Advanced OS Lab 2 - Booting a PC


This lab is split into three parts. The first part concentrates on getting familiarized with x86 assembly language, the Bochs x86 emulator, and the PC's power-on bootstrap procedure. The second part examines the boot loader for our class kernel, which resides in the boot directory of the lab2 tree. Finally, the third part delves into the initial template for our class kernel itself, named JOS, which resides in the kernel directory.

Software Setup

The files you will need for this lab are available in ~class/src/lab2.tar.gz. To install them in your account, do this:

% tar xzf ~class/lab2.tar.gz
% cd lab2
% setenv GCCPREFIX i386-jos-elf-

We have set up the appropriate compilers and simulators for you on the class machines. If you are instead working on your own machine, the tools page has directions on how to set up bochs and gcc for use with the lab.

Hand-In Procedure

When you are ready to hand in your lab, run gmake handin in the lab2 directory. This will submit all the source code in your lab2 directory.

You do not need to turn in answers to any of the questions in the text of the lab. (Do answer them for yourself though! They will help with the rest of the lab.)

We will be grading your solutions with a grading program. You can run gmake grade to test your solutions with the grading program.

Part 1: PC Bootstrap

The purpose of the first exercise is to introduce you to x86 assembly language and the PC bootstrap process, and to get you started with the Bochs debugger. You will not have to write any code for this part of the lab, but you should go through it anyway for your own understanding and be prepared to answer the questions posed below.

Getting Started with x86 assembly

If you are not already familiar with x86 assembly language, you will quickly become familiar with it during this course! The PC Assembly Language Book is an excellent place to start. Hopefully, the book contains mixture of new and old material for you.

Warning: Unfortunately the examples in the book are written for the NASM assembler, whereas we will be using the GNU assembler. NASM uses the so-called Intel syntax while GNU uses the AT&T syntax. While semantically equivalent, an assembly file will differ quite a lot, at least superficially, depending on which syntax is used. Luckily the conversion between the two is pretty simple, and is covered in Brennan's Guide to Inline Assembly.

Exercise 1. Read or at least carefully scan the entire PC Assembly Language book, except that you should skip all sections after 1.3.5 in chapter 1, which talk about features of the NASM assembler that do not apply directly to the GNU assembler. You may also skip chapters 5 and 6, and all sections under 7.2, which deal with processor and language features we won't use in class.

Also read the section "The Syntax" in Brennan's Guide to familiarize yourself with the most important features of GNU assembler syntax.

Certainly the definitive reference for x86 assembly language programming is Intel's instruction set architecture reference, which you can find on the class reference page in two flavors: an HTML edition of the old 80386 Programmer's Reference Manual, which is much shorter and easier to navigate than more recent manuals but describes all of the x86 processor features that we will make use of in class; and the full, latest and greatest IA-32 Intel Architecture Software Developer's Manuals from Intel, covering all the features of the most recent processors that we won't need in class but you may be interested in learning about. An equivalent (but even longer) set of manuals is available from AMD, which also covers the new 64-bit extensions now appearing in both AMD and Intel processors.

You should read the recommended chapters of the PC Assembly book, and "The Syntax" section in Brennan's Guide now. Save the Intel/AMD architecture manuals for later or use them for reference when you want to look up the definitive explanation of a particular processor feature or instruction.

Simulating the x86

Instead of developing the operating system on a real, physical personal computer (PC), we use a simulator, which emulates a complete PC faithfully (i.e., the code you write for the simulator, boots on a real PC too). Using a simulator simplifies debugging; you can, for example, set break points inside of the simulated x86, which is difficult to do with the silicon-version of an x86.

In class we will use the Bochs PC Emulator. This emulator has been around for quite a while, and is slow and quirky but has a great many useful features. Another freely available PC emulator is QEMU, which is much faster than Buchs but has less mature debugging facilities. You are welcome to give QEMU a try (or any of the commercially available PC virtual machine programs), but our class lab assignments will assume (and sometimes require) that you are running under Bochs.

To get started, extract the Lab 2 files into your own directory on a class machine as described above in "Software Setup", then type gmake in the lab2 directory to build the minimal class boot loader and kernel you will start with. (It's a little generous to call the code we're running here a "kernel," but we'll flesh it out throughout the semester.)

