Each assignment indicates how difficult it is:
Easy: less than an hour. These exercise are typically often warm-up exercises for subsequent exercises.
Moderate: 1-2 hours.
Hard: More than 2 hours. Often these exercises don't require much code, but the code is tricky to get right.
The exercises in general require not many lines of code (tens to a few hundred lines), but the code is conceptually complicated and often details matter a lot. So, make sure you do the assigned reading for the labs, read the relevant files through, consult the documentation (the RISC-V manuals etc. are on the reference page) before you write any code. Only when you have a firm grasp of the assignment and solution, then start coding. When you start coding, implement your solution in small steps (the assignments often suggest how to break the problem down in smaller steps) and test whether each steps works before proceeding to the next one.
Warning: don't start a lab the night before a lab is due; it much more time efficient to do the labs in several sessions spread over multiple days. The manifestation of a bug in operating system kernel can be bewildering and may require much thought and careful debugging to understand and fix.
vim. There are some alternatives such as emacs and nano. However, I think using vim is your best alternative. My first code editor was a punched card. I wrote code using punched cards from 1974 until 1981. After punched cards, I used a keyboard with scrolling paper to edit code. You could list lines, add lines, and edit lines. In 1983, I wrote code on a VT100 connected to a VAX, using the EDT editor. Sometime in the late 1980's or early 1990's, I began using vim. I initially selected emacs, but it was not available on all computers so I selected vim. Once you get used to vim, you can edit source code efficiently.
.vimrc.vimrc is read by vim to apply your specific features. The following is my .vimrc thoughts, followed by the contents of my .vimrc file.
Viewing line numbers as you edit are sometimes nice. The set number and set nonumber manipulate line numbers.
I do not like tabs in my code so I map tabs to 4 spaces.
Incremental search in text editors displays real-time search results as you type search strings.
Highlight search highlights text as you type search strings.
I map Ctl-l to list all of the files in a vertical split window. You can move to a file in the window, hit enter, and the file is opened in a new tab.
My .vimrc file.
" custom vim settings go into this file set number set tabstop=4 set shiftwidth=4 set expandtab set incsearch set hlsearch syntax on set mouse=a let g:netrw_banner = 0 let g:netrw_liststyle = 0 " Values for netrw_browse_split " 1 open file in new horizontal split " 2 open file in new vertical split " 3 open file in new tab " 4 open in previous window let g:netrw_browse_split = 3 let g:netrw_altv = 1 let g:netrw_winsize = 15 let g:netrw_list_hide = '^\.\=/\=$,.DS_Store,.git,*\.o,\.*\.swp,.h' let g:netrw_hide = 1 " Uncomment the following commands to have Vexp called on Vim startup "" augroup ProjectDrawer "" autocmd! "" autocmd VimEnter * :Vexplore "" augroup END map <c-l> :Vex<CR>
When editing source code for a program that is in multiple .c files, you want to bounce from one .c file to another discovering the definitions of functions and data. The ctags program creates a tags file, which allows you to begin editing in one file and easily jump to another file that contains the definitions of functions and data. There are various ways to create a tags file, but the simplist is to place source code in a directoy and run the ctags program. The Xv6 program has two primary directories of source code. The kernel directory contains to OS kernel source code. The user directory contains the utility program source code. You can create a tags file in both directories. For example, to create a tags file in the kernel directory.
$ cd kernel
$ tags *.c *.h
When you use vim to edit a file in the kernel directory, vim reads the tags file. Now you can use Ctl-] and Ctl-t to bounce from the current file being shown to another file containing the definition of the function or data. Suppose I am editing file.c, and my cursor is on a line calling the function acquire as mimiced below.
struct file*
filedup(struct file *f)
{
acquire(&ftable.lock);
Pressing Ctl-] opens the file spinlock.c and positions the cursor on the function acquire. When you are finished examining the function acquire, you return to file.c by pressing Ctl-t. Both file.c and spinlock.c remain a buffers in your vim editing session.
The tags file many have multiple entries for a tag, and Ctl-] jumps to the first match. You can view additional matches by pressing g] or g Ctl-], which show all matches and allow you to select a destination. Other options are for you to enter :tselect or :tjump, which also show all matches and allow you to select a destination.
bash has tab completion. The shell /bin/sh does not provide tab completion.
