直接打印到文本视频内存时出现意外输出

Question

我正在用C开发一个内核,并在屏幕上创建了一些在视频内存上打印的内容.我预计视频内存中的第一个字节将是要打印的字符,第二个字节将告诉颜色.但我的程序有一些不同但它的工作原理!! 这是非常意外和不寻常的.

我的内核代码 -

#define VIDEO_MEM 0xb8000

void write_string( int colour, const unsigned char *string );

void main()
{
    unsigned char *vid = (unsigned char*) VIDEO_MEM;
    int i=0;
    for (i = 0; i < 2000; i++)
    {
        *vid = ' ';
        *(vid+2) = 0x1f;
        vid += 2;
    }
    write_string(0x1f,"The Kernel has been loaded successfully!!");
}

void write_string( int colour, const unsigned char *string ) {
    unsigned char *vid = (unsigned char*) VIDEO_MEM;
    while(*string != 0)
    {
        *(vid) = *string;
        *(vid+2) = colour;
        ++string;
        vid+=2;
    }
}

它打印字符*vid和颜色打开*(vid+2)然后增加vid2.它应该替换并打印下一个字符*(vid+2).所以,颜色应该去,但它仍然有效.

此外,颜色应该打开 *(vid+1)

当我使用*(vid+1)而不是*(vid+2)打印字符串时,屏幕显示向下箭头字符(使用ACII代码0x1f,我想成为颜色)替换整个字符串.

为什么代码表现如此不寻常？

有人可以帮忙吗？

编辑

我编辑了我的代码,现在它打印字符串.但是出现了另一个问题.我添加了对特定行号打印的支持.但现在这会将字符串向后移动一个字符.

void write_string( int colour, const unsigned char *string, int pos ) {
    unsigned char *vid = (unsigned char*) VIDEO_MEM;
    vid+=pos*160;
    while(*string != 0)
    {
        *vid = colour;
        *(vid+1) = *string;
        ++string;
        vid+=2;
    }

}

所以,如果我告诉它在第10行打印,它会在第9行的最后一个字符上打印第一个字符然后继续.

我还有一个字符打印功能,只是打印花括号(})而不是给定的字符,并且在给定位置的后面也是一个字符(就像write_string函数中的错误一样).它也不会改变作为参数给出的字符背景颜色.

void putChar(char character, short col, short row, char attr) {
    unsigned char* vid_mem = (unsigned char *) VIDEO_MEM;
    int offset = (row*80 + col)*2;
    vid_mem += offset;
    if(!attr) {
        attr = 0x0f;
    }
    *vid_mem = (attr<<8)+character;
}

---

编辑2

我的启动加载器:

[org 0x7c00]

KERNEL equ 0x1000

mov [BOOT_DRIVE],dl

mov bp,0x9000
mov sp,bp

mov bx, msgReal
call print_string

call load_kernel

call switch_to_pm

jmp $

%include 'boot/bios.ASM'

%include 'boot/gdt.ASM'
%include 'boot/protected_mode.ASM'
%include 'boot/print32.ASM'

[bits 16]
load_kernel:
    mov bx,msgKernel
    call print_string

    mov bx, KERNEL
    mov dh, 15
    mov dl, [BOOT_DRIVE]
    call disk_load
    ret

[bits 32]

BEGIN_PM:
    mov ebx, msgProt
    call print_string32
    call KERNEL
    jmp $

BOOT_DRIVE db 0
msgReal db "Booted in 16-bit mode",0
msgProt db "Successfully switched to 32-bit mode",0
msgKernel db "Loading the kernel onto memory",0

times 510-($-$$) db 0
dw 0xaa55

bios.ASM -

;BIOS Functions
[bits 16]

print_string:
    pusha
    mov cx,bx
    mov ah,0x0e
    printStringStart:
    mov al,[bx]
    cmp al,0
    je done
    int 0x10
    inc bx
    jmp printStringStart
    done:
    popa
    ret

print_word:
    pusha
    mov ax,0x0000
    mov cl,0x10
    mov al,bh
    div cl
    call printDig
    mov al,bh
    and al,0x0f
    call printDig
    mov ax,0x0000
    mov al,bl
    div cl
    call printDig
    mov al,bl
    and al,0x0f
    call printDig
    popa
    ret

printDig:
    cmp al,0x9
    jg alpha
    add al,'0'
    mov ah,0x0e
    int 0x10
    jmp pDigDone
    alpha:
    sub al,0xa
    add al,'A'
    mov ah,0x0e
    int 0x10
    pDigDone:
    ret

hex_prefix: db '0x',0

disk_load:
    push dx
    mov ah,0x02
    mov al,dh
    mov ch,0x00
    mov dh,0x00
    mov cl,0x02
    int 0x13
    jc disk_error
    pop dx
    cmp dh,al
    jne disk_error
    ret

disk_error:
    mov ah,0x0e
    mov al,'X'
    int 0x10
    mov bx,errMsg
    call print_string
    jmp $

errMsg:
    db "Disk Read Error....."
    times 80-20 db " "
    db 0

gdt.ASM -

gdt_start:
gdt_null:
    dd 0x0
    dd 0x0

gdt_code:
    dw 0xffff
    dw 0x0
    db 0x0
    db 10011010b
    db 11001111b
    db 0x0

gdt_data:
    dw 0xffff
    dw 0x0
    db 0x0
    db 10010010b
    db 11001111b
    db 0x0
gdt_end:

gdt_descriptor:
    dw gdt_end - gdt_start - 1
    dd gdt_start

CODE_SEG equ gdt_code - gdt_start
DATA_SEG equ gdt_data - gdt_start

protected_mode.ASM -

[bits 16]
switch_to_pm:
    cli
    lgdt [gdt_descriptor]
    mov eax, cr0
    or eax, 0x1
    mov cr0, eax
    jmp CODE_SEG:init_pm

