Assembly 在x86实模式汇编中编写中断处理程序

我正在学习使用汇编在x86实模式下的中断处理。我下面的例子取自：

但是当我编译并运行上面的代码时

$ as --32 -o main.o main.S -g
$ ld -T linker.ld -o main.img --oformat binary -m elf_i386 -nostdlib main.o
$ qemu-system-i386 -hda main.img

我得到以下错误：

qemu-system-i386: Trying to execute code outside RAM or ROM at 0xf00fff53
This usually means one of the following happened:

(1) You told QEMU to execute a kernel for the wrong machine type, and it crashed on startup (eg trying to run a raspberry pi kernel on a versatilepb QEMU machine)
(2) You didn't give QEMU a kernel or BIOS filename at all, and QEMU executed a ROM full of no-op instructions until it fell off the end
(3) Your guest kernel has a bug and crashed by jumping off into nowhere

This is almost always one of the first two, so check your command line and that you are using the right type of kernel for this machine.
If you think option (3) is likely then you can try debugging your guest with the -d debug options; in particular -d guest_errors will cause the log to include a dump of the guest register state at this point.

Execution cannot continue; stopping here.

我错过了什么？为什么需要

mov%cs，0x02

，或者它到底在做什么

我试着在gdb下调试这个，当我逐行执行步骤时，我没有在gdb下遇到这个错误，这是wierd，我还在检查

编辑

这就是

BEGIN

的定义：

.macro BEGIN
    .local after_locals
    .code16
    cli
    /* Set %cs to 0. */
    ljmp $0, $1f
    1:
    xor %ax, %ax
    /* We must zero %ds for any data access. */
    mov %ax, %ds
    mov %ax, %es
    mov %ax, %fs
    mov %ax, %gs
    mov %ax, %bp
    /* Automatically disables interrupts until the end of the next instruction. */
    mov %ax, %ss
    /* We should set SP because BIOS calls may depend on that. TODO confirm. */
    mov %bp, %sp
    /* Store the initial dl to load stage 2 later on. */
    mov %dl, initial_dl
    jmp after_locals
    initial_dl: .byte 0
after_locals:
.endm

---

我只能假设您在教程中最初介绍的代码中引入了bug。例如，您说您使用以下工具进行组装：

as --32 -o main.o main.S -g

如果包含教程中显示的

common.h

，则此命令将失败，如下所示：

出现这些错误是因为编写教程代码的方式要求在汇编代码上运行C预处理器。最简单的方法是使用GCC将代码以汇编程序的形式传递到后端进行汇编：

gcc -c -g -m32 -o main.o main.S

GCC将使用扩展名为

.S

的任何文件扩展名，并在通过AS汇编程序之前在

.S

上运行C预处理器。或者，您可以使用

cpp

直接运行C预处理器，然后单独运行

As

要使用GCC构建

main.img

，您可以使用以下命令：

gcc -c -g -m32 -o main.o main.S
ld -T linker.ld -o main.img --oformat binary -m elf_i386 -nostdlib main.o

要使用C预处理器构建它，您可以执行以下操作：

cpp main.S > main.s
as -g --32 -o main.o main.s
ld -T linker.ld -o main.img --oformat binary -m elf_i386 -nostdlib main.o

在使用QEMU运行时，代码按预期工作，使用：

qemu-system-i386 -hda main.img

输出应类似于：

关于CS和实模式IVT的问题 您询问了此代码：

/* Set address of the handler for interrupt 0. */
movw $handler, 0x00
/* Set code segment of the handler for interrupt 0. */
mov %cs, 0x02
int $0

在实模式下，默认的IBM-PC中断向量表（IVT）是从物理地址0x00000（0x0000:0x0000）到0x00400（0x0000:0x0400）的前1024字节内存。IVT中的每个条目为4个字节（每个条目4个字节*256个中断=1024个字节）。指令指针（IP）的一个字（2字节）（也称为偏移量），其中中断向量位于该字后，后跟一个包含该段的字（2字节）

中断0从内存0x000000（0x0000:0x0000）的IVT最底部开始。中断1从0x00004（0x0000:0x0004）开始。。。中断255从0x003FC（0x0000:0x03FC）开始

