Jos - L6 notes-优快云博客

本文详细解析了xv6操作系统中的系统调用实现机制，包括open函数的具体流程、参数传递方式以及如何处理除零错误和未映射内存访问等问题。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

Q: xv6 Sheet 16: init.c calls open("console"). How is that implemented?

A: open -> STUB(open) in usys.S -> trigger a interrupt T_SYSCALL -> idt[T_SYSCALL] -> vectors[T_SYSCALL] -> vector48 in vectors.S -> alltraps in trapasm.S -> trap() in trap.c -> syscall() in syscall.c -> sys_open() in sysfile.c

Q: xv6 usys.S (not in book). (No saving of registers. Why?)

A: It's one out of following three, tracking the code, all the information are reserved in trap frame, so No.3 ：

法一：利用registers儲存參數, No
- 優：快速
- 缺：不適用於參數多時。
  ∵register數目有限，參數不能太多
法二：將參數存在memory中的某個表格或Block，同時將此表格的起始位址存在一個register中，將此register傳給OS：
- 優：適用於參數多時
  ∵ 可存參數較多
- 缺：速度較慢
  ∵不限制參數的數目與長度
法三：利用system stack保存參數
- push(保存)參數
- OS利用POP取出參數
- 不限制參數的數目及長度

Q:xv6 Sheet 28: in vectors.S briefly, then in alltraps. Step through to call trap,
examine registers and stack. How will the kernel find the argument to open?

A: Stored in trap frame.

Q: (Briefly) xv6 Sheet 52: sys_open uses argstr and argint to get its arguments. How do
they work?

A: Get the arguments from the stack of trap frame.

52 // Fetch the argno'th word-sized system call argument as an integer.
53 int
54 argint(int argno, int *ip)
55 {
56   struct proc *p = curproc[cpu()];
57
58   return fetchint(p, p->tf->esp + 4 + 4*argno, ip);
59 }

Q: What happens if a user program divides by zero or accesses unmapped memory?
Exception. Same path as system call until trap.
What happens if kernel divides by zero or accesses unmapped memory?
A: That will cause kernel panic