A Simple x86 Bootloader

A very primitive bootloader created with NASM assembly language.

Summary

• In real mode memory is addressed using a logical address rather than a physical address.
• Given a logical address starting at A, with offset B, reconstructed physical address would be (A * 0x10 + B).

Creating Data Segment

• Code resides at 0x7C00, so DS may begin at 0x7C0

mov ax, 0x7C0
mov ds, ax

Creating Stack Segment

• We create the stack segment right after the 512 bytes of the bootloader, i.e., from 0x7E0 and stack pointer is set initially to point to 0x2000.

mov ax, 0x7E0
mov ss, ax
mov sp, 0x2000

Clear Screen - Int 10/AH=07h Interrupt

• AH = 07h
• AL = number of lines by which to scroll down (00h = clear entire window)
• BH = attribute uses to write blank lines on top of the window
• CH, CL = row, column of window's upper left corner
• DH, DL = row, column of window's lower right corner

Moving the Cursor - Int 10/AH=02h Interrupt

• AH = 02h
• BH = page number
• We have set page number as [bp + 4] because, contents of bp takes up 2 bytes and the argument takes up two bytes (Also note that the new bp is the old sp).

Printing message - Int 10/AH=0Eh Interrupt

• AH = 0Eh
• AL = character to write
• BH = page number