Everything you always wanted to know about USB booting but were too USB (Unable or Sophisticated or Busy) to ask.

Have you ever seen this error message when trying to boot from a floppy disk?

Quote:

Remove disks or other media. Press any key to restart

I guess that floppy is not bootable.
Or how about this message when booting from a CD?

Quote:

Boot from ATAPI CD-ROM : Press any key to boot from CD. . .

If you don't touch the keyboard, that CD won't boot.
Or maybe you've seen this error message when booting from a USB flash drive?

Quote:

Disk error Press any key to restart

Gee wiz, another boot failure, right? Wrong!

USB Booting Secret #1: Everything boots!
In fact, all of the above messages came directly or indirectly from the boot sector of the device.
That's the way the PC works, based on the BIOS setup (that's up to you) the BIOS will load the first sector of the
Initial Program Load device to 7C00h, perform a few checks and then jump to 7C00h. Everything boots.
(Of course there are old computers with USB that can't boot a flash drive, but we're not concerned about those here).

USB Booting Secret #2: What happens in the boot sector, stays in the boot sector!
When it comes to USB booting there are basically 4 types of computers:

PC without USB, or with USB and can't boot USB because of old BIOS firmware.
PC that always boots USB with floppy disk emulation.
PC that always boots USB with hard drive emulation.
PC that examines the boot sector to determine the type of emulation and/or with BIOS selection setup.

Because #1 - everything boots and #2 - booting occurs no matter what's in the boot sector (it does of course have to
pass a few simple BIOS checks), we can come up with a simple USB booting method designed specifically for
OS development.

The floppy disk is dead, long live the floppy disk!

Tip # 1
The FAT12 floppy disk format is simple, easy and universal.
Transfering files to the USB is easy and it will run from any PC that can boot from USB.
Use CHS for access and don't assume floppy disk geometry.

Tip # 2
Ask and you shall receive.
Later on, in the second stage, use int 13h function ah=8 to obtain and save the BIOS emulated geometry. This
information can used later to calculate CHS.

Code:

push dx   push es   mov ah, 8   int 13h                       ; get drive parameters   mov [DRIVE_TYPE], bl   and cx, 3Fh                   ; maximum sector number   mov [SECTORS_PER_TRACK], cx   mov [NUMBER_OF_DRIVES], dl   movzx dx, dh                  ; maximum head number   add dx, 1   mov [NUMBER_OF_HEADS], dx   pop es   pop dx   mov [DRIVE_NUMBER], dl

Presented here is simple boot sector / boot loader.
The next 35 sectors are loaded to 0:600h. The loaded second stage (filled with "nop" to make it 17 1/2 K in size)
displays a message to the screen.
5 lines of data statements follow to create a basic FAT12 file system structure.

Code:

; Original code by Mike Gonta, Public Domain 2011 LOAD_ADDRESS                      equ 600h use16 org 7C00h   jmp start   nop   db '        '   dw 512                        ; bytes per sector   db 1                          ; sectors per cluster   dw 36                         ; reserved sector count   db 2                          ; number of FATs   dw 16*14                      ; root directory entries   dw 18*2*80                    ; sector count   db 0F0h                       ; media byte   dw 9                          ; sectors per fat   dw 18                         ; sectors per track   dw 2                          ; number of heads   dd 0                          ; hidden sector count   dd 0                          ; number of sectors huge   db 0                          ; drive number   db 0                          ; reserved   db 29h                        ; signature   dd 0                          ; volume ID   db '           '              ; volume label   db 'FAT12   '                 ; file system type    start:   xor ax, ax   mov ds, ax   mov es, ax   mov ss, ax   mov sp, 7C00h        mov bx, LOAD_ADDRESS   mov cx, 2                     ; track 0, sector 2   xor dh, dh                    ; side 0   mov ax, 223h                  ; read 35 sectors   test dl, dl   jne .1   mov ax, 211h                  ; read 17 sectors   call read   jc exit   add bx, 512*17   mov cx, 1                     ; track 0, sector 1   mov dh, 1                     ; side 1   mov ax, 212h                  ; read 18 sectors .1:   call read   jc exit   jmp 0:LOAD_ADDRESS    read:   mov bp, 2                     ; 3 tries .1:   push ax   int 13h   jnc .2   sub bp, 1   jc .2   xor ax, ax   int 13h   pop ax   jmp .1 .2:   pop bp   ret   print:   mov ah, 0Eh   xor bh, bh .1:   mov al, [si]   lea si, [si+1]   test al, al   je .2   int 10h   jmp .1 .2:   ret    exit:   mov si, boot_drive_read_error   call print   mov ah, 10h   int 16h   int 19h     boot_drive_read_error:   db 'Boot drive read error!', 13, 10   db 'Press any key to restart.', 13, 10, 0   times 510-($-$$)                db 0                                   dw 0AA55h use16 org LOAD_ADDRESS stage2:   times 512*34 db 90h   mov si, message   call stage2_print   mov ah, 10h   int 16h   int 19h    stage2_print:   mov ah, 0Eh   xor bh, bh .1:   mov al, [si]   lea si, [si+1]   test al, al   je .2   int 10h   jmp .1 .2:   ret    message:   db 'USB booting is easy', 13, 10   db 'Press any key to restart.', 13, 10, 0     times (512*35)-($-$$)           db 0    fat1:                           ; empty FAT12 file system   db 0F0h, 0FFh, 0FFh   times 512*9-($-fat1)            db 0 fat2:   db 0F0h, 0FFh, 0FFh   times 512*9-($-fat2)            db 0 root:   times 512*14-($-root)           db 0