% cd lab2
% gmake
as kern/entry.S
cc kern/init.c
cc kern/console.c
cc kern/monitor.c
cc lib/printf.c
cc lib/printfmt.c
cc lib/readline.c
cc lib/string.c
ld obj/kern/kernel
as boot/boot.S
cc boot/main.c
mk boot/boot
boot block is 424 bytes (max 510)
mk obj/kern/bochs.img

Now you're ready to run Bochs. The necessary configuration file for Bochs, named .bochsrc, is already supplied for you in the lab2 directory. This .bochsrc includes a command to make Bochs use the file obj/kern/bochs.img, created above, as the contents of the simulated PC's "virtual hard disk" once Bochs starts running. This simulated hard disk image contains both our boot loader (obj/boot/boot) and our kernel (obj/kern/kernel).

% bochs
                       Bochs x86 Emulator 2.1.1
                           February 08, 2004
00000000000i[     ] reading configuration from .bochsrc
Bochs Configuration: Main Menu
This is the Bochs Configuration Interface, where you can describe the
machine that you want to simulate.  Bochs has already searched for a
configuration file (typically called bochsrc.txt) and loaded it if it
could be found.  When you are satisfied with the configuration, go
ahead and start the simulation.
You can also start bochs with the -q option to skip these menus.
1. Restore factory default configuration
2. Read options from...
3. Edit options
4. Save options to...
5. Begin simulation
6. Quit now
Please choose one: [5]
Bochs has read the file .bochsrc describing the virtual x86 PC it will emulate for our kernel, and it is stopping to give you an opportunity to change the settings if desired before beginning the actual simulation. Since the configuration settings are already correct, just press Enter to start the simulation. (As Bochs points out, you can bypass this step in the future by typing 'bochs -q' instead of just 'bochs'.)
Next at t=0
(0) [0x000ffff0] f000:fff0 (unk. ctxt): jmp f000:e05b             ; ea5be000f0
Bochs has now started the simulated machine, and is ready to execute the first instruction. An X window should have popped up to show the "virtual display" of the simulated PC. The window is blank because the simulated PC hasn't actually booted yet - it's frozen in the state a real PC would be in just after being turned on and coming out of hardware reset but before executing any instructions.

The text that Bochs printed on your normal shell window, and the <bochs:1> prompt, is part of the Bochs debugging interface, which you can use to control and examine the state of the simulated PC. The main reference for this debugging interface that you should become familiar with is the section Using Bochs internal debugger in the Bochs User Manual. You can always get a reminder of the names of the most common commands by typing help:

<bochs:1> help
help - show list of debugger commands
help 'command'- show short command description
-*- Debugger control -*-
    help, q|quit|exit, set, instrument, show, trace-on, trace-off,
    record, playback, load-symbols, slist
-*- Execution control -*-
    c|cont, s|step|stepi, p|n|next, modebp
-*- Breakpoint management -*-
    v|vbreak, lb|lbreak, pb|pbreak|b|break, sb, sba, blist,
    bpe, bpd, d|del|delete
-*- CPU and memory contents -*-
    x, xp, u|disas|disassemble, r|reg|registers, setpmem, crc, info, dump_cpu,
    set_cpu, ptime, print-stack, watch, unwatch, ?|calc

For now, just type c to "continue" (i.e., start) execution from the current point. Some text should now appear in the Bochs window, ending with:

Booting from Hard Disk...
480 decimal is XXX octal!
entering test_backtrace 5
entering test_backtrace 4
entering test_backtrace 3
entering test_backtrace 2
entering test_backtrace 1
entering test_backtrace 0
leaving test_backtrace 0
leaving test_backtrace 1
leaving test_backtrace 2
leaving test_backtrace 3
leaving test_backtrace 4
leaving test_backtrace 5
Welcome to the JOS kernel monitor!
Type 'help' for a list of commands.

Everything after 'Booting from Hard Disk...' was printed by our skeletal JOS kernel; the K> is the prompt printed by the small monitor, or interactive control program, that we've included in the kernel. These four lines printed by the kernel will also appear in the regular shell window from which you ran Bochs. This is because for testing and lab grading purposes we have set up the JOS kernel to write its console output not only to the virtual VGA display (as seen in the Bochs window), but also to the simulated PC's virtual parallel port, which Bochs outputs to its own standard output because of a particular line we included in our .bochsrc. (Identify that line!)