[bits 32]
init_pm:
    mov ax, DATA_SEG
    mov ds, ax
    mov ss, ax
    mov es, ax
    mov fs, ax
    mov gs, ax

    mov ebp,0x90000
    mov esp,0x90000

    call BEGIN_PM

print32.ASM -

[bits 32]

VIDEO_MEM equ 0xb8000
DEF_COLOR equ 0x0f

print_string32:
    pusha
    mov edx,VIDEO_MEM

print_string32_loop:
    mov al, [ebx]
    mov ah, DEF_COLOR

    cmp al,0
    je print_string32_end

    mov [edx],ax

    inc ebx
    add edx,2
    jmp print_string32_loop

print_string32_end:
    popa
    ret

我还在内核之前添加了一个kernel_start.asm文件,同时链接到调用main函数 -

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

这是我的make文件 -

C_SOURCES = $(wildcard drivers/*.c kernel/*.c)
HEADERS = $(wildcard kernel/*.h drivers/*.h)

OBJ = ${C_SOURCES:.c=.o}

all: os-image

os-image: boot/boot_sector.bin kernel.bin
    cat $^ > $@

kernel.bin: kernel/kernel_start.o ${OBJ}
    ld -o $@ -Ttext 0x1000 $^ --oformat binary

%.o : %.c
    gcc -std=c99 -Wall -pedantic -ffreestanding -c $< -o $@

%.o : %.asm
    nasm $< -f elf64 -o $@

%.bin : %.asm 
    nasm $< -f bin -o $@

clean:
    rm -fr kernel/*.o
    rm -fr drivers/*.o   
    rm -fr boot/*.bin
    rm -fr os-image *.bin *.o

Answer 1

通过其他答案和评论中建议的更改,您的问题似乎对我来说似乎不可重现.以下代码适用于我.我试图保持你的编码方式,这样才对你有意义:

#define VIDEO_MEM 0xb8000

void write_string( unsigned char colour, const char *string );
void write_string_line( unsigned char colour, const char *string, int pos );
void putChar(char character, short col, short row, unsigned char attr);

/* Place this at top of file as first code in kernel.o */
__asm__ ("call main\r\n" \
         "cli\r\n" \
         "hlt\r\n"
         );

void main()
{
    volatile unsigned char *vid = (unsigned char*) VIDEO_MEM;
    int i=0;
    for (i = 0; i < 2000; i++)
    {
        *vid = ' ';
        *(vid+1) = 0x1f;
        vid += 2;
    }
    write_string(0x1f,"The Kernel has been loaded successfully!!");
    write_string_line(0x1f,"Testing Here!!",1);
    putChar('Z',3,3,0xf3);
}

void write_string( unsigned char colour, const char *string ) {
    volatile unsigned char *vid = (unsigned char*) VIDEO_MEM;
    while(*string != 0)
    {
        *(vid) = *string;
        *(vid+1) = colour;
        ++string;
        vid+=2;
    }
}

void write_string_line( unsigned char colour, const char *string, int pos ) {
    volatile unsigned char *vid = (unsigned char*) VIDEO_MEM;
    vid+=pos*160;
    while(*string != 0)
    {
        *vid = *string;
        *(vid+1) = colour;
        ++string;
        vid+=2;
    }

}

void putChar(char character, short col, short row, unsigned char attr) {
    volatile unsigned char* vid_mem = (unsigned char *) VIDEO_MEM;
    int offset = (row*80 + col)*2;
    vid_mem += offset;
    if(!attr) {
        attr = 0x0f;
    }
    *(unsigned short int *)vid_mem = (attr<<8)+character;
    /* This would do the same as line above
    *vid_mem     = character;
    *(vid_mem+1) = attr;
    */
}

我__asm__在开头添加了以确保代码是第一个出现在生成的目标文件中的代码.它可能没有它.我已经修改了你所有的*vid指针volatile.由于视频是内存映射IO,因此您不希望编译器在优化时可能会删除屏幕写入.可能你的代码没有用volatile,但是在这里添加它是为了避免潜在的问题.

运行BOCHS时,此代码生成此屏幕输出:

如果您使用此处提供的代码并且它不起作用,则表明您遇到的问题可能与您在引导加载程序中写入的读取磁盘的代码相关,启用A20,设置GDT,进入保护模式,以及然后调用你的C代码.根据您编译和链接内核的方式,也可能出现问题.

未定义行为的可能原因

在EDIT 2中提供了所有代码和make文件之后,很明显一个重要问题是大多数代码都被编译并链接到64位对象和可执行文件.该代码不能在32位保护模式下工作.

在make文件中进行以下调整:

• 使用GCC进行编译时,您需要添加-m32选项
• 与GNU Assembler(as)一起组装时,需要使用32位对象--32
• 与LD链接时,您需要添加-melf_i386选项
• 在使用针对32位对象的NASM进行组装时,您需要更改-f elf64为-f elf32

使用来自主机环境的64位编译器和工具链的首选方法是为i686或i386创建交叉编译器工具链.