Hi,

I try to switch to protected mode, with the following code:

Code:

org $0000     cld ;-------- Set up segments ---------;     start:         mov ax, $7c0         mov ds, ax         mov ax, $9000         mov es, ax         mov cx, 256 ;----------- Print the welcome message -----------          mov bl, $8      ; gray color     mov si, hello     call putStr      ;---------- Print the "stage1" and "stage2" ----------;     mov bl, $f      ; wihte color     mov si, stg1     call putStr     mov si, stg2     call putStr      ;---------- switch to PMode ---------; toPM:     cli     lgdt [gdt_info]     mov eax, cr0     or al, 1     mov cr0, eax     jmp far $8:PMode      ;------------- Global Descriptor Table (from Linux 0.01 bootldr) ------------- gdt:         dw   0,0,0,0         ; dummy         dw   0x07FF          ; 8Mb - limit=2047 (2048*4096=8Mb)         dw   0x0000          ; base address=0         dw   0x9A00          ; code read/exec         dw   0x00C0          ; granularity=4096, 386         dw   0x07FF          ; 8Mb - limit=2047 (2048*4096=8Mb)         dw   0x0000          ; base address=0         dw   0x9200          ; data read/write         dw   0x00C0          ; granularity=4096, 386 gdtend:          gdt_info:     dw gdtend-gdt-1     dd gdt ; ---- FUNCTIONS ---- ; putStr:     lodsb         ; al = [ds:si]     or al, al     ; al == 0 ?     jz done     mov ah, $e    ; print char     int $10     jmp putStr   done:     ret      ;-------- DATA --------; hello:  db "  -  Hi! Bootloader -", 13, 10, 13, 10, 0 stg1:   db "*   stage 1: loading stage 2", 13, 10, 0 stg2:   db "**  stage 2: switching to protected mode...", 13, 10, 0 stg3:   db "*** stage 3: loading the kernel", 13, 10, 0            times 510 - ($-$$) db $42 dw 0xAA55 PMode:     mov dword [0xb8000], "done"  ;; print out "done" from video memory @@: jmp @b

All that I get is a blinking screen from qemu. It outputs my "stage1" and "stage2" strings, and reboot very fastly, forever.

I think my PMode switching is bad and makes the troubles. Am I wrong ? What do you think?

Regards.

Your ORG is 0x0000 but your code is loaded at 0x00007C00. Either set your jump to PMode to point to 0x00007C00+PMode, or set your GDT table for a base of 0x00007C00.

Edit: Another option is to ORG at 0x7c00 and run everything from a 0x0000 base.

I'm going to guess you're booting from a hdd or floppy, where the boot sector size is 512 bytes. Your PMode code is outside this area and so needs to be loaded from the disk first (the code is not actually there at run time). Right after the PMode switch you'll want to set up ds,es,ss to be valid GDT data selectors (which looks to be 0x10).

Quote:

Edit: Another option is to ORG at 0x7c00 and run everything from a 0x0000 base.

Quote:

Your PMode code is outside this area and so needs to be loaded from the disk first

Ok, I've added the org directive correctly and I have inserted my MPode in the first 512 bytes. It's work, now.
So I have to switch to PMode after be loaded outside of the first 512 bytes because I have to use the int $13 to load anything from the disk (and pmode don't allow BIOS int). aright?

stage 1: jump outside
stage 2: switching to pmode
stage 3: loading a kernel

Is it a good idea?

Thanks you

step 1: jump outside
step 2: loading a kernel
step 3: switching to pmode

There's a number of ways you can do this.

e.g. In my project, the HDD version has a "stub" (stage 0) boot sector that uses the BIOS to load the first stage and the CD ISO version boots directly from the first stage*; both use PIO ATA drivers to load the second stage and execute it.

The second stage, still in RMode, enables and checks A20, enumerates the BIOS memory map, enumerates and enables VESA (if available)** and switches to PMode. Again, this has a 32bit PIO ATA driver which is almost the same as the 16bit one, which it uses to scan for its file system and load the core and its drivers before switching to it.


*This difference is because I can't fit my first stage code into a 512 byte HDD sector (is it possible, I just didn't want two versions of the code) but can for the 2048 byte CD sector.

**These are all the things you might want to do in RMode that can't be done in PMode.

----

In my case "stages 2 & 3" are the same since switching to PMode is relatively basic compared to all the other initialisation. BUT there are other projects I've seen that switch immediately to PMode, load their kernels and execute it in their boot sectors.

The point here is it can vary where you choose to divide the different operations AND what you want to do. There is an issue of functionality vs size during the boot process that affects these decisions.

what is sure is that 512 bytes is not enough to (be partition root) AND (load&launch kernel from file system) AND (switch pm). it can do only one of them at a time.

better would be to load and launch kernel from file system in a second stage, because the first one of only 512 bytes will just be the partition table.
second stage will load kernel and launch it
and then, kernel is in the mode you want, you can launch applications now!

applications can be any privilege level, it depends on the kernel to determine this iopl at loading.

etc...

hem, maybe it is possible in fact, it depends on how is made the file system.

