Tutorial Week 4

Questions

    R3000 and assembly

  1. What is a branch delay?

  2. The goal of this question is to have you reverse engineer some of the C compiler function calling convention (instead of reading it from a manual). The following code contains 6 functions that take 1 to 6 integer arguments. Each function sums its arguments and returns the sum as a the result.

    #include <stdio.h> /* function protoypes, would normally be in header files */ int arg1(int a); int arg2(int a, int b); int arg3(int a, int b, int c); int arg4(int a, int b, int c, int d); int arg5(int a, int b, int c, int d, int e ); int arg6(int a, int b, int c, int d, int e, int f); /* implementations */ int arg1(int a) { return a; } int arg2(int a, int b) { return a + b; } int arg3(int a, int b, int c) { return a + b + c; } int arg4(int a, int b, int c, int d) { return a + b + c + d; } int arg5(int a, int b, int c, int d, int e ) { return a + b + c + d + e; } int arg6(int a, int b, int c, int d, int e, int f) { return a + b + c + d + e + f; } /* do nothing main, so we can compile it */ int main() { }

    The following code is the disassembled code that is generated by the C compiler (with certain optimisations turned of for the sake of clarity).

    004000f0 <arg1>: 4000f0: 03e00008 jr ra 4000f4: 00801021 move v0,a0 004000f8 <arg2>: 4000f8: 03e00008 jr ra 4000fc: 00851021 addu v0,a0,a1 00400100 <arg3>: 400100: 00851021 addu v0,a0,a1 400104: 03e00008 jr ra 400108: 00461021 addu v0,v0,a2 0040010c <arg4>: 40010c: 00852021 addu a0,a0,a1 400110: 00861021 addu v0,a0,a2 400114: 03e00008 jr ra 400118: 00471021 addu v0,v0,a3 0040011c <arg5>: 40011c: 00852021 addu a0,a0,a1 400120: 00863021 addu a2,a0,a2 400124: 00c73821 addu a3,a2,a3 400128: 8fa20010 lw v0,16(sp) 40012c: 03e00008 jr ra 400130: 00e21021 addu v0,a3,v0 00400134 <arg6>: 400134: 00852021 addu a0,a0,a1 400138: 00863021 addu a2,a0,a2 40013c: 00c73821 addu a3,a2,a3 400140: 8fa20010 lw v0,16(sp) 400144: 00000000 nop 400148: 00e22021 addu a0,a3,v0 40014c: 8fa20014 lw v0,20(sp) 400150: 03e00008 jr ra 400154: 00821021 addu v0,a0,v0 00400158 <main>: 400158: 03e00008 jr ra 40015c: 00001021 move v0,zero
    1. arg1 (and functions in general) returns its return value in what register?
    2. Why is there no stack references in arg2?
    3. What does jr ra do?
    4. Which register contains the first argument to the function?
    5. Why is the move instruction in arg1 after the jr instruction.
    6. Why does arg5 and arg6 reference the stack?

  3. The following code provides an example to illustrate stack management by the C compiler. Firstly, examine the C code in the provided example to understand how the recursive function works.

    #include <stdio.h> #include <unistd.h> char teststr[] = "\nThe quick brown fox jumps of the lazy dog.\n"; void reverse_print(char *s) { if (*s != '\0') { reverse_print(s+1); write(STDOUT_FILENO,s,1); } } int main() { reverse_print(teststr); }

    The following code is the disassembled code that is generated by the C compiler (with certain optimisations turned off for the sake of clarity).

    1. Describe what each line in the code is doing.
    2. What is the maximum depth the stack can grow to when this function is called?
    004000f0 <reverse_print>: 4000f0: 27bdffe8 addiu sp,sp,-24 4000f4: afbf0014 sw ra,20(sp) 4000f8: afb00010 sw s0,16(sp) 4000fc: 80820000 lb v0,0(a0) 400100: 00000000 nop 400104: 10400007 beqz v0,400124 <reverse_print+0x34> 400108: 00808021 move s0,a0 40010c: 0c10003c jal 4000f0 <reverse_print> 400110: 24840001 addiu a0,a0,1 400114: 24040001 li a0,1 400118: 02002821 move a1,s0 40011c: 0c1000af jal 4002bc <write> 400120: 24060001 li a2,1 400124: 8fbf0014 lw ra,20(sp) 400128: 8fb00010 lw s0,16(sp) 40012c: 03e00008 jr ra 400130: 27bd0018 addiu sp,sp,24
  4. Why is recursion or large arrays of local variables avoided by kernel programmers?

    5. Threads

  5. Compare cooperative versus preemptive multithreading?
  6. Describe user-level threads and kernel-level threads. What are the advantages or disadvantages of each approach?

  7. A web server is constructed such that it is multithreaded. If the only way to read from a file is a normal blocking read system call, do you think user-level threads or kernel-level threads are being used for the web server? Why?

  8. Assume a multi-process operating system with single-threaded applications. The OS manages the concurrent application requests by having a thread of control within the kernel for each process. Such a OS would have an in-kernel stack assocaited with each process.

    Switching between each process (in-kernel thread) is performed by the function switch_thread(cur_tcb,dst_tcb). What does this function do?

    9. Kernel Entry and Exit

  9. What is the EPC register? What is it used for?
  10. What happens to the KUc and IEc bits in the STATUS register when an exception occurs? Why? How are they restored?
  11. What is the value of ExcCode in the Cause register immediately after a system call exception occurs?
  12. Why must kernel programmers be especially careful when implementing system calls?
  13. The following questions are focused on the case study of the system call convention used by OS/161 on the MIPS R3000 from the lecture slides.
    1. How does the 'C' function calling convention relate to the system call interface between the application and the kernel?
    2. What does the most work to preserve the compiler calling convention, the system call wrapper, or the OS/161 kernel.
    3. At minimum, what additional information is required beyond that passed to the system-call wrapper function?
  14. In the example given in lectures, the library function read invoked the read system call. Is it essential that both have the same name? If not, which name is important?
  15. To a programmer, a system call looks like any other call to a library function. Is it important that a programmer know which library function result in system calls? Under what circumstances and why?

  16. Describe a plausible sequence of activities that occur when a timer interrupt results in a context switch.