程序集使用不同的语法执行带偏移的跳远 [英] Assembly executing a long jump with an offset with different syntax

问题描述

我正在为内核编写GDT，并且一切进展顺利，我正在关注本教程.

I am writing a GDT for a Kernel and all is going well, I'm following this tutorial.

http://www.osdever.net/bkerndev/Docs/gdt.htm

将C代码链接到汇编代码时，他使用了这段代码.

When link the C code to the assembly code he uses this piece of code.

; This will set up our new segment registers. We need to do
; something special in order to set CS. We do what is called a
; far jump. A jump that includes a segment as well as an offset.
; This is declared in C as 'extern void gdt_flush();'
global _gdt_flush     ; Allows the C code to link to this
extern _gp            ; Says that '_gp' is in another file
_gdt_flush:
lgdt [_gp]        ; Load the GDT with our '_gp' which is a special pointer
    mov ax, 0x10      ; 0x10 is the offset in the GDT to our data segment
    mov ds, ax
    mov es, ax
    mov fs, ax
    mov gs, ax
    mov ss, ax
    jmp 0x08:flush2   ; 0x08 is the offset to our code segment: Far jump!
flush2:
ret               ; Returns back to the C code!

但是，到目前为止，我的汇编语法有所不同，这是我在boot.s文件中所拥有的一部分.

However, my assembly syntax is different here is what I have so far as part of my boot.s file.

.global gdt_flush     /*Allows the C code to link to this*/
.extern gp            /*Says that '_gp' is in another file*/
_gdt_flush:
    lgdt gp        /*; Load the GDT with our '_gp' which is a special pointer*/
    mov %ax, 0x10     /* ; 0x10 is the offset in the GDT to our data segment*/
    mov %ds, %ax
    mov %es, %ax
    mov %fs, %ax
    mov %gs, %ax
    mov %ss, %ax
    jmp flush2   /*; 0x08 is the offset to our code segment: Far jump!*/
flush2:
ret               /*; Returns back to the C code!*/

我的问题是如何将该指令的语法转换为我正在使用的格式?

My question is how do I translate the syntax of this instruction into the format I am using?

他的:jmp 0x08:flush2 ; 0x08 is the offset to our code segment: Far jump!

矿井:(long l?)jmp ????flush2 /*; 0x08 is the offset to our code segment: Far jump!*/

推荐答案

一些事情.远距离跳伞的AT& T语法是:

A few things. The AT&T syntax for a far jump is:

jmp $0x08,$flush2 

在这种情况下，标签前面必须带有$.像0x08这样的立即值也需要$.这行代码没有按照您的想象做:

The label in this case needs to be preceded by a $. Immediate values like 0x08 also need a $. This line doesn't do what you think it does:

mov %ax, 0x10

关于AT& T语法的重要事项是，与Intel语法不同，操作数是相反的.源操作数是第一个，目标操作是后一个.其次，x86/x86-64上AT& T语法中的立即值必须在它们前面加上$符号，否则它们实际上被视为内存操作数.您的指令实际上将 AX 的16位内容移到了内存地址0x00000010上，这不是您想要的.您想要的是:

Important thing about AT&T syntax is that unlike Intel Syntax the operands are reversed. Source operand is first and destination operation is after. Secondly, immediate values in AT&T syntax on x86/x86-64 need to have a $ sign prepended to them or they are actually treated as a memory operand. Your instruction actually moved the 16-bit contents of AX to the memory address 0x00000010 which is not what you intended. What you wanted was:

mov $0x10, %ax

这会将立即值0x10移动到 AX .操作数取反的问题也适用于您的所有行，例如:

This moves the immediate value 0x10 to AX. The problem with operands being reversed also applies to all your lines like:

mov %ds, %ax

应该是:

mov %ax, %ds

---

我通常更喜欢调用您的函数load_gdt.我通常喜欢通过以下代码传递段值(CS和DS)和GDTR的地址:

---

I usually prefer calling your function load_gdt. I'm usually a fan of passing the segment values (CS and DS) and the address of the GDTR with code like:

load_gdt:
    mov 4(%esp), %edx    # EDX is 1st argument - GDT record pointer
    mov 8(%esp), %eax    # EAX is 2nd argument - Data Selector
    lgdt (%edx)          # Load GDT with GDT record pointer passed as 1st argument
    mov %eax, %ds        # Reload all the data descriptors with Data selector (2nd arg)
    mov %eax, %es
    mov %eax, %gs
    mov %eax, %fs
    mov %eax, %ss

    pushl 12(%esp)      # Create FAR pointer on stack using Code selector (3rd argument)
    push $.setcs         # Offset of FAR JMP will be setcs label below
    ljmp *(%esp)        # Do the FAR JMP to next instruction to set CS with Code selector,
                        #    and set the EIP (instruction pointer) to offset of setcs
.setcs:
    add $8, %esp        # Restore stack (remove 2 DWORD values we put on stack to
                        #     create FAR Pointer)
    ret

C 原型如下所示:

void load_gdt(struct gdt_ptr *gdt_ptr, unsigned int data_sel, unsigned int code_sel);

---

如果要将GNU汇编器与Intel语法的变体一起使用，则可以尝试将此指令添加到所有汇编文件的顶部:

---

If you want to use GNU assembler with a variant of Intel Syntax you could try adding this directive to the top of all your assembly files:

.intel_syntax noprefix

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