##Exception 系统中为每种可能的异常都分配了一个唯一的非整数的异常号(exception number),其中一些号码是由处理器的设计者分配的，如divide by zero, page faults, memory access violations, break- points, and arithmetic overflows；另外一些是由os kernel的设计者分配的，如system calls and signals from external I/O devices.
当系统启动时，操作系统会分配和初始化一个jump table,我们称为exception table. table的kth entry(k is exception number)包含address of the handler for exception k. Exception table的地址存放在exception table base register.
An exception is akin to a procedure call, but with some important differences :
- 如果从用户程序转移到内核, 那么用户程序的状态会被压到内核栈中.
- 异常处理程序运行在kernel mode
##不同的异常
###interrupt 中断是异步发生的,是来自处理外部的I/O设备信号的结果.
###trap & system call trap是有意的异常(active),是执行一条指令的结果, 主动触发. trap最重要的用途是system call.
read, fork, execve, exit都需要system call, 这些函数是os提供的服务, 必须运行在kernel mode.
###fault 一个经典的fault是page fault exception. 因此fault可能返回到当前指令.
###abort abort是不可恢复的致命错误造成的结果，通常是一些硬件错误.
##Linux/IA32系统中的异常
| 异常号 | 描述 | 类别 |
|---|---|---|
| 0 | Divide error | fault |
| 13 | General protection fault | fault |
| 14 | Page fault | fault |
| 18 | Machine check | abort |
| 32-127 | OS-defined exceptions | Interrupt or trap |
| 128 (0x80) | System call | trap |
| 129–255 | OS-defined exceptions | Interrupt or trap |
##Linux/IA32系统中的system call linux提供上百种系统调用. 每个系统调用都有一个唯一的整数号,对应于一个到内核中跳转表的偏移量.
| Number | Name | Description | Number | Name | Description | |:-------:|:--------:|: ------:|----------|:-------------:|------:| | 1 | exit | Terminate process | 27 | alarm | Set signal delivery alarm clock | | 2 | fork | Create new process | 29 | pause | Suspend process until signal arrives | | 3 | read | Read file | 37 | kill | Send signal to another process | | 4 | write | Write file | 48 | signal | Install signal handler |
System calls are provided on IA32 systems via a trapping instruction called int n, where n can be the index of any of the 256 entries in the IA32 exception table. Historically, system calls are provided through exception 128 (0x80).
system call的参数是通过register而不是stack进行传递的. %eax包含系统调用号, %ebx, %ecx, %edx, %esi, %edi, %ebp包含最多六个任意的参数. stack pointer %esp不能使用, 因为进入kernel mode, kernel会overwrite掉.
.section .data
string:
.ascii "hello, world\n"
string_end:
.equ len, string_end - string
.section .text
.globl main
main:
\\First, call write(1, "hello, world\n", 13)
movl $4, %eax \\System call number 4
movl $1, %ebx \\stdout has descriptor 1
movl $string, %ecx \\Hello world string
movl $len, %edx \\String length
int $0x80 \\System call code
\\Next, call exit(0)
movl $1, %eax \\System call number 0
movl $0, %ebx \\Argument is 0
int $0x80 \\System call code