Writing a Simple Operating System — from Scratch

CHAPTER 3. BOOT SECTOR PROGRAMMING (IN 16-BIT REAL

MODE) 29

; Read some sectors from the boot disk using our disk_read function

[org 0x7c00]

mov [BOOT_DRIVE], dl ; BIOS stores our boot drive in DL, so it’s

; best to remember this for later.

mov bp, 0x8000 ; Here we set our stack safely out of the

mov sp, bp ; way , at 0x8000

mov bx, 0x9000 ; Load 5 sectors to 0x0000(ES):0 x9000(BX)

mov dh, 5 ; from the boot disk.

mov dl, [BOOT_DRIVE]

call disk_load

mov dx, [0 x9000] ; Print out the first loaded word , which

call print_hex ; we expect to be 0xdada , stored

; at address 0x9000

mov dx, [0 x9000 + 512] ; Also , print the first word from the

call print_hex ; 2nd loaded sector: should be 0xface

jmp $

%include "../print/print_string.asm" ; Re -use our print_string function

%include "../hex/print_hex.asm" ; Re-use our print_hex function

%include "disk_load.asm"

; Include our new disk_load function

; Global variables

BOOT_DRIVE: db 0

; Bootsector padding

times 510-($-$$) db 0

dw 0xaa55

; We know that BIOS will load only the first 512-byte sector from the disk ,

; so if we purposely add a few more sectors to our code by repeating some

; familiar numbers , we can prove to ourselfs that we actually loaded those

; additional two sectors from the disk we booted from.

times 256 dw 0xdada

times 256 dw 0xface