I am trying to read a 2nd sector of the floppy to get enough memory to switch to PMode as you have suggested.
Here is my code, the carry flag which denote an error is always up after trying to read a 2nd sector:

Code:

; ; yet another try ; SIZE = 1                ; number of sectors to load (512 bytes is enough) ;------------------------ STAGE 1 ------------------------; org $7c00     mov [bootdrv], dl   ; save the drive number (normally 0, floppy drive A)          mov si, str0     call putstr         ; print out the welcome msg          mov si, str1     call putstr         ; print out "step 1...."     xor ax, ax          ; reset disk drive     int $13             ; dl is already set          mov ah, 2           ; read sectors from drive     mov al, SIZE        ; how many sectors?     mov ch, 0           ; read the first track     mov cl, 2           ; read the 2nd sector     mov dh, 0           ; head 0     mov dl, [bootdrv]   ; load on the current drive     int $13             ; if error occured then CF = 1     jc @f          jmp stage2       ;; fail     @@:     mov si, fuck     call putstr     jmp $      ;-- functions -- putstr:     lodsb         ; al = [ds:si]     or al, al     ; al == 0 ?     jz @f     mov ah, $e    ; print char     int $10     jmp putstr @@: ret      ;-- data -- str0: db "- bootloader -", 10, 13, 0 str1: db "> step 1: jumping outside the bootsector...", 0 str2: db "> step 2", 0 done: db "done", 10, 13, 0 fuck: db 10, 13, "failed to read the sector. FUCK! :@", 0 bootdrv: db 0 times 510-($-$$) db $00 dw $aa55 ;------------------------ STAGE 2 ------------------------; stage2:     mov si, done     call putstr         ; print "done"

I think there are bad parameters to the 2nd function of the int $13 which makes this failure. I tried to modify some values (track number, sector number) but the carry flag is still up indicating an error.

emc: Where's your destination buffer es:bx setup?

Your are trying to load the second sector to linear address 0x7e00.
es:bx needs to be setup according to your destination.

http://en.wikipedia.org/wiki/INT_13H#INT_13h_AH.3D02h:_Read_Sectors_From_Drive

Regards
Mac2004

My destination buffer must be $7c00+512 (absolute address) and it's $07e00, so BX has to contain $7e00 and ES has to contain $000 and we have [$0000:$7e00].

Code:

0000 +  7e00   ($7e00 = $7c00+512) --------- = 07e00

I tried that but the problem still occurs

This is ripped out of some old code from my previous loader:

Code:

use16         org 0x7C00         cli         jmp 0x0000:@f     @@:         xor ax,ax         mov ds,ax         mov es,ax         mov ss,ax         mov sp,0x7C00         mov byte [disk.id],dl         sti         mov ax,1      ; LBA         mov bx,stage2 ; Dst         mov si,1      ; Size         call disk.read        ;jc .error         jmp stage2         ; in         ; ax = LBA         ; si = Count         ; es:bx = Dst         ;         ; out         ; CF disk.read:         push ax dx si     .next:         or si,si         jz .done         call disk.readlba         jc .done         mov dx,es         add bx,0x0200         jnc @f         add dx,0x1000     @@:         mov es,dx         inc ax         dec si         jmp .next     .done:         pop si dx ax         ret         ; in         ; ax = LBA         ; es:bx = Dst         ; si = Count         ;         ; out         ; CF disk.readlba:         push ax cx dx di         mov di,0x0005         call disk.lba_chs     @@:         stc         dec di         jz .done         call disk.reset         jc .done         mov ax,0x0201         int 0x13         jc @b     .done:         pop di dx cx ax         ret         ;in         ;         ;out         ; CF disk.reset:         push ax di         mov di,0x0005 ; Attempts     @@:         stc         dec di         jz .done         xor ah,ah         int 0x13         jc @b     .done:         pop di ax         ret         ; in         ; ax = LBA         ;         ; out         ; ch = C         ; cl = S         ; dh = H         ; dl = Drive ID disk.lba_chs:         xor dx,dx         div word [disk.s_c]         inc dx         push dx         xor dx,dx         div word [disk.h_c]         pop cx         mov ch,al         mov dh,dl         mov dl,byte [disk.id]         ret         disk.s_c dw 18         disk.h_c dw  2         disk.id  db  0         times (0x0200 - ($ - 0x7C00) - 2) db 0         dw 0xAA55 stage2:         mov ax,0x0003         int 0x10         cli         hlt times (0x0200 - ($ and 0x01FF)) db 0

The sectors per cylinder (disk.s_c) and heads per cylinder (disk.h_c) values are set up for a floppy disk so need to be modified for a hard disk (this example will actually work for both).