sectors read. Compare it.
    jne $
    popa
    ;call switch_to_pm ; disable interrupts, load GDT,  etc. Finally jumps to 'BEGIN_PM'
    cli ; 1. disable interrupts
    lgdt [gdt_descriptor] ; 2. load the GDT descriptor
    mov eax, cr0
    or eax, 0x1 ; 3. set 32-bit mode bit in cr0
    mov cr0, eax
    jmp CODE_SEG:init_pm ; 4. far jump by using a different segment
    jmp $ ; Never executed
gdt_start: ; don't remove the labels, they're needed to compute sizes and jumps
    ; the GDT starts with a null 8-byte
    dd 0x0 ; 4 byte
    dd 0x0 ; 4 byte
; GDT for code segment. base = 0x00000000, length = 0xfffff
; for flags, refer to os-dev.pdf document, page 36
gdt_code: 
    dw 0xffff    ; segment length, bits 0-15
    dw 0x0       ; segment base, bits 0-15
    db 0x0       ; segment base, bits 16-23
    db 10011010b ; flags (8 bits)
    db 11001111b ; flags (4 bits) + segment length, bits 16-19
    db 0x0       ; segment base, bits 24-31
; GDT for data segment. base and length identical to code segment
; some flags changed, again, refer to os-dev.pdf
gdt_data:
    dw 0xffff
    dw 0x0
    db 0x0
    db 10010010b
    db 11001111b
    db 0x0
gdt_end:
; GDT descriptor
gdt_descriptor:
    dw gdt_end - gdt_start - 1 ; size (16 bit), always one less of its true size
    dd gdt_start ; address (32 bit)
; define some constants for later use
CODE_SEG equ gdt_code - gdt_start
DATA_SEG equ gdt_data - gdt_start
[bits 32] ; using 32-bit protected mode
; this is how constants are defined
VIDEO_MEMORY equ 0xb8000
WHITE_ON_BLACK equ 0x0f ; the color byte for each character
[bits 32]
init_pm: ; we are now using 32-bit instructions
    mov ax, DATA_SEG ; 5. update the segment registers
    mov ss, ax
    mov es, ax
    mov fs, ax
    mov gs, ax
    ;mov ax, 11c4h
    mov ds, ax
    mov ebp, 0x90000 ; 6. update the stack right at the top of the free space
    mov esp, ebp
    call KERNEL_OFFSET ; Give control to the kernel
    jmp $ ; Stay here when the kernel returns control to us (if ever)
BOOT_DRIVE db 0 ; It is a good idea to store it in memory because 'dl' may get overwritten
; padding
times 510 - ($-$$) db 0
dw 0xaa55

内核.c

void write_string( int colour, const char *string )
{
    volatile char *video = (volatile char*)0xB8000;
    while( *string != 0 )
    {
        *video++ = *string++;
        *video++ = colour;
    }
}

void ClearScreen()
{
    char* video = (char*) 0xB8000;
    int i = 0;
    while(i < 80*25)
    {
        *video++ = 0;
        *video++ = 0;
        i++;
    }
}

char* str1 = "from global";

void start() 
{
    ClearScreen();
    char str2[] = "from stack";
    write_string(7, str1); //This is not working
    write_string(7, str2); //This is working but I don't want that
}

kernel_entry.asm

[bits 32]
[extern _start]
call _start
jmp $

生成文件

GCC = gcc.exe
NASM = nasm.exe
LD = ld.exe
OBJCOPY = objcopy.exe
QEMU = qemu-system-i386.exe

build:
    $(GCC) -ffreestanding -c kernel.c -o kernel.o
    $(NASM) kernel_entry.asm -f elf -o kernel_entry.o
    $(LD) -T NUL -o kernel.tmp -Ttext 0x1000 kernel_entry.o kernel.o
    $(OBJCOPY) -O binary -j .text kernel.tmp kernel.bin
    $(NASM)  bootsect.asm -f bin -o bootsect.bin
    copy /b bootsect.bin + kernel.bin os-image.bin
    $(QEMU) -fda os-image.bin

clean:
    del kernel.o
    del kernel_entry.o
    del kernel.tmp
    del kernel.bin
    del bootsect.bin
    del os-image.bin

Answer 1

如果您想编写自己的引导加载程序和应用程序的启动代码，则必须由初始化代码执行将 rdata(或rodata)复制到内存的操作。bss 部分也是如此，启动代码有责任将该部分的全局变量所在的整个内存重置为 0。