指示：

/* Set address of the handler for interrupt 0. */
movw $handler, 0x00

/* Set code segment of the handler for interrupt 0. */
mov %cs, 0x02

int $0

将处理程序的16位偏移量移动到内存地址DS:0x0000。对于16位寻址，DS始终是一个隐含段，除非寄存器BP出现在内存引用中（即

（%BP）

），否则假定该段为SS

在

BEGIN

宏中将DS设置为0x0000，因此DS:0x00是0x0000:0x0000，这是中断0的段：偏移地址的IP（偏移）部分。指示：

/* Set address of the handler for interrupt 0. */
movw $handler, 0x00

/* Set code segment of the handler for interrupt 0. */
mov %cs, 0x02

int $0

CS在

BEGIN

宏中设置为0x0000。此指令将0x0000移动到内存地址DS:0x02（0x0000:0x0002）。0x0000:0x0002是中断0地址的段部分。在此指令之后，中断0的IVT条目现在指向引导扇区中的

处理程序
代码。指示：

/* Set address of the handler for interrupt 0. */
movw $handler, 0x00

/* Set code segment of the handler for interrupt 0. */
mov %cs, 0x02

int $0

调用现在指向处理程序的中断0。它应该在屏幕上显示
a
，然后在打印
b
的
int$0
之后继续执行代码，然后停止


最小完全可验证示例代码
你的问题缺少一个最小的完整的可验证的例子。我修改了
common.h
以只包含您编写的代码所需的宏，并保持其他内容不变：

链接器.ld：

SECTIONS
{
    /* We could also pass the -Ttext 0x7C00 to as instead of doing this.
     * If your program does not have any memory accesses, you can omit this.
     */
    . = 0x7c00;
    .text :
    {
        __start = .;

        /* We are going to stuff everything
         * into a text segment for now, including data.
         * Who cares? Other segments only exist to appease C compilers.
         */
        *(.text)

        /* Magic bytes. 0x1FE == 510.
         *
         * We could add this on each Gas file separately with `.word`,
         * but this is the perfect place to DRY that out.
         */
        . = 0x1FE;
        SHORT(0xAA55)

        /* This is only needed if we are going to use a 2 stage boot process,
         * e.g. by reading more disk than the default 512 bytes with BIOS `int 0x13`.
         */
        *(.stage2)

        /* Number of sectors in stage 2. Used by the `int 13` to load it from disk.
         *
         * The value gets put into memory as the very last thing
         * in the `.stage` section if it exists.
         *
         * We must put it *before* the final `. = ALIGN(512)`,
         * or else it would fall out of the loaded memory.
         *
         * This must be absolute, or else it would get converted
         * to the actual address relative to this section (7c00 + ...)
         * and linking would fail with "Relocation truncated to fit"
         * because we are trying to put that into al for the int 13.
         */
        __stage2_nsectors = ABSOLUTE((. - __start) / 512);

        /* Ensure that the generated image is a multiple of 512 bytes long. */
        . = ALIGN(512);
        __end = .;
        __end_align_4k = ALIGN(4k);
    }
}

/* I really want this for the local labels.
 *
 * The major downside is that every register passed as argument requires `<>`:
 * http://stackoverflow.com/questions/19776992/gas-altmacro-macro-with-a-percent-sign-in-a-default-parameter-fails-with-oper/
 */
.altmacro

/* Helpers */

/* Push registers ax, bx, cx and dx. Lightweight `pusha`. */
.macro PUSH_ADX
    push %ax
    push %bx
    push %cx
    push %dx
.endm

/* Pop registers dx, cx, bx, ax. Inverse order from PUSH_ADX,
 * so this cancels that one.
 */
.macro POP_DAX
    pop %dx
    pop %cx
    pop %bx
    pop %ax
.endm