The kernel monitor is currently pretty boring; it only knows about two not particularly useful commands:

K> help
help - display this list of commands
kerninfo - display information about the kernel
K> kerninfo
Special kernel symbols:
  start  f0100020 (virt)  00100020 (phys)
  etext  f01013a8 (virt)  001013a8 (phys)
  edata  f010ac08 (virt)  0010ac08 (phys)
  end    f010b268 (virt)  0010b268 (phys)
Kernel executable memory footprint: 45KB

The help command is obvious, and we will shortly discuss the meaning of what the kerninfo command prints. Although simple, it's important to note that this kernel monitor is running "directly" on the "raw (virtual) hardware" of the simulated PC. This means that you should be able to copy the contents of obj/kern/bochs.img onto the first few sectors of a real hard disk, insert that hard disk into a real PC, turn it on, and see exactly the same thing on the PC's real screen as you did above in the Bochs window. (We don't recommend you do this on a real machine with useful information on its hard disk, though, because copying bochs.img onto the beginning of its hard disk will trash the master boot record and the beginning of the first partition, effectively causing everything previously on the hard disk to be lost!)

Exercise 2. Scan through the Bochs internal debugger section of the Bochs user manual to get a feel for these commands and their syntax. Play with the commands a little: do some stepping and tracing through the code, examining CPU registers and memory and disassembling instructions at different points, without worrying too much yet about what the code is actually doing. While the kernel monitor is waiting for user input (or at any other time the simulation is running), you can always hit CTRL-C in the shell window from which you ran Bochs in order to halt the simulation and break back into the Bochs debugger. Be sure you understand the distinction between which software you're interacting with when you type commands in the kernel monitor versus in the Bochs debugger.

The PC's Physical Address Space

We will now dive into a bit more detail about how a PC starts up. A PC's physical address space is hard-wired to have the following general layout:

+------------------+  <- 0xFFFFFFFF (4GB)
|      32-bit      |
|  memory mapped   |
|     devices      |
|                  |

|                  |
|      Unused      |
|                  |
+------------------+  <- depends on amount of RAM
|                  |
|                  |
| Extended Memory  |
|                  |
|                  |
+------------------+  <- 0x00100000 (1MB)
|     BIOS ROM     |
+------------------+  <- 0x000F0000 (960KB)
|  16-bit devices, |
|  expansion ROMs  |
+------------------+  <- 0x000C0000 (768KB)
|   VGA Display    |
+------------------+  <- 0x000A0000 (640KB)
|                  |
|    Low Memory    |
|                  |
+------------------+  <- 0x00000000
The first PCs, which were based on the 16-bit Intel 8088 processor, were only capable of addressing 1MB of physical memory. The physical address space of an early PC would therefore start at 0x00000000 but end at 0x000FFFFF instead of 0xFFFFFFFF. The 640KB area marked "Low Memory" was the only random-access memory (RAM) that an early PC could use; in fact the very earliest PCs only could be configured with 16KB, 32KB, or 64KB of RAM!

The 384KB area from 0x000A0000 through 0x000FFFFF was reserved by the hardware for special uses such as video display buffers and firmware held in nonvolatile memory. The most important part of this reserved area is the Basic Input/Output System (BIOS), which occupies the 64KB region from 0x000F0000 through 0x000FFFFF. In early PCs the BIOS was held in true read-only memory (ROM), but current PCs store the BIOS in updateable flash memory. The BIOS is responsible for performing basic system initialization such as activating the video card and checking the amount of memory installed. After performing this initialization, the BIOS loads the operating system from some appropriate location such as floppy disk, hard disk, CD-ROM, or the network, and passes control of the machine to the operating system.

When Intel finally "broke the one megabyte barrier" with the 80286 and 80386 processors, which supported 16MB and 4GB physical address spaces respectively, the PC architects nevertheless preserved the original layout for the low 1MB of physical address space in order to ensure backward compatibility with existing software. Modern PCs therefore have a "hole" in physical memory from 0x000A0000 and 0x00100000, dividing RAM into "low" or "conventional memory" (the first 640KB) and "extended memory" (everything else). In addition, some space at the the very top of the PC's 32-bit physical address space, above all physical RAM, is now commonly reserved by the BIOS for use by 32-bit PCI devices.

With recent x86 processors it is now possible in fact for PCs to have more than 4GB of physical RAM, which means that RAM can extend further above 0XFFFFFFFF. In this case the BIOS must therefore arrange to leave a second hole in the system's RAM at the top of the 32-bit addressable region, in order to leave room for these 32-bit devices to be mapped. Because of design limitations JOS will actually be limited to using only the first 256MB of a PC's physical memory anyway, so for now we will pretend that all PCs still have "only" a 32-bit physical address space. But dealing with complicated physical address spaces and other aspects of hardware organization that evolved over many years is one of the important practical challenges of OS development.


