COS 318 : Operating system
Fall 2002, Princeton University
Project 2: Bootup Mechanism

This project involves writing the bootup code for a simple operating system that we will be developing for the rest of the semester. In this project, the bootup code will be in the so called real mode (instead of protected mode).

A PC can be booted up in two modes: cold boot or warm boot. Cold boot means you power down and power up the machine by pressing the power button of a PC. Warm boot means that you press <ctrl><alt><del> keys together. Either way, the PC will reset the processor and run a piece of code in the BIOS to try to read the first sector from the floppy drive if a diskette is in the drive, otherwise, it will try to read a sector from the hard disk drive. Once this sector is read in, the CPU will jump to the beginning of the loaded code.

Your job in this project is to implement two programs: bootblock.s and createimage.c. The bootblock.s is the boot code that resides on the boot sector and its responsibility is to load the rest of the operating system image from other sectors to the memory. The createimage code is a tool (in this case a Linux tool) to create a bootable operating system image including the bootblock and the rest of the operating system.

The "bootblock" and "createimage" from this assignment will be used throughout the semester.

You should use the code available in /u/cs318/1_pre/ as a starting point for your project. We provide you with five files:

Since our entire project will be using the floppy drive, please do not read/modify the hard disk.

Detailed requirements/hints for bootblock.s

This file should contain the code that will be written on the boot sector of the floppy disk. This has to be written in x86 assembly and shouldn't exceed 512 bytes (1 sector). This code will be responsible for loading in the rest of the operating system, setting up a stack for the kernel and then invoking it. You are required to handle kernels of size up to 36 sectors. You will get extra credit if your bootblock can handle sizes greater than 36 sectors.

The bootblock gets loaded at 0x07c0:0000. Your bootblock should load the OS starting at 0x0000:1000. In real mode, you have 640 KBytes starting at 0x0000:0000. The low memory area has some system data like the interrupt vectors and BIOS data. So, to avoid overwriting them, you should only use memory above 0x0000:1000 for this assignment.

To design and implement this, you need to learn about x86 architecture, CPU resets and how to read a sector from the floppy drive with BIOS (described below). We have provided you with a trivial and useless kernel (kernel.s) to test whether your implementation works.

Note: You can assume that the entire OS is less or equal to 36 sectors for this project.

Useful information:

Detailed requirements/hints for createimage.c

This program is a Linux tool to create a bootable image on a floppy diskette. To make your life a bit easy, we have provided you with a man page. You can view the man page by typing

man -M /u/cs318/man createimage

Please ignore the "-vm" option for this project.

You should be aware that two adjacent segments in an executable file may not be contiguous when they are loaded into memory, you should pad before each segment.   When copying segments from executable files into image file, you should pad after each segment to make sure that its length can be divided by 512 bytes.   The length of a segment in executable file may expand when loaded into memory.

You should read:

Submitting Programming Assignments

Submit your final solution electronically using the following command:

/u/cs318/bin/318submit 2 README Makefile bootblock.s createimage.c

The submit command copies your files to the directory /u/cs318/submit/UserLogin/number and lists all the files that you have submitted for assignment number. UserLogin is your user account name. If you execute submit after the project deadline, your files are placed in directory number_late. You can run submit more than once, and you can submit partial lists of files.

Each group needs to submit only one copy.  This means that only one of you needs to submit.

Low-level Details

Display memory

During booting, you can write directly to the screen by writing to the display RAM which is mapped starting at 0xb800:0000. Each location on the screen requires two bytes---one to specify the attribute (Use 0x07) and the second for the character itself. The text screen is 80x25 characters. So, to write to i-th row and j-th column, you write the 2 bytes starting at offset ((i-1)*80+(j-1))*2.

So, the following code sequence writes the character 'K' (ascii 0x4b) to the top left corner of the screen.
movw 0xb800,%bx

movw %bx,%es

movw $0x074b,%es:(0x0)

This code sequence is very useful for debugging.

BIOS Int 0x13 Function 2 (From Undocumented PC)

Reads a number of 512 bytes diskette sectors starting from a specified location. The data read is placed into RAM at the location specified by ES:BX. The buffer must be sufficiently large to hold the data AND must not cross a 64K linear address boundary.

Called with:
ah = 2

al = number of sectors to read, 1 to 36

ch = track number, 0 to 79

cl = sector number, 1 to 36

dh = head number, 0 or 1

dl = drive number, 0 to 3

es:bx = pointer where to place information read from diskette

Returns:
ah = return status (0 if successful)

al = number of sectors read

carry = 0 successful, = 1 if error occurred

BIOS Int 0x10 Function 0x0e (From Undocumented PC)

This function sends a character to the display at the current cursor position on the active display page. As necessary, it automatically wraps lines, scrolls and interprets some control characters for specific actions. Note : Linefeed is 0x0a and carriage return is 0x0d.

Called with:
ah = 0x0e

al = character to write

bh = active page number (Use 0x00)

bl = foreground color (graphics mode only) (Use 0x02)

Returns:
character displayed

CS318 Staff