/* Structural. */

/* Setup a sane initial state.
 *
 * Should be the first thing in every file.
 *
 * Discussion of what is needed exactly: http://stackoverflow.com/a/32509555/895245
 */
.macro BEGIN
    LOCAL after_locals
    .code16
    cli
    /* Set %cs to 0. TODO Is that really needed? */
    ljmp $0, $1f
    1:
    xor %ax, %ax
    /* We must zero %ds for any data access. */
    mov %ax, %ds
    /* TODO is it really need to clear all those segment registers, e.g. for BIOS calls? */
    mov %ax, %es
    mov %ax, %fs
    mov %ax, %gs
    /* TODO What to move into BP and SP?
     * http://stackoverflow.com/questions/10598802/which-value-should-be-used-for-sp-for-booting-process
     */
    mov %ax, %bp
    /* Automatically disables interrupts until the end of the next instruction. */
    mov %ax, %ss
    /* We should set SP because BIOS calls may depend on that. TODO confirm. */
    mov %bp, %sp
    /* Store the initial dl to load stage 2 later on. */
    mov %dl, initial_dl
    jmp after_locals
    initial_dl: .byte 0
after_locals:
.endm

/* BIOS */

.macro CURSOR_POSITION x=$0, y=$0
    PUSH_ADX
    mov $0x02, %ah
    mov $0x00, %bh
    mov \x, %dh
    mov \y, %dl
    int $0x10
    POP_DAX
.endm

/* Clear the screen, move to position 0, 0. */
.macro CLEAR
    PUSH_ADX
    mov $0x0600, %ax
    mov $0x7, %bh
    mov $0x0, %cx
    mov $0x184f, %dx
    int $0x10
    CURSOR_POSITION
    POP_DAX
.endm

/* Print a 8 bit ASCII value at current cursor position.
 *
 * * `c`: r/m/imm8 ASCII value to be printed.
 *
 * Usage:
 *
 * ....
 * PUTC $'a
 * ....
 *
 * prints `a` to the screen.
 */
.macro PUTC c=$0x20
    push %ax
    mov \c, %al
    mov $0x0E, %ah
    int $0x10
    pop %ax
.endm

.include "common.h"
BEGIN
    CLEAR
    /* Set address of the handler for interrupt 0. */
    movw $handler, 0x00
    /* Set code segment of the handler for interrupt 0. */
    mov %cs, 0x02
    int $0
    PUTC $'b
    hlt
handler:
    PUTC $'a
    iret

通用.h：

SECTIONS
{
    /* We could also pass the -Ttext 0x7C00 to as instead of doing this.
     * If your program does not have any memory accesses, you can omit this.
     */
    . = 0x7c00;
    .text :
    {
        __start = .;

        /* We are going to stuff everything
         * into a text segment for now, including data.
         * Who cares? Other segments only exist to appease C compilers.
         */
        *(.text)

        /* Magic bytes. 0x1FE == 510.
         *
         * We could add this on each Gas file separately with `.word`,
         * but this is the perfect place to DRY that out.
         */
        . = 0x1FE;
        SHORT(0xAA55)

        /* This is only needed if we are going to use a 2 stage boot process,
         * e.g. by reading more disk than the default 512 bytes with BIOS `int 0x13`.
         */
        *(.stage2)

        /* Number of sectors in stage 2. Used by the `int 13` to load it from disk.
         *
         * The value gets put into memory as the very last thing
         * in the `.stage` section if it exists.
         *
         * We must put it *before* the final `. = ALIGN(512)`,
         * or else it would fall out of the loaded memory.
         *
         * This must be absolute, or else it would get converted
         * to the actual address relative to this section (7c00 + ...)
         * and linking would fail with "Relocation truncated to fit"
         * because we are trying to put that into al for the int 13.
         */
        __stage2_nsectors = ABSOLUTE((. - __start) / 512);

        /* Ensure that the generated image is a multiple of 512 bytes long. */
        . = ALIGN(512);
        __end = .;
        __end_align_4k = ALIGN(4k);
    }
}

/* I really want this for the local labels.
 *
 * The major downside is that every register passed as argument requires `<>`:
 * http://stackoverflow.com/questions/19776992/gas-altmacro-macro-with-a-percent-sign-in-a-default-parameter-fails-with-oper/
 */
.altmacro

