直接打印到文本视频内存时出现意外输出 [英] Unexpected output when printing directly to text video memory

问题描述

我正在用 C 开发内核，并创建了一些可以在视频内存的屏幕上打印的内容.我希望视频内存中的第一个字节是要打印的字符，第二个字节告诉颜色.但我的程序有一些不同的东西，但它有效！！这是非常出乎意料和不寻常的.

I am developing a kernel in C and created something to print on screen on video memory. I expected that the first byte in video memory would be the character to print and the second byte tells the color. But my program has something different but it works!! It is very unexpected and unusual.

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;
    }
}

它在 *vid 上打印字符，在 *(vid+2) 上打印颜色，然后将 vid 增加 2.它然后应该替换并打印 *(vid+2) 上的下一个字符.所以，颜色应该会消失，但它仍然有效.

It prints the character on *vid and the color on *(vid+2) and then increments the vid by 2. It should then replace and print the next char on *(vid+2). So, the color should go but it still works.

另外，颜色应该在*(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(带有 ACII 代码 0x1f 我想成为颜色)替换整个字符串.

When I use *(vid+1) instead of *(vid+2) to print the string, the screen shows down arrow characters (with ACII code 0x1f which I wanted to be the color) replacing the entire string.

为什么代码表现得如此异常??

Why does the code behave so unusual??

有人可以帮忙吗?

编辑

我已经编辑了我的代码，现在它会打印字符串.但另一个问题出现了.我添加了对在特定行号上打印的支持.但是现在这会将字符串向后移动一个字符.

I have edited my code and now it prints string. But another problem arose. I added a support for printing on particular line number. But now this shifts the string backwards by one character.

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.

So, If I tell it to print on line 10, it prints the first character on the last character of the 9th line and then continues.

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

I also have a character printing function that justs prints curly braces (}) instead of the given character and that too one character backwards of the given position (like the error in the write_string function). Also it doen't change the character background color given as argument.

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;
}

<小时>

编辑 2

我的引导加载程序:

[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 -

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 -

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

在链接调用主函数时，我还在内核之前添加了一个 kernel_start.asm 文件 -

I also add a kernel_start.asm file just before the kernel while linking to call the main function -

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

这是我的制作文件 -

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

推荐答案

根据其他答案和评论中建议的更改，您的问题对我来说似乎无法重现.以下代码对我有用.我试图保持您的编码方式，以便对您有意义:

With the changes suggested in other answer and comments, your problem doesn't seem to be reproducible for me. The following code works for me. I've tried to maintain how you coded it just so it makes sense to you:

#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
" 
         "cli
" 
         "hlt
"
         );

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;
    */
}

我在开头添加了 __asm__ 以确保代码首先出现在生成的目标文件中.没有它它可能会工作.我已将您所有的 *vid 指针修改为 volatile .由于视频是内存映射 IO，您不希望编译器在优化时可能删除屏幕写入.很可能您的代码在没有 volatile 的情况下也能工作，但在此处添加它以避免潜在问题是合适的.

I've added the __asm__ at the beginning to make sure that code is the first to appear in the generated object file. It likely works without it. I've modified all your *vid pointers to be volatile . Since video is memory mapped IO you don't want to have the compiler potentially remove screen writes when it optimizes. Likely your code will work without volatile, but it is proper to add it here to avoid potential problems.

当运行 BOCHS 时，这段代码会产生这个屏幕输出:

When run BOCHS this code produces this screen output:

如果您使用此处提供的代码但它不起作用，则表明您遇到的问题可能与您在引导加载程序中编写的读取磁盘、启用 A20、设置 GDT、输入保护的代码有关模式，然后调用您的 C 代码.根据您编译和链接内核的方式，也可能出现问题.

If you use the code provided here and it doesn't work that would suggest the issue you are having is likely related to the a code you write in your bootloader that read the disk, enabled A20, set the GDT, entered protected mode, and then called into your C code. It is also possible problems could occur depending on how you compile and link your kernel.

在 EDIT 2 中提供了所有代码和 make 文件之后，很明显一个重大问题是大部分代码被编译并链接到 64 位对象和可执行文件.该代码在 32 位保护模式下不起作用.

After all the code and the make file were made available in EDIT 2 it became clear that one significant problem was that most of the code was compiled and linked to 64-bit objects and executables. That code won't work in 32-bit protected mode.

在 make 文件中进行这些调整:

In the make file make these adjustments:

• 使用GCC编译时需要添加-m32选项
• 当使用 GNU Assembler (as) 组装面向 32 位对象时，您需要使用 --32
• 当与LD链接时，你需要添加-melf_i386选项
• 当使用 NASM 组装面向 32 位对象时，您需要将 -f elf64 更改为 -f elf32

• When compiling with GCC you need to add -m32 option
• When assembling with GNU Assembler (as) targeting 32-bit objects you need to use --32
• When linking with LD you need to add the -melf_i386 option
• When assembling with NASM targeting 32-bit objects you need to change -f elf64 to -f elf32

从宿主环境使用 64 位编译器和工具链的一个更好的选择是创建一个 i686 或 i386 的交叉编译器工具链.

A preferable option to using a 64-bit compiler and tool chain from the host environment is to create a cross compiler toolchain for i686 or i386.

这篇关于直接打印到文本视频内存时出现意外输出的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！