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diff --git a/web/xv6-intro.html b/web/xv6-intro.html new file mode 100644 index 0000000..3669866 --- /dev/null +++ b/web/xv6-intro.html @@ -0,0 +1,163 @@ +<title>Homework: intro to xv6</title> +<html> +<head> +</head> +<body> + +<h1>Homework: intro to xv6</h1> + +<p>This lecture is the introduction to xv6, our re-implementation of + Unix v6. Read the source code in the assigned files. You won't have + to understand the details yet; we will focus on how the first + user-level process comes into existence after the computer is turned + on. +<p> + +<b>Hand-In Procedure</b> +<p> +You are to turn in this homework during lecture. Please +write up your answers to the exercises below and hand them in to a +6.828 staff member at the beginning of lecture. +<p> + +<p><b>Assignment</b>: +<br> +Fetch and un-tar the xv6 source: + +<pre> +sh-3.00$ wget http://pdos.csail.mit.edu/6.828/2007/src/xv6-rev1.tar.gz +sh-3.00$ tar xzvf xv6-rev1.tar.gz +xv6/ +xv6/asm.h +xv6/bio.c +xv6/bootasm.S +xv6/bootmain.c +... +$ +</pre> + +Build xv6: +<pre> +$ cd xv6 +$ make +gcc -O -nostdinc -I. -c bootmain.c +gcc -nostdinc -I. -c bootasm.S +ld -N -e start -Ttext 0x7C00 -o bootblock.o bootasm.o bootmain.o +objdump -S bootblock.o > bootblock.asm +objcopy -S -O binary bootblock.o bootblock +... +$ +</pre> + +Find the address of the <code>main</code> function by +looking in <code>kernel.asm</code>: +<pre> +% grep main kernel.asm +... +00102454 <mpmain>: +mpmain(void) +001024d0 <main>: + 10250d: 79 f1 jns 102500 <main+0x30> + 1025f3: 76 6f jbe 102664 <main+0x194> + 102611: 74 2f je 102642 <main+0x172> +</pre> +In this case, the address is <code>001024d0</code>. +<p> + +Run the kernel inside Bochs, setting a breakpoint +at the beginning of <code>main</code> (i.e., the address +you just found). +<pre> +$ make bochs +if [ ! -e .bochsrc ]; then ln -s dot-bochsrc .bochsrc; fi +bochs -q +======================================================================== + Bochs x86 Emulator 2.2.6 + (6.828 distribution release 1) +======================================================================== +00000000000i[ ] reading configuration from .bochsrc +00000000000i[ ] installing x module as the Bochs GUI +00000000000i[ ] Warning: no rc file specified. +00000000000i[ ] using log file bochsout.txt +Next at t=0 +(0) [0xfffffff0] f000:fff0 (unk. ctxt): jmp far f000:e05b ; ea5be000f0 +(1) [0xfffffff0] f000:fff0 (unk. ctxt): jmp far f000:e05b ; ea5be000f0 +<bochs> +</pre> + +Look at the registers and the stack contents: + +<pre> +<bochs> info reg +... +<bochs> print-stack +... +<bochs> +</pre> + +Which part of the stack printout is actually the stack? +(Hint: not all of it.) Identify all the non-zero values +on the stack.<p> + +<b>Turn in:</b> the output of print-stack with +the valid part of the stack marked. Write a short (3-5 word) +comment next to each non-zero value explaining what it is. +<p> + +Now look at kernel.asm for the instructions in main that read: +<pre> + 10251e: 8b 15 00 78 10 00 mov 0x107800,%edx + 102524: 8d 04 92 lea (%edx,%edx,4),%eax + 102527: 8d 04 42 lea (%edx,%eax,2),%eax + 10252a: c1 e0 04 shl $0x4,%eax + 10252d: 01 d0 add %edx,%eax + 10252f: 8d 04 85 1c ad 10 00 lea 0x10ad1c(,%eax,4),%eax + 102536: 89 c4 mov %eax,%esp +</pre> +(The addresses and constants might be different on your system, +and the compiler might use <code>imul</code> instead of the <code>lea,lea,shl,add,lea</code> sequence. +Look for the move into <code>%esp</code>). +<p> + +Which lines in <code>main.c</code> do these instructions correspond to? +<p> + +Set a breakpoint at the first of those instructions +and let the program run until the breakpoint: +<pre> +<bochs> vb 0x8:0x10251e +<bochs> s +... +<bochs> c +(0) Breakpoint 2, 0x0010251e (0x0008:0x0010251e) +Next at t=1157430 +(0) [0x0010251e] 0008:0x0010251e (unk. ctxt): mov edx, dword ptr ds:0x107800 ; 8b1500781000 +(1) [0xfffffff0] f000:fff0 (unk. ctxt): jmp far f000:e05b ; ea5be000f0 +<bochs> +</pre> +(The first <code>s</code> command is necessary +to single-step past the breakpoint at main, otherwise <code>c</code> +will not make any progress.) +<p> + +Inspect the registers and stack again +(<code>info reg</code> and <code>print-stack</code>). +Then step past those seven instructions +(<code>s 7</code>) +and inspect them again. +Convince yourself that the stack has changed correctly. +<p> + +<b>Turn in:</b> answers to the following questions. +Look at the assembly for the call to +<code>lapic_init</code> that occurs after the +the stack switch. Where does the +<code>bcpu</code> argument come from? +What would have happened if <code>main</code> +stored <code>bcpu</code> +on the stack before those four assembly instructions? +Would the code still work? Why or why not? +<p> + +</body> +</html> |