Project 4: Writing a Bootloader from Scratch
Project 4: Writing a Bootloader from Scratch 15–410 Operating Systems April 23, 2007 1 Introduction 1.1 Overview In this project, you (and your partner) will be writing a bootloader that is ca- pable of booting your 410 kernel. In doing so, you will complete a full stack kernel+bootloader that is ideally capable of booting on commodity hardware (although such isn’t a strict requirement of this project). Furthermore, by writ- ing a bootloader, you will be exposed to a different style of systems program- ming in a slightly unusual environment. Fortunately there will be no concur- rency, no paging, and no protection issues to worry about. Unfortunately, there is also no memory manager and no libc—the only support code available are functions provided by the BIOS, and the ones you write yourself. Due to the constrained environment that the bootloader operates in, this project will heavily rely on services provided by the PC BIOS. Most of this project will involve low level programming in 16-bit real mode. As GCC does not emit real mode compliant code, most of this project will be programmed in assembly. However, portions of the bootloader will also be programmed in 32- bit protected mode with paging disabled, an environment that much resembles the operating environment of Project 1. 1.2 Goals • To learn (a small amount of) programming in x86 real mode, including the use of PC BIOS services. • To become familiar with reading sectors from a floppy disk, including the BIOS interface for interacting with the floppy disk controller as well as the floppy disk CHS (cylinder, head, sector) layout.
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