Before starting on this assignment, make sure you have gone through the guide to setting your system up for development (available in the Resources section in NYU classes). You will need the following for this assignment:

All of these should be installed already as part of Homework 1. You should also download the Makefile I have written for this assignment, which will allow you to build your bootloader by just typing make guess.

Bare Metal Assembly

When the computer first boots, the BIOS initializes the hardware, and then passes control to the bootloader. At this point, the CPU is in what's called 16-bit real mode, meaning that it's essentially compatible with the original 8086 PC. Generally, the OS bootloader will try to get out of real mode as quickly as possible, but for now we'll write a small program to get used to assembly programming and get a taste of how things work early on in boot.

Once the BIOS has done basic initialization of hardware, it will try to read a boot sector off of the boot medium. This could be a floppy disk, a hard drive, or whatever else the BIOS can figure out how to access (most modern BIOSes even allow booting off of the network). The BIOS reads 512 bytes from this device, checks to make sure it ends with the two bytes 0x55 0xAA, loads it at address 0x7C00, and then begins execution.

If we want to write a program that executes this early in boot, we will have to write it in 16-bit assembly and then assemble it into a binary boot sector. We can do this with GNU as (also known as gas; it's the standard open-source assembler, and is also used by xv6). Here's an example program that just prints "Hello world" to the console and then waits forever. It uses BIOS interrupts to set up the right video mode and then prints out a message character by character.

.code16         # Use 16-bit assembly
.globl start    # This tells the linker where we want to start executing

start: 
    movw $message, %si  # load the offset of our message into %si
    movb $0x00,%ah      # 0x00 - set video mode
    movb $0x03,%al      # 0x03 - 80x25 text mode
    int $0x10           # call into the BIOS

print_char: 
    lodsb           # loads a single byte from (%si) into %al and increments %si
    testb %al,%al   # checks to see if the byte is 0
    jz done         # if so, jump out (jz jumps if ZF in EFLAGS is set)
    movb $0x0E,%ah  # 0x0E is the BIOS code to print the single character
    int $0x10       # call into the BIOS using a software interrupt
    jmp print_char  # go back to the start of the loop

done: 
    jmp done        # loop forever

# The .string command inserts an ASCII string with a null terminator
message:
    .string    "Hello world"

# This pads out the rest of the boot sector and then puts
# the magic 0x55AA that the BIOS expects at the end, making sure
# we end up with 512 bytes in total.
# 
# The somewhat cryptic "(. - start)" means "the current address
# minus the start of code", i.e. the size of the code we've written
# so far. So this will insert as many zeroes as are needed to make
# the boot sector 510 bytes log, and 

.fill 510 - (. - start), 1, 0
.byte 0x55
.byte 0xAA

Make sure to read the comments (anything after a # symbol) and fully understand what each statement does.

If you put that code into a file called hello.s in the same directory as the Makefile, you can then create a boot sector named hello from it by typing make hello, which will invoke the assembler and linker to produce the 512 byte boot sector:

You can see that it's 512 bytes using ls, and (if you like) look at the raw bytes that will be loaded into memory with xxd:

Homework: Guessing Game

Write an x86 boot sector that implements a simple guessing game. You should ask the user for a number from 0 to 9, read their response, and then tell them if they got it right or wrong. If they got it right, print a success message and then loop forever.

Submit a commented assembly file named guess.s that assembles to a 512-byte boot sector. The boot sector should be able to run in QEMU with the command line:

Prerequisites

Bare Metal Assembly

Homework: Guessing Game

Further Reading

Submit