Homework: intro to xv6

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.

Hand-In Procedure

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.

Fetch and un-tar the xv6 source:

sh-3.00$ wget http://pdos.csail.mit.edu/6.828/2008/src/xv6-rev2.tar.gz 
sh-3.00$ tar xzvf xv6-rev2.tar.gz
Build xv6:
$ 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
Find the address of the main function by looking in kernel.asm:
% grep main kernel.asm
00102454 <mpmain>:
001024d0 <main>:
  10250d:       79 f1                   jns    102500 <main+0x30>
  1025f3:       76 6f                   jbe    102664 <main+0x194>
  102611:       74 2f                   je     102642 <main+0x172>
In this case, the address is 001024d0.

Run the kernel inside Bochs, setting a breakpoint at the beginning of main (i.e., the address you just found).

$ 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> b 0x1024d0
<bochs> c
(0) Breakpoint 1, 0x0x001024d0 in ?? ()
Next at t=895814
(0) [0x001024d0] 0008:0x001024d0 (unk. ctxt): lea ecx, dword ptr ss:[esp+0x4] ; 8d4c2404
(1) [0xfffffff0] f000:fff0 (unk. ctxt): jmp far f000:e05b         ; ea5be000f0
The details of what you see are likely to differ from the above output. Look at the registers and the stack contents:
<bochs> info reg
<bochs> print-stack
Which part of the stack printout is actually the stack? (Hint: not all of it.) Identify all the non-zero values on the stack.

Turn in: 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.

Make a copy of .bochsrc called uni, find the line that starts with cpu: count=2, and change the 2 to 1, and write out the file. Then run

bochs -q -f uni

This will run bochs with just one CPU. Set a breakpoint at segment 3, offset 0, and set it running:

<bochs> vb 0x1B:0
<bochs> c
(0) Breakpoint 1, 0x0020b000 (0x001b:0x00000000)
Next at t=5312876
(0) [0x0020b000] 001b:0x00000000 (unk. ctxt): push 0x00000024           ; 6824000000

The code that is running is from initcode.S; the push 0x00000024 is the first instruction of the first user-level process. 0x24 is the address of some data; the process is pushing the address so that the kernel can later fetch the data.

Turn in: What physical address does the 0x24 translate to? That is, if the code were at this point to load 32 bits from address 0x24, from what physical address would the data be fetched? What is the data at that address?

Hint: You can use info gdt to find out about segments. You can use the x command to look at physical memory.