This tutorial shows how to write Intel assembler code for a first-stage boot loader for a toy operating system. We review the structure of the boot sector, and write code for resetting the disk system, rebooting, and writing a string to the screen.
Now that we know the structure of the boot sector’s boot parameter block (BPB) and extended boot parameter block (EBPB), we can start writing our first boot loader code in GNU assembler. (If you need a refresher, please have a look at the first part of this series about the structure of the boot sector).
This article is part of a series on toy operating system development.
This tutorial shows how to write Intel assembler code for a first-stage boot loader for a toy operating system. We review the structure of the boot sector, and write code for resetting the disk system, rebooting, and writing a string to the screen.
Now that we know the structure of the boot sector’s boot parameter block (BPB) and extended boot parameter block (EBPB), we can start writing our first boot loader code in GNU assembler. (If you need a refresher, please have a look at the first part of this series about the structure of the boot sector).
This article is part of a series on toy operating system development.
First code in GNU assembler
We’ll be using the GNU assembler, since it’s free, comes with a boatload of options, supports AT&T and Intel assembly syntax and plays nice with gcc and ld later on. Some of the preprocessor directives used may need some explanation, but all code will be in straightforward Intel syntax.
Here’s some boilerplate code to get started:
.code16
.intel_syntax noprefix
.text
.org 0x0
LOAD_SEGMENT = 0x1000
.global main
main:
jmp short start
nop
# BPB and EBPB here
start:
# rest of codeThe pile of preprocessor instructions at the top tell the assembler to assemble code for real mode. Since all (intel-based) computers start up in real mode with 16-bit instructions, we won’t be able to write 32-bit code here yet. We also instruct GNU assembler that we’ll be using Intel syntax (e.g. mov ax, 1 instead of movw $1, %ax – some prefer the latter, but most readers of this text will be familiar with Intel). The origin of our code will be 0x0, i.e. all absolute addresses start at 0x0, which will be convenient.
Then there’s the main entry point of our code, which corresponds to the first byte of actual output when assembled. The code under the label main simply jumps over the BPB and EBPB located at offset 0x3, resuming execution at the label start. We’ll flesh out the BPB/EBPB in a bit, since it’ll have to have a very exact size.
We’ve also defined a constant LOAD_SEGMENT, which is the segment where we’ll be loading our second stage boot loader (more about that later).
The Boot Parameter Block
The structure of the boot parameter block can be coded like this:
bootsector:
iOEM: .ascii "DevOS " # OEM String
iSectSize: .word 0x200 # bytes per sector
iClustSize: .byte 1 # sectors per cluster
iResSect: .word 1 # #of reserved sectors
iFatCnt: .byte 2 # #of FAT copies
iRootSize: .word 224 # size of root directory
iTotalSect: .word 2880 # total # of sectors if over 32 MB
iMedia: .byte 0xF0 # media Descriptor
iFatSize: .word 9 # size of each FAT
iTrackSect: .word 9 # sectors per track
iHeadCnt: .word 2 # number of read-write heads
iHiddenSect: .int 0 # number of hidden sectors
iSect32: .int 0 # # sectors for over 32 MB
iBootDrive: .byte 0 # holds drive that the boot sector came from
iReserved: .byte 0 # reserved, empty
iBootSign: .byte 0x29 # extended boot sector signature
iVolID: .ascii "seri" # disk serial
acVolumeLabel: .ascii "MYVOLUME " # volume label
acFSType: .ascii "FAT16 " # file system typeThe fields in this structure correspond to the specification in the first part of this text about the structure of the boot sector, and since they’re nicely labelled, we’ll be able to refer to them later on.
Real-mode Segments
After the start label, we can write some actual code. Let’s start by defining our real mode data segments:
cli # Turn off interrupts
mov iBootDrive, dl # save what drive we booted from (should be 0x0)
mov ax, cs # CS = 0x0, since that's where boot sector is (0x07c00)
mov ds, ax # DS = CS = 0x0
mov es, ax # ES = CS = 0x0
mov ss, ax # SS = CS = 0x0
mov sp, 0x7C00 # Stack grows down from offset 0x7C00 toward 0x0000.
sti # Enable interruptsHere, we mask interrupts so that interrupt calls don’t mess up our sector declarations. We set ES = DS = SS = CS = 0x0, and make the stack grow down from 0x7C00 (our boot loader was loaded at 0x7C00). When done, we turn the interrupts back on. It’s important to note that the BIOS places the number of the boot drive in the DL register. We store it in our BPB for later use.
Resetting the disk system
Next, we need to prepare the floppy drive for use. This is done through BIOS interrupt 0x13, subfunction 0. We call it with the boot drive in DL:
mov dl, iBootDrive # drive to reset
xor ax, ax # subfunction 0
int 0x13 # call interrupt 13h
jc bootFailure # display error message if carry set (error)If the reset fails, the carry flag will be set and we jump to a label where we handle a boot failure by showing a message, waiting for a key press and rebooting. Come to think of it, we’ll need a way to print a string to the screen.
Printing a string
We’ll add a short function that uses BIOS interrupt 0x10, sub-function 9 to print characters to the screen. The calling code must point DS:SI to the null-terminated string to be printed.
.func WriteString
WriteString:
lodsb # load byte at ds:si into al (advancing si)
or al, al # test if character is 0 (end)
jz WriteString_done # jump to end if 0.
mov ah, 0xe # Subfunction 0xe of int 10h (video teletype output).
mov bx, 9 # Set bh (page nr) to 0, and bl (attribute) to white (9).
int 0x10 # call BIOS interrupt.
jmp WriteString # Repeat for next character.