Close and restart Bochs, so that you once again see the first instruction to be executed:
Next at t=0
(0) [0x000ffff0] f000:fff0 (unk. ctxt): jmp f000:e05b             ; ea5be000f0

From this output you can conclude a few things:

Why does Bochs start like this? This is how Intel designed the 8088 processor, which IBM used in their original PC. Because the BIOS in a PC is "hard-wired" to the physical address range 0x000f0000-0x000fffff, this design ensures that the BIOS always gets control of the machine first after power-up or any system restart - which is crucial because on power-up there is no other software anywhere in the machine's RAM that the processor could execute. The Bochs simulator comes with its own BIOS, which it maps at this location in the processor's simulated physical address space. On processor reset, the (simulated) processor sets CS to 0xf000 and the IP to 0xfff0, and consequently, execution begins at that (CS:IP) segment address. But how did the segmented address 0xf000:fff0 turn into the physical 0x000ffff0 we mentioned above?

To answer that we need to know a bit about real mode addressing. In real mode (the mode that PC starts off in), address translation works according to the formula: physical address = 16 * segment + offset. So, when the PC sets CS to 0xf000 and IP to 0xfff0, the physical address referenced is:

   16 * 0xf000 + 0xfff0   # in hex multiplication by 16 is
   = 0xf0000 + 0xfff0     # easy--just append a 0.
   = 0xffff0 

We can see that the PC starts executing 16 bytes from the end of the BIOS code. Therefore we shouldn't be surprised that the first thing that the BIOS does is jmp backwards to an earlier location in the BIOS; after all how much could it accomplish in just 16 bytes?

Exercise 3. Use the Bochs debugger to trace into the ROM BIOS for a few more instructions, and try to guess what it might be doing. You might find the common I/O address assignments table in Phil Storrs PC Hardware book handy, as well as other materials on the class reference materials page. No need to figure out all the details - just the general idea of what the BIOS is doing first.

When the BIOS runs, it sets up an interrupt descriptor table and initializes various devices such as the VGA display. This is where the "Plex86/Bochs VGABios" messages you see in the Bochs window come from. How can you find out exactly where in the BIOS this is happening? It happens that while in text mode, the VGA display is mapped in memory at address 0xb8000, and you can use a data watchpoint, or a breakpoint that fires when a particular memory location is read or written (instead of executed), to find out when and where the BIOS is writing these messages to the display.

To set a data watchpoint:

<bochs:1> watch write 0xb8000
<bochs:2> watch stop
<bochs:3> c
The first line sets the watchpoint. The second line instructs bochs to stop the simulation whenever a watchpoint fires.

Exercise 4. Set a watchpoint in the video display memory as instructed above. How many times is the watchpoint hit between when Bochs starts running and when the BIOS transfers control to the boot loader? Is it the "main" ROM BIOS in the 0x000f0000-0x000fffff region that is performing this task, or something else? Think about why the PC is designed this way.

After initializing the PCI bus and all the important devices the BIOS knows about, it searches for a bootable device such as a floppy, hard drive, or CD-ROM. Eventually, when it finds a bootable disk, the BIOS reads the boot loader from the disk and transfers control to it.

Part 2: The Boot Loader

Floppy and hard disks for PCs are by historical convention divided up into 512 byte regions called sectors. A sector is the disk's minimum transfer granularity: each read or write operation must be one or more sectors in size and aligned on a sector boundary. If the disk is bootable, the first sector is called the boot sector, since this is where the boot loader code resides. When the BIOS finds a bootable floppy or hard disk, it loads the 512-byte boot sector into memory at physical addresses 0x7c00 through 0x7dff, and then uses a jmp instruction to set the CS:IP to 0000:7c00, passing control to the boot loader. Like the BIOS load address, these addresses are fairly arbitrary - but they are fixed and standardized for PCs.

The ability to boot from a CD-ROM came much later during the evolution of the PC, and as a result the PC architects took the opportunity to rethink the boot process slightly. As a result, the way a modern BIOS boots from a CD-ROM is a bit more complicated (and more powerful). CD-ROMs use a sector size of 2048 bytes instead of 512, and the BIOS can load a much larger boot image from the disk into memory (not just one sector) before transferring control to it. For more information, see the "El Torito" Bootable CD-ROM Format Specification.

