Submit your solutions before the beginning of the lecture (i.e., 1pm) on the due date mentioned on the schedule page to the submission web site.
$ add -f 6.828Fetch the xv6 source:
$ mkdir 6.828 $ cd 6.828 $ git clone git://github.com/mit-pdos/xv6-public.git Cloning into 'xv6-public'... ... $Build xv6 on Athena:
$ cd xv6-public $ make ... gcc -O -nostdinc -I. -c bootmain.c gcc -nostdinc -I. -c bootasm.S ld -m elf_i386 -N -e start -Ttext 0x7C00 -o bootblock.o bootasm.o bootmain.o objdump -S bootblock.o > bootblock.asm objcopy -S -O binary -j .text bootblock.o bootblock ... $
If you are not using Athena for 6.828 JOS labs, but build on your own machine, see the instructions on the tools page. If you have a build infrastructure on your own machine for lab 1, then you should be able to use that infrastructure for building xv6 too.
Find the address of
_start, the entry point of the kernel:
$ nm kernel | grep _start 8010a48c D _binary_entryother_start 8010a460 D _binary_initcode_start 0010000c T _startIn this case, the address is
Run the kernel inside QEMU GDB, setting a breakpoint
_start (i.e., the address
you just found).
$ make qemu-gdb ... <leave "make qemu-gdb" running, and in a new terminal, navigate to the same directory and run the following. If you are trying this by logging into athena.dialup.mit.edu, check the hostname to make sure that you are running both the commands on the same physical machine.> $ gdb GNU gdb (Ubuntu 7.7.1-0ubuntu5~14.04.2) 7.7.1 Copyright (C) 2014 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html> This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Type "show copying" and "show warranty" for details. This GDB was configured as "x86_64-linux-gnu". Type "show configuration" for configuration details. For bug reporting instructions, please see: <http://www.gnu.org/software/gdb/bugs/>. Find the GDB manual and other documentation resources online at: <http://www.gnu.org/software/gdb/documentation/>. For help, type "help". Type "apropos word" to search for commands related to "word". + target remote localhost:26000 The target architecture is assumed to be i8086 [f000:fff0] 0xffff0: ljmp $0xf000,$0xe05b 0x0000fff0 in ?? () + symbol-file kernel (gdb) br * 0x0010000c Breakpoint 1 at 0x10000c (gdb) c Continuing. The target architecture is assumed to be i386 => 0x10000c: mov %cr4,%eax Breakpoint 1, 0x0010000c in ?? () (gdb)
The details of what you see are likely to differ from the above output, depending on the version of gdb you are using, but gdb should stop at the breakpoint, and it should be the above mov instruction. Your gdb may also complain that auto-loading isn't enabled. In that case, it will print instructions on how to enable auto-loading, and you should follow those instructions.
(gdb) info reg ... (gdb) x/24x $esp ... (gdb)
Write a short (3-5 word) comment next to each non-zero value on the stack explaining what it is. Which part of the stack printout is actually the stack? (Hint: not all of it.)
You might find it convenient to consult the files bootasm.S, bootmain.c, and bootblock.asm (which contains the output of the compiler/assembler). The reference page has pointers to x86 assembly documentation, if you are wondering about the semantics of a particular instruction. Your goal is to understand and explain the contents of the stack that you saw above, just after entering the xv6 kernel. One way to achieve this would be to observe how and where the stack gets setup during early boot and then track the changes to the stack up until the point you are interested in. Here are some questions to help you along:
Submit: The output of x/24x $esp with the valid part of the stack marked, plus your comments, in a file named hwN.txt (where N is the homework number as listed on the schedule).