Principle and introduction of disk boot program

First, Introduction

• System boot or restart

1. BIOS Power On Self test-post. The BIOS performs a jump instruction at the memory address of ffff:0000h, jumps to the self-test program that is cured in ROM, and checks the system hardware (including memory).
2. Reads the master boot record (MBR). When the BIOS checks that the hardware is healthy and matches the settings in the CMOS, the available boot devices are detected in the CMOS setup sequence for the boot device. The BIOS reads the first sector (that is, the MBR sector) of the corresponding boot device into the memory address at 0000:7c00h.
3. Check that the 0000:7DFEH-0000:7DFFH (MBR end flag bit) is equal to 55AAH, if not equal to go to try another boot device, if no boot device meets the requirements, then display "No ROM BASIC" and then panic.
4. When a boot device is detected to meet the requirements, the BIOS gives control to the appropriate boot device. The MBR that initiates the device copies itself to the 0000:0600h, and then resumes execution.
5. Start the bootstrapper based on the boot code in the MBR.

Ii. several issues

• Why do I need a disk boot program


为什么需要磁盘引导程序而不是直接载入操作系统？或许是因为历史原因，BIOS最初是为最原始的8位PC所创，8位的PC磁盘很小，所以BIOS只能先载入磁盘引导程序然后通过磁盘引导程序导入操作系统。


• Why does the disk boot program load from 1 sectors, 0 heads, 0 tracks to 7c00h


“引导程序加载器--int19”。由于BIOS加载磁盘引导程序时需要调用系统中断int19来加载引导程序，而这个中断指定了将磁盘1扇区，0磁头，0磁道加载到7c00H处。


• Why the disk boot program can only be 512 bytes maximum


“A bootstrap must be exactly512 bytes long because of the two byte check and the one sector limitation.”
“一个引导程序必须为512字节，因为他的大小有一个扇区的限制，最后两个字节需要设置为‘0xAA55‘。”


• Why is the disk bootstrapper 16-bit


因为DOS系统是16位，X86系统都向后兼容引导时的16位模式。

Principle and introduction of disk boot program