For class, however, we will use the conventional hard drive boot mechanism, which means that our boot loader must fit into a measly 512 bytes. The boot loader consists of one assembly language source file, boot/boot.S, and one C source file, boot/main.c Look through these source files carefully and make sure you understand what's going on. The boot loader must perform two main functions:

  1. First, the boot loader switches the processor from real mode to 32-bit protected mode, because it is only in this mode that software can access all the memory above 1MB in the processor's physical address space. Protected mode is described briefly in sections 1.2.7 and 1.2.8 of PC Assembly Language, and in great detail in the Intel architecture manuals. At this point you only have to understand that translation of segmented addresses (segment:offset pairs) into physical addresses happens differently in protected mode, and that after the transition offsets are 32 bits instead of just 16.
  2. Second, the boot loader reads the kernel from the hard disk by directly accessing the IDE disk device registers via the x86's special I/O instructions. If you would like to understand better what the particular I/O instructions here actually mean, check out the "IDE hard drive controller" section on the class reference page. You will not need to learn much about programming specific devices in this class: writing device drivers is in practice a very important part of OS development, but from a conceptual or architectural viewpoint it is also one of the least interesting.

After you understand the boot loader source code, look at the file obj/boot/boot.asm. This file is a disassembly of the boot loader that our GNUmakefile creates after compiling the boot loader. This disassembly file makes it easy to see exactly where in physical memory all of the boot loader's code resides, and makes it easier to track what's happening while stepping through the boot loader in Bochs.

You can set breakpoints in Bochs with the b command. You have to give the base explicitly, so say something like b 0x7c00 for hexadecimal. A full command overview is at From the debugger, you can continue execution using the c and s commands: c causes Bochs to continue execution indefinitely; and s allows you to step through the instructions, executing exactly n instructions (a default of 1 if the parameter n is not specified) before suspending execution again. trace-on and trace-off can be used to set tracing before using the other commands.

To examine instructions in memory (besides the immediate next one to be executed, which Bochs prints automatically), you use the u command. This command has the syntax u/n addr, where n is the number of consecutive instructions to disassemble and addr is the memory address at which to start disassembling.

Exercise 5. Set a breakpoint at address 0x7c00, which is where the boot sector will be loaded. Continue execution until that break point. Trace through the code in boot/boot.S, using the source code and the disassembly file obj/boot/boot.asm to keep track of where you are. Also use the u command in Bochs to disassemble sequences of instructions in the boot loader, and compare the original boot loader source code with both the GNU disassembly in obj/boot/boot.asm and the Bochs disassembly from the u command.

Trace into cmain() in boot/main.c, and then into read_sector(). Identify the exact assembly instructions that correspond each of the statements in read_sector(). Trace through the rest of read_sector() and back out into cmain(), and identify the begin and end of the for loop that reads the remaining sectors of the kernel from the disk. Find out what code will run when the loop is finished, set a breakpoint there, and continue to that breakpoint. Then step through the remainder of the boot loader.

Be able to answer the following questions:

Challenge! Make JOS boot under Bochs from a simulated CD-ROM. You will need to learn about the mkisofs utility and will have to modify the .bochsrc appropriately.

Loading the Kernel

We will now look in further detail at the C language portion of the boot loader, in boot/main.c. To make sense out of boot/main.c you'll need to know what an ELF binary is. When you compile and link a C program such as the JOS kernel, the compiler transforms each C source ('.c') file into an object ('.o') file containing assembly language instructions encoded in the compact binary format expected by the hardware. The linker then combines all of the compiled object files into a single binary image such as obj/kern/kernel, which in this case is a binary in the ELF format, which stands for "Executable and Linkable Format".

Full information about this format is available in the ELF specification on our reference page, but you will not need to delve very deeply into the details of this format in this class. Although as a whole the format is quite powerful and complex, most of the complex parts are for supporting dynamic loading of shared libraries, which we will not do in this class.

For purposes of this class, you can consider the contents of an ELF executable to be (mostly) just a short, fixed-length header with important loading information, followed by several program sections, which are just contiguous chunks of code or data intended to be loaded byte-for-byte into memory at a fixed, pre-computed address before transferring control to the program. The loader does nothing to the code or data at load time; it must be ready to go.

