/sys/i386/i386/locore.s分析笔记

kernel的入口地址是/sys/i386/i386/locore.s中定义的btext：
200  /**********************************************************************
201   *
202   * This is where the bootblocks start us, set the ball rolling...
203   *
204   */
205  NON_GPROF_ENTRY(btext)
从/boot/kernel/kernel中可以读出btext的链接地址：
# readelf -a /boot/kernel/kernel | grep btext
  6870: c0458a30     0 FUNC    GLOBAL DEFAULT    5 btext
26381: c0458a30     0 FUNC    GLOBAL DEFAULT    5 btext
因此，在物理地址0x458a30处设置断点，单步跟踪locore.s中的初始化代码。程序运行至此
的cpu主要寄存器的内容如下：
rax: 0x00000000:00458a30 rcx: 0x00000000:a0200000
rdx: 0x00000000:000488a0 rbx: 0x00000000:00458a30
rsp: 0x00000000:0009e844 rbp: 0x00000000:00094884
rsi: 0x00000000:000610e4 rdi: 0x00000000:0005b9cc
r8 : 0x00000000:00000000 r9 : 0x00000000:00000000
r10: 0x00000000:00000000 r11: 0x00000000:00000000
r12: 0x00000000:00000000 r13: 0x00000000:00000000
r14: 0x00000000:00000000 r15: 0x00000000:00000000
rip: 0x00000000:00458a30
eflags 0x00000002
首先是向0x472写入0x1234，告知bios下次为热引导：
216  /* Tell the bios to warmboot next time */
217          movw    $0x1234,0x472
构建一个新的栈帧：
220  /* Set up a real frame in case the double return in newboot is executed. */
221          pushl   %ebp
222          movl    %esp, %ebp
此时cpu主要寄存器的内容如下：
rax: 0x00000000:00458a30 rcx: 0x00000000:a0200000
rdx: 0x00000000:000488a0 rbx: 0x00000000:00458a30
rsp: 0x00000000:0009e840 rbp: 0x00000000:0009e840
rsi: 0x00000000:000610e4 rdi: 0x00000000:0005b9cc
r8 : 0x00000000:00000000 r9 : 0x00000000:00000000
r10: 0x00000000:00000000 r11: 0x00000000:00000000
r12: 0x00000000:00000000 r13: 0x00000000:00000000
r14: 0x00000000:00000000 r15: 0x00000000:00000000
rip: 0x00000000:00458a3c
eflags 0x00000002
将PSL_KRENEL赋给eflags：
224  /* Don't trust what the BIOS gives for eflags. */
225          pushl   $PSL_KERNEL
226          popfl
PSL_KERNEL是在/sys/i386/include/psl.h中定义的：
60  /*
61   * The i486 manual says that we are not supposed to change reserved flags,
62   * but this is too much trouble since the reserved flags depend on the cpu
63   * and setting them to their historical values works in practice.
64   */
65  #define PSL_RESERVED_DEFAULT    0x00000002
66
67  /*
68   * Initial flags for kernel and user mode.  The kernel later inherits

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