Fixed bug reported by revolution.
Edited source code (removed extraneous code) to simplify presentation.
more USB Booting Secrets
How to transfer a bootable image to the USB flash drive

[DRIVE_NUMBER]=0 always. Looks like a bug to me.

Mike Gonta wrote:

Use CHS for access and don't assume floppy disk geometry.

Why do you always assume 18 sectors per track?

revolution wrote:

[DRIVE_NUMBER]=0 always. Looks like a bug to me.

That's just a placeholder in the assembled code which is updated with the saved BIOS drive number in dl.

revolution wrote:

Why do you always assume 18 sectors per track?

Also just a place holder which is updated with the return parameters of int 13h, ah=8.
This same code boots and runs on any PC that can boot USB (it will also work on a floppy disk).

The bug is here:

Code:

push dx   push es ...   pop dx   mov [DRIVE_NUMBER], dl ;dl=0   pop es ;es=????

Mike Gonta wrote:

Also just a place holder which is updated with the return parameters of int 13h, ah=8.

Code:

mov ax, 211h                  ; read 17 sectors   call read   jc exit   add bx, 512*17   mov cx, 1                     ; track 0, sector 1   mov dh, 1                     ; side 1   mov ax, 212h                  ; read 18 sectors

Looks rather fixed at 18 sectors to me.

revolution wrote:

The bug is here: Code:   push dx   push es ...   pop dx   mov [DRIVE_NUMBER], dl ;dl=0   pop es ;es=????

Thank you.
Fixed it. I wasn't paying attention to the order of the pushes.

revolution wrote:

Mike Gonta wrote: Also just a place holder which is updated with the return parameters of int 13h, ah=8. Code:   mov ax, 211h                  ; read 17 sectors   call read   jc exit   add bx, 512*17   mov cx, 1                     ; track 0, sector 1   mov dh, 1                     ; side 1   mov ax, 212h                  ; read 18 sectors Looks rather fixed at 18 sectors to me.

This is the part that reads from the flash drive that boots with floppy drive emulation. The test for the drive number is
just before it.

Code:

mov ax, 223h                  ; read 35 sectors   test dl, dl   jne .1

If dl=80h (hard drive emulation) the code jumps past this code to here.

Code:

.1:   call read   jc exit

And reads 35 sectors.
The code to convert LBA to CHS based on the obtained values for SECTORS_PER_TRACK and NUMBER_OF_HEADS is
trivial but not required for loading 35 sectors from LBA=1.

I tested Mike's code on 2 different machines (amd64 3500+ and 4000+). Boot example seems to work nicely.

Should this thread be added to the FAQ section? These booting issues
are frequently asked questions.

regards
Mac2004

Mac2004 wrote:

I tested Mike's code on 2 different machines (amd64 3500+ and 4000+). Boot example seems to work nicely.

Hi Mac2004,
Thanks for testing.
The small step from antiquated floppy disk to USB flash drive seems to be a gigantic hurtle for the OS dev beginner.

Mike Gonta wrote:

Mac2004 wrote: I tested Mike's code on 2 different machines (amd64 3500+ and 4000+). Boot example seems to work nicely. Hi Mac2004, Thanks for testing. The small step from antiquated floppy disk to USB flash drive seems to be a gigantic hurtle for the OS dev beginner.

Most of modern bioses support usb booting nicely.

No problem Thank you for releasing the code.

Regards
Mac2004