An ELF binary starts with a fixed-length ELF header, followed by a variable-length program header listing each of the program sections to be loaded. The C definitions for these ELF headers are located in inc/elf.h. The relevant sections for our purposes are named as follows:

These conventional section names obviously reflect the processor's viewpoint: anything that humans would consider "text", such as ASCII strings generated by the C compiler from string literals in the source code, will actually be found in the '.rodata' section.

While the linker is computing the memory layout of a program, it reserves memory space for all uninitialized global variables, such as just 'int x;', in yet another, special section called '.bss' that immediately follows the data section in memory. Since this section is supposed to be "uninitialized", however - or rather, initialized to a default value of all zeros, as required for all global variables in C - there is no need for the contents of this section to be stored in the ELF binary file. Instead, the linker simply records the address and size of the bss section in the ELF program header along with the sizes of the other sections to be loaded, and leaves it to the loader (or in some cases the program itself) to zero the bss section.

You can display a full list of the names, sizes, and link addresses of all the sections in the kernel executable by typing:

% i386-jos-elf-objdump -h obj/kern/kernel

obj/kern/kernel:     file format elf32-i386

Idx Name          Size      VMA       LMA       File off  Algn
  0 .text         0000105e  f0100020  f0100020  00001020  2**2
  1 .rodata       00000443  f0101080  f0101080  00002080  2**2
  2 .data         00008b80  f0102000  f0102000  00003000  2**12
                  CONTENTS, ALLOC, LOAD, DATA

You will see many more sections than the ones we listed above, but the others are not important for our purposes. Most of the others are to hold debugging information, which is typically included in the program's executable file but not actually loaded into memory by the program loader.

Besides the section information, there is one more field in the ELF header that is important to us, named e_entry. This field holds the link address of the entry point in the program: the memory address in the program's text section at which the program is supposed to begin executing.

To examine memory in the Bochs simulator, you use the x command, which has the same syntax as gdb's. The command overview (linked above) has full details. For now, it is enough to know that the recipe x/nx addr prints n words of memory at addr. (Note that both 'x's in the command are lowercase.)

Warning: The size of a word is not a universal standard. To Bochs, a word is four bytes. In GNU assembly, a word is two bytes (the 'w' in xorw, which stands for word, means 2 bytes).

Exercise 6. Reset the machine (exit bochs and start it again). Examine the 8 words of memory at 0x00100000 at the point the BIOS enters the boot loader, and then again at the point the boot loader enters the kernel. Why are they different? What is there at the second breakpoint? (You do not really need to use Bochs to answer this question. Just think.)

Link vs. Load Address

The load address of a binary is the memory address at which a binary is actually loaded. For example, the BIOS is loaded by the PC hardware at address 0xf0000. So this is the BIOS's load address. Similarly, the BIOS loads the boot sector at address 0x7c00. So this is the boot sector's load address.

The link address of a binary is the memory address for which the binary is linked. Linking a binary for a given link address prepares it to be loaded at that address. A program's link address in practice becomes subtly encoded within the binary in a multitude of ways, with the result that if a binary is not loaded at the address that it is linked for, things will not work.

In one sentence: the link address is the location where a binary assumes it is going to be loaded, while the load address is the location where a binary actually is loaded. It's up to us to make sure that they turn out to be the same.

Look at the linker commands in boot/Makefrag and kern/Makefrag used to build our boot loader and kernel, and you will find a (different) -Ttext option in each case, specifying the link address for the binary.

Exercise 7. Trace through the first few instructions of the boot loader again and identify the first instruction that would "break" or otherwise do the wrong thing if you were to get the boot loader's link address wrong. Then change the link address in boot/Makefrag to something wrong, recompile the boot loader, and trace into it again to see what happens. Don't forget to change it back afterwards!

When object code contains no absolute addresses that ``subtly encode'' the link address in this fashion, we say that the code is position-independent: it will behave correctly no matter where it is loaded. GCC can generate position-independent code using the -fpic option, and this feature is used extensively in modern shared libraries that use the ELF executable format. Position independence typically has some performance cost, however, because it restricts the ways in which the compiler may choose instructions to access the program's data. We will not use position-independent code at all in 6.828, simply because we have no pressing need to.

Part 3: The Kernel

We will now start to examine the minimal JOS kernel in a bit more detail. (And you will finally get to write some code!) Like the boot loader, the kernel begins with some assembly language code that sets things up so that C language code can execute properly.