To see what shell you are using, enter $ echo $SHELL
To change your default shell (this example code changes it to bash), enter the following:
$ chsh -s /bin/bash
pwd: enter-your-pw
After this you will have to logout and log back in to use your new shell.
Alternatively, you can run bash from your existing shell by entering the following:
$ /bin/bash
C Pointers - Make sure you understand C and pointers. The book "The C programming language (second edition)" by Kernighan and Ritchie is a succinct description of C. Some useful pointer exercises are here. Unless you are already thoroughly versed in C, do not skip or skim the pointer exercises above. If you do not really understand pointers in C, you will suffer untold pain and misery in the labs, and then eventually come to understand them the hard way. Trust us; you don't want to find out what "the hard way" is.
A few pointer common idioms are in particular worth remembering:
int *p = (int*)100, then
(int)p + 1 and (int)(p + 1)
are different numbers: the first is 101 but
the second is 104.
When adding an integer to a pointer, as in the second case,
the integer is implicitly multiplied by the size of the object
the pointer points to.p[i] is defined to be the same as *(p+i),
referring to the i'th object in the memory pointed to by p.
The above rule for addition helps this definition work
when the objects are larger than one byte.&p[i] is the same as (p+i), yielding
the address of the i'th object in the memory pointed to by p.Although most C programs never need to cast between pointers and integers, operating systems frequently do. Whenever you see an addition involving a memory address, ask yourself whether it is an integer addition or pointer addition and make sure the value being added is appropriately multiplied or not.
Git Checkpoints - If you have an exercise partially working, checkpoint your progress by committing your code. If you break something later, you can then roll back to your checkpoint and go forward in smaller steps. To learn more about Git, take a look at the Git user's manual, or, you may find this CS-oriented overview of Git useful.
Lab Test Failures - If you fail a test, make sure you understand why your code fails the test. Insert print statements until you understand what is going on.
Print Statements - You may find that your print statements may produce much output that you would like to search through; one way to do that is to run make qemu inside of script (run man script on your machine), which logs all console output to a file, which you can then search. Don't forget to exit script.
GDB - In many cases, print statements will be sufficient, but sometimes being able to single step through some assembly code or inspecting the variables on the stack is helpful. To use gdb with xv6, run make make qemu-gdb in one window, run gdb-multiarch (or riscv64-linux-gnu-gdb) in another window (if you are using Athena, make sure that the two windows are on the same Athena machine), set a break point, followed by followed by 'c' (continue), and xv6 will run until it hits the breakpoint. See GDB - the GNU Debugger for helpful tips on using GDB with our Xv6 labs. (If you start gdb and see a warning of the form 'warning: File ".../.gdbinit" auto-loading has been declined', edit ~/.gdbinit to add "add-auto-load-safe-path...", as suggested by the warning.)
Assembly Code - If you want to see what the assembly is that the compiler generates for the kernel or to find out what the instruction is at a particular kernel address, see the file kernel.asm, which the Makefile produces when it compiles the kernel. (The Makefile also produces .asm for all user programs.)
Kernel Panics - If the kernel panics, it will print an error message listing the value of the program counter when it crashed; you can search kernel.asm to find out in which function the program counter was when it crashed, or you can run addr2line -e kernel/kernel pc-value (run man addr2line for details). If you want to get backtrace, restart using gdb: run 'make qemu-gdb' in one window, run gdb (or riscv64-linux-gnu-gdb) in another window, set breakpoint in panic ('b panic'), followed by followed by 'c' (continue). When the kernel hits the break point, type 'bt' to get a backtrace.
Kernel Hangs - If your kernel hangs (e.g., due to a deadlock) or cannot execute further (e.g., due to a page fault when executing a kernel instruction), you can use gdb to find out where it is hanging. Run run 'make qemu-gdb' in one window, run gdb (riscv64-linux-gnu-gdb) in another window, followed by followed by 'c' (continue). When the kernel appears to hang hit Ctrl-C in the qemu-gdb window and type 'bt' to get a backtrace.