WriteString_done:
retw
.endfuncWe can now define the bootFailure label:
diskerror: .asciz "Disk error. "
bootFailure:
lea si, diskerror
call WriteString
call RebootGreat. We’ve got code to reset the floppy drive, and if it fails, there’s code that prints failure strings and reboots. Although, we still have to write a Reboot function.
Rebooting
Here is some code that prints a “Press any key to reboot” message, waits for a keystroke, and reboots the machine.
rebootmsg: .asciz "Press any key to reboot\r\n"
.func Reboot
Reboot:
lea si, rebootmsg # Load address of reboot message into si
call WriteString # print the string
xor ax, ax # subfuction 0
int 0x16 # call bios to wait for key
.byte 0xEA # machine language to jump to FFFF:0000 (reboot)
.word 0x0000
.word 0xFFFF
.endfuncHere, we use BIOS interrupt 0x16, sub-function 0 to read a key (any key). We then add a far jump to 0xffff:0000 which causes the machine to reboot.
Putting it all together
When we combine the functions above into a single source file, we end up with this:
.code16
.intel_syntax noprefix
.text
.org 0x0
LOAD_SEGMENT = 0x1000 ; load the boot loader to segment 1000h
.global main
main:
jmp short start # jump to beginning of code
nop
bootsector:
iOEM: .ascii "DevOS " # OEM String
iSectSize: .word 0x200 # bytes per sector
iClustSize: .byte 1 # sectors per cluster
iResSect: .word 1 # #of reserved sectors
iFatCnt: .byte 2 # #of FAT copies
iRootSize: .word 224 # size of root directory
iTotalSect: .word 2880 # total # of sectors if over 32 MB
iMedia: .byte 0xF0 # media Descriptor
iFatSize: .word 9 # size of each FAT
iTrackSect: .word 9 # sectors per track
iHeadCnt: .word 2 # number of read-write heads
iHiddenSect: .int 0 # number of hidden sectors
iSect32: .int 0 # # sectors for over 32 MB
iBootDrive: .byte 0 # holds drive that the boot sector came from
iReserved: .byte 0 # reserved, empty
iBootSign: .byte 0x29 # extended boot sector signature
iVolID: .ascii "seri" # disk serial
acVolumeLabel: .ascii "MYVOLUME " # volume label
acFSType: .ascii "FAT16 " # file system type
.func WriteString
WriteString:
lodsb # load byte at ds:si into al (advancing si)
or al, al # test if character is 0 (end)
jz WriteString_done # jump to end if 0.
mov ah, 0xe # Subfunction 0xe of int 10h (video teletype output)
mov bx, 9 # Set bh (page nr) to 0, and bl (attribute) to white (9)
int 0x10 # call BIOS interrupt.
jmp WriteString # Repeat for next character.
WriteString_done:
retw
.endfunc
.func Reboot
Reboot:
lea si, rebootmsg # Load address of reboot message into si
call WriteString # print the string
xor ax, ax # subfuction 0
int 0x16 # call bios to wait for key
.byte 0xEA # machine language to jump to FFFF:0000 (reboot)
.word 0x0000
.word 0xFFFF
.endfunc
start:
# Setup segments:
cli
mov iBootDrive, dl # save what drive we booted from (should be 0x0)
mov ax, cs # CS = 0x0, since that's where boot sector is (0x07c00)
mov ds, ax # DS = CS = 0x0
mov es, ax # ES = CS = 0x0
mov ss, ax # SS = CS = 0x0
mov sp, 0x7C00 # Stack grows down from offset 0x7C00 toward 0x0000.
sti
# Display "loading" message:
lea si, loadmsg
call WriteString
# Reset disk system.
# Jump to bootFailure on error.
mov dl, iBootDrive # drive to reset
xor ax, ax # subfunction 0
int 0x13 # call interrupt 13h
jc bootFailure # display error message if carry set (error)
# End of loader, for now. Reboot.
call Reboot
bootFailure:
lea si, diskerror
call WriteString
call Reboot
# PROGRAM DATA
loadmsg: .asciz "Loading OS...\r\n"
diskerror: .asciz "Disk error. "
rebootmsg: .asciz "Press any key to reboot.\r\n"
.fill (510-(.-main)), 1, 0 # Pad with nulls up to 510 bytes (excl. boot magic)
BootMagic: .int 0xAA55 # magic word for BIOSPoints of note
- The WriteString and Reboot sections are functions, that we’ll want to call various times in the other boot code that we’ll write soon. They are not part of the main body of code. That is why they are placed before the start label, so that execution will jump over them.
- The loader will not print a “Loading OS…” message right after it sets the segments.
- After having reset the disk system successfully, the loader will reboot. We’re doing that only because we haven’t written any more code yet that does interesting things.
- The source code ends with a
.fillpreprocessor directive. This causes the assembler to fill up the output file with null bytes all the way to offset 510. The final two bytes contain the magic word required by some BIOSes. Compilation will now yield a file of exactly 512 bytes, which is what we need for our boot sector.
Summary
We’ve written assembler code that prepares data and stack segments and resets the floppy drive. We’ve also added functions for writing text to the screen, waiting for a keypress, and rebooting.
I don’t know about you, but I’m just about ready to see this code in action! In the next section on setting up a toolchain and using Bochs, we’ll see how we can actually compile and test this code.