Using segmentation to work around position dependence

Did you notice above that while the boot loader's link and load addresses match perfectly, there appears to be a (rather large) disparity between the kernel's link and load addresses? Go back and check both and make sure you can see what we're talking about.

Operating system kernels often like to be linked and run at very high virtual address, such as 0xf0100020, in order to leave the lower part of the processor's virtual address space for user programs to use. The reason for this arrangement will become clearer in the next lab. Most machines don't even have that much physical memory, however. (How much would it be exactly?)

Since we can't actually load the kernel at physical address 0xf0100020, we will use the processor's memory management hardware to map virtual address 0xf0100020 - the link address at which the kernel code expects to run - to physical address 0x00100020 - where the boot loader actually loaded the kernel. This way, although the kernel's virtual address is high enough to leave plenty of address space for user processes, it will be loaded in physical memory at the 1MB point in the PC's RAM, just above the BIOS ROM.

In fact, we will actually map the entire bottom 256MB of the PC's physical address space, from 0x00000000 through 0x0fffffff, to virtual addresses 0xf0000000 through 0xffffffff respectively. You should now be able to see why the JOS kernel is limited to using only the first 256MB of physical memory in a PC.

The x86 processor has two distinct memory management mechanisms that we could use to achieve this mapping: segmentation and paging. Both are described in full detail in the 80386 Programmer's Reference Manual or the IA-32 Developer's Manual, Volume 3. When the JOS kernel first starts up, we'll initially use segmentation to establish our desired virtual-to-physical mapping, because it is quick and easy - and the x86 processor requires us to set up the segmentation hardware in any case, because it can't be disabled!

Exercise 8. Use Bochs to trace into the JOS kernel, and identify the exact point at which the new virtual-to-physical mapping takes effect. Then examine the Global Descriptor Table (GDT) that the code uses to achieve this effect, and make sure you understand what's going on.

What is the first instruction after this point that would fail to work properly if this virtual-to-physical mapping wasn't in place? Comment out or otherwise intentionally "break" the segmentation setup code in kern/entry.S, trace into it in Bochs, and see if you were right.

Formatted Printing to the Console

Most people take functions like printf() for granted, sometimes even thinking of them as "primitives" of the C language. But in an OS kernel, all I/O of any kind that we do, we have to implement ourselves!

Read through lib/printf.c, lib/printfmt.c, and kern/console.c, and make sure you understand their relationship. It will become clear in later labs why the first two source files are located in the separate lib directory.

Exercise 9. We have omitted a small fragment of code - the code necessary to print octal numbers using patterns of the form "%o". Find and fill in this code fragment.

Be able to answer the following questions:

  1. Explain the interface between printf.c and console.c. Specifically, what function does console.c export? How is this function used by printf.c?
  2. Explain the following from console.c:
    1       if (crt_pos >= CRT_SIZE) {
    2               int i;
    3               memcpy(crt_buf, crt_buf + CRT_COLS, CRT_SIZE << 1);
    4               for (i = CRT_SIZE - CRT_COLS; i < CRT_SIZE; i++)
    5                       crt_buf[i] = 0x0700 | ' ';
    6               crt_pos -= CRT_COLS;
    7       }
  3. For the following questions you might wish to consult the notes for Lecture 2. These notes cover GCC's calling convention on the x86.

    Trace the execution of the following code step-by-step:

    int x = 1, y = 3, z = 4;
    warn("x %d, y %x, z %d\n", x, y, z);
  4. Run the following code.
        u_int i = 0x00646c72;
        warn("H%x Wo%s", 57616, &i);
    What is the output? Explain how this output is arrived out in the step-by-step manner of the previous exercise.

    The output depends on that fact that the x86 is little-endian. If the x86 were instead big-endian what would you set i to in order to yield the same output? Would you need to change 57616 to a different value?

    Here's a description of little- and big-endian and a more whimsical description.

  5. In the following code, what is going to be printed after 'y='? (note: the answer is not a specific value.) Why does this happen?
        warn("x=%d y=%d", 3);
  6. Let's say that GCC changed its calling convention so that it passed arguments in declaration order (i.e., the opposite of reverse order). How would you have to change printf or its interface so that it would still be possible to pass it a variable number of arguments?
Challenge! Enhance the console to allow text to be printed in different colors. The traditional way to do this would be to make it interpret ANSI escape sequences embedded in the text strings printed to the console, but you may use any mechanism you like. There is plenty of information on the references page and elsewhere on the web on programming the VGA display hardware. If you're feeling really adventurous, you could try switching the VGA hardware into a graphics mode and making the console draw text onto the graphical frame buffer.

