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标题:bochs单步执行的问题
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bochs单步执行的问题
(0) Breakpoint 1, 0x0000000000007c00 in ?? ()
Next at t=17555471
(0) [0x000000007c00] 0000:7c00 (unk. ctxt): jmp .+33 (0x00007c24)     ; e92100
<bochs:3> regs
rax: 00000000_0000aa55 rcx: 00000000_00090000
rdx: 00000000_00000000 rbx: 00000000_00000000
rsp: 00000000_0000ffd6 rbp: 00000000_00000000
rsi: 00000000_000e0000 rdi: 00000000_0000ffac
r8 : 00000000_00000000 r9 : 00000000_00000000
r10: 00000000_00000000 r11: 00000000_00000000
r12: 00000000_00000000 r13: 00000000_00000000
r14: 00000000_00000000 r15: 00000000_00000000
rip: 00000000_00007c00
eflags 0x00000082: id vip vif ac vm rf nt IOPL=0 of df if tf SF zf af pf cf
<bochs:4> sreg
es:0x0000, dh=0x00009300, dl=0x0000ffff, valid=1
        Data segment, base=0x00000000, limit=0x0000ffff, Read/Write, Accessed
cs:0x0000, dh=0x00009300, dl=0x0000ffff, valid=1
        Data segment, base=0x00000000, limit=0x0000ffff, Read/Write, Accessed
ss:0x0000, dh=0x00009300, dl=0x0000ffff, valid=7
        Data segment, base=0x00000000, limit=0x0000ffff, Read/Write, Accessed
ds:0x0000, dh=0x00009300, dl=0x0000ffff, valid=1
        Data segment, base=0x00000000, limit=0x0000ffff, Read/Write, Accessed
fs:0x0000, dh=0x00009300, dl=0x0000ffff, valid=1
        Data segment, base=0x00000000, limit=0x0000ffff, Read/Write, Accessed
gs:0x0000, dh=0x00009300, dl=0x0000ffff, valid=1
        Data segment, base=0x00000000, limit=0x0000ffff, Read/Write, Accessed
ldtr:0x0000, dh=0x00008200, dl=0x0000ffff, valid=1
tr:0x0000, dh=0x00008b00, dl=0x0000ffff, valid=1
gdtr:base=0x00000000000f9a37, limit=0x30
idtr:base=0x0000000000000000, limit=0x3ff
<bochs:5> n
Next at t=17555472
(0) [0x000000007c24] 0000:7c24 (unk. ctxt): mov ax, cs                ; 8cc8
<bochs:6> regs
rax: 00000000_0000aa55 rcx: 00000000_00090000
rdx: 00000000_00000000 rbx: 00000000_00000000
rsp: 00000000_0000ffd6 rbp: 00000000_00000000
rsi: 00000000_000e0000 rdi: 00000000_0000ffac
r8 : 00000000_00000000 r9 : 00000000_00000000
r10: 00000000_00000000 r11: 00000000_00000000
r12: 00000000_00000000 r13: 00000000_00000000
r14: 00000000_00000000 r15: 00000000_00000000
rip: 00000000_00007c24
eflags 0x00000082: id vip vif ac vm rf nt IOPL=0 of df if tf SF zf af pf cf
<bochs:7> sreg
es:0x0000, dh=0x00009300, dl=0x0000ffff, valid=1
        Data segment, base=0x00000000, limit=0x0000ffff, Read/Write, Accessed
cs:0x0000, dh=0x00009300, dl=0x0000ffff, valid=1
        Data segment, base=0x00000000, limit=0x0000ffff, Read/Write, Accessed
ss:0x0000, dh=0x00009300, dl=0x0000ffff, valid=7
        Data segment, base=0x00000000, limit=0x0000ffff, Read/Write, Accessed
ds:0x0000, dh=0x00009300, dl=0x0000ffff, valid=1
        Data segment, base=0x00000000, limit=0x0000ffff, Read/Write, Accessed
fs:0x0000, dh=0x00009300, dl=0x0000ffff, valid=1
        Data segment, base=0x00000000, limit=0x0000ffff, Read/Write, Accessed
gs:0x0000, dh=0x00009300, dl=0x0000ffff, valid=1
        Data segment, base=0x00000000, limit=0x0000ffff, Read/Write, Accessed
ldtr:0x0000, dh=0x00008200, dl=0x0000ffff, valid=1
tr:0x0000, dh=0x00008b00, dl=0x0000ffff, valid=1
gdtr:base=0x00000000000f9a37, limit=0x30
idtr:base=0x0000000000000000, limit=0x3ff

以上是用bochs对一段简单的引导代码的观察,当代码被加载到0x7c00处时,cs值为0x0000,ax值为aa55(引导盘标志),我的疑问是在执行n(单步执行)显示mov ax,cs后
ax的值为何还是aa55,继续执行n(单步执行)显示如下
<bochs:8> n
Next at t=17555473
(0) [0x000000007c26] 0000:7c26 (unk. ctxt): mov ds, ax                ; 8ed8
<bochs:9> reg
rax: 00000000_00000000 rcx: 00000000_00090000
rdx: 00000000_00000000 rbx: 00000000_00000000
rsp: 00000000_0000ffd6 rbp: 00000000_00000000
rsi: 00000000_000e0000 rdi: 00000000_0000ffac
r8 : 00000000_00000000 r9 : 00000000_00000000
r10: 00000000_00000000 r11: 00000000_00000000
r12: 00000000_00000000 r13: 00000000_00000000
r14: 00000000_00000000 r15: 00000000_00000000
rip: 00000000_00007c26
eflags 0x00000082: id vip vif ac vm rf nt IOPL=0 of df if tf SF zf af pf cf
<bochs:10>

这次ax的值变成了0x0000,这让我很奇怪,难道不是应该在上一步mov ax,cs 之后就变成0x0000么???

另一个问题是mov ax,cs的地址是0000:7c24 好奇怪啊 难道不应该是0000:7c02么?而且接着的一条指令是在0000:7c24的基础上增加了两个字节到了0000:7c26
最后一个问题从[000000007c00]这个来看,bochs模拟器似乎是在48位寻址模式下,怎样能设置成32位寻址?我用的bochs版本2.6.9

感觉汇编背后充满了各种繁琐的东西,而且都是书上没有提到的,或者提到了一句半句也根本不清楚其中缘由的。吐槽一下intel的x86为了兼容寻址模式简直丑陋。
2017-11-09 22:21







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