/* Helpers */

/* Push registers ax, bx, cx and dx. Lightweight `pusha`. */
.macro PUSH_ADX
    push %ax
    push %bx
    push %cx
    push %dx
.endm

/* Pop registers dx, cx, bx, ax. Inverse order from PUSH_ADX,
 * so this cancels that one.
 */
.macro POP_DAX
    pop %dx
    pop %cx
    pop %bx
    pop %ax
.endm


/* Structural. */

/* Setup a sane initial state.
 *
 * Should be the first thing in every file.
 *
 * Discussion of what is needed exactly: http://stackoverflow.com/a/32509555/895245
 */
.macro BEGIN
    LOCAL after_locals
    .code16
    cli
    /* Set %cs to 0. TODO Is that really needed? */
    ljmp $0, $1f
    1:
    xor %ax, %ax
    /* We must zero %ds for any data access. */
    mov %ax, %ds
    /* TODO is it really need to clear all those segment registers, e.g. for BIOS calls? */
    mov %ax, %es
    mov %ax, %fs
    mov %ax, %gs
    /* TODO What to move into BP and SP?
     * http://stackoverflow.com/questions/10598802/which-value-should-be-used-for-sp-for-booting-process
     */
    mov %ax, %bp
    /* Automatically disables interrupts until the end of the next instruction. */
    mov %ax, %ss
    /* We should set SP because BIOS calls may depend on that. TODO confirm. */
    mov %bp, %sp
    /* Store the initial dl to load stage 2 later on. */
    mov %dl, initial_dl
    jmp after_locals
    initial_dl: .byte 0
after_locals:
.endm

/* BIOS */

.macro CURSOR_POSITION x=$0, y=$0
    PUSH_ADX
    mov $0x02, %ah
    mov $0x00, %bh
    mov \x, %dh
    mov \y, %dl
    int $0x10
    POP_DAX
.endm

/* Clear the screen, move to position 0, 0. */
.macro CLEAR
    PUSH_ADX
    mov $0x0600, %ax
    mov $0x7, %bh
    mov $0x0, %cx
    mov $0x184f, %dx
    int $0x10
    CURSOR_POSITION
    POP_DAX
.endm

/* Print a 8 bit ASCII value at current cursor position.
 *
 * * `c`: r/m/imm8 ASCII value to be printed.
 *
 * Usage:
 *
 * ....
 * PUTC $'a
 * ....
 *
 * prints `a` to the screen.
 */
.macro PUTC c=$0x20
    push %ax
    mov \c, %al
    mov $0x0E, %ah
    int $0x10
    pop %ax
.endm

.include "common.h"
BEGIN
    CLEAR
    /* Set address of the handler for interrupt 0. */
    movw $handler, 0x00
    /* Set code segment of the handler for interrupt 0. */
    mov %cs, 0x02
    int $0
    PUTC $'b
    hlt
handler:
    PUTC $'a
    iret


建议
GDB（GNU调试器）不理解实模式段：偏移量寻址。用GDB调试实模式代码是非常有问题的，我不推荐这样做。您应该考虑使用真正的模式代码调试，因为它理解真实模式，段：偏移寻址，并且更适合调试引导加载程序或在进入32位保护模式或长模式之前运行的任何代码。p> 您是否先设置了
%ds
？使用BOCHS的内置调试器；它理解实模式分段，我认为可以为您解析/转储IDT/IVT之类的东西。下一次：请不要发布错误消息的图片。相反，请将错误消息以文本形式复制/粘贴到问题中。@fuz:已更正。real mode无法访问物理地址0xF00FFF53；因此，要么Qemu的错误消息是错误的（例如，可能应该是“0xF00F:0xFF53”），要么您没有处于实模式。如果你认为你在真正的模式，而你不是，那么你可以期待事情会变得非常错误…我的问题是，像这样用GNU汇编程序编写的教程，你需要了解AT&t语法，这本身就是一个问题。我的问题是-您向我们展示的代码是否与教程完全相同，或者您是否对其进行了更改，但没有将这些更改放在问题中？如果这不完全是你正在使用的代码，你需要让我们知道