The Stack

In the final exercise of this lab, we will explore in more detail the way the C language uses the stack on the x86, and in the process write a useful new kernel monitor function that prints a backtrace of the stack: a list of the saved Instruction Pointer (IP) values that can be used to determine the exact context in which a particular piece of C code was called.

Exercise 10. Determine where the kernel initializes its stack, and exactly where in memory its stack is located. How does the kernel reserve space for its stack? And at which "end" of this reserved area is the stack pointer initialized to point to?

In C programs on the x86, both the esp (stack pointer) and ebp (base pointer) registers typically have special meanings. The stack pointer points to the current dividing point in the stack area between "free" stack space and "in use" stack space. Since the stack grows down on the x86 (and, historically, most other processors, with a few exceptions), at a given time the stack pointer points to the first "in use" byte of the stack; everything below that location in the region reserved for the stack is considered "free". Pushing values onto the stack decreases the stack pointer, and popping values off the stack increases the stack pointer. Various x86 processor instructions, such as call are "hard-wired" to use the stack pointer register in specific ways.

The ebp, in contrast, is associated with the stack primarily by software convention. On entry to a C function, the function's prologue code normally saves the previous function's base pointer by pushing it onto the stack, and then copies the current esp value into ebp for the duration of the function. If all the functions in a program consistently obey this convention, then at any given point during the program's execution, it is possible to trace back through the stack by following the chain of saved ebp pointers and determining exactly what nested sequence of function calls caused this particular point in the program to be reached. This capability can be particularly useful, for example, when a particular function causes an assert failure or panic because bad arguments were passed to it, but you aren't sure who passed the bad arguments. With stack backtrace functionality, you can trace back and find the offending function.

Exercise 11. To become familiar with the C calling conventions on the x86, find the address of the test_backtrace function in obj/kern/kernel.asm, set a breakpoint there in Bochs, and examine what happens each time it gets called after the kernel starts. How many 32-bit words does each recursive nesting level of test_backtrace push on the stack, and what are those words?

The above exercise should give you the information you need to implement a stack backtrace function, which you should call mon_backtrace(). A prototype for this function is already waiting for you in kern/monitor.c. You can do it entirely in C, but you may find the read_ebp() function in inc/x86.h useful. You'll also have to hook this new function into the kernel monitor's command list so that it can actually be invoked interactively by the user.

The backtrace function should display a listing of function call frames in the following format:

Stack backtrace:
  ebp f0109e58  eip f0100a62  args 00000001 f0109e80 f0109e98 f0100ed2 00000031
  ebp f0109ed8  eip f01000d6  args 00000000 00000000 f0100058 f0109f28 00000061

The first line printed reflects the currently executing function, namely mon_backtrace itself, the second line reflects the function that called mon_backtrace, the third line reflects the function that called that one, and so on. You should print all the outstanding stack frames, not just a specific number for example. By studying kern/entry.S you'll find that there is an easy way to tell when to stop.

Within each line, the ebp value indicates the base pointer into the stack used by that function: i.e., the position of the stack pointer just after the function was entered and the function prologue code set up the base pointer. The listed eip value is the function's return instruction pointer: the instruction address (hopefully in the kernel's text section) where control will return to when the function returns. This address typically indicates exactly the point from which the function in question was called. (More accurately, the return eip usually points to the instruction immediately following the relevant call instruction - why?) Finally, the five hex values listed after args are the first five arguments to the function in question, which would have been pushed on the stack just before the function was called. If the function was called with fewer than five arguments, of course, then not all five of these values will be useful. (Why can't the backtrace code detect how many arguments there actually are? How could this limitation be fixed?)

Exercise 12. Implement the backtrace function as specified above. When you think you have it working right, run gmake grade to see if its output conforms to what our grading script expects, and fix it if it doesn't. After you have handed in your Lab 2 code, you are welcome to change the output format of the backtrace function any way you like.
Challenge! Modify the JOS boot loader to load the symbol table from the kernel's ELF binary in addition to the text and data sections, and then modify your stack backtrace function to look up and display the name of each function called in the backtrace in addition to the return eip. You may find this enhancement particularly useful for debugging in future labs.

This completes the lab. Type gmake handin in the lab2 directory to submit your solutions.