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From Bootloader to Kernel? https://forum.osdev.org/viewtopic.php?f=1&t=642 |
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Author: | Fear [ Wed May 24, 2006 11:00 pm ] |
Post subject: | From Bootloader to Kernel? |
I wrote a simple (and I mean simple) kernel in C, and a simple bootloader in NASM. The bootloader is in binary format which means, no externals. So, how do I get into my kernel, if I can't call it from my bootloader. Thanks in advance. |
Author: | obandu [ Wed May 24, 2006 11:00 pm ] |
Post subject: | Re: From Bootloader to Kernel? |
I have the same problem. I compile mine in DJGPP. What is the right way to compile in DJGPP. What switches should one use when linking with ld without it turning into a Windows executable. |
Author: | osprogram [ Wed May 24, 2006 11:00 pm ] |
Post subject: | Re: From Bootloader to Kernel? |
@Fear: You have to have your bootloader load your kernel into memory and then "jmp" to the memory address you loaded it to. (You wrote it in assembly, right?) Some more info about your bootloader might help, but I don't this this should be too difficult. @obandu: I use COFF format for my DJGPP output. Do you need to mix in assembly as well? Even if you don't, you should read "Mixing Assembly and C" over in the howtos section. It has a good example in the beginning of what you'll need to do. Link: http://www.osdev.com/howtos/1/index.html More specifically, you should probably make a batch file for compiling things along these lines: (for C only) gcc -ffreestanding -c -o kernel.o kernel.c ld -Ttext 0x100000 --oformat binary -o kernel.bin kernel.o You may have to add lines for more than one file: (for C and (N)asm) gcc -ffreestanding -c -o mix_c.o mix_c.c nasm -f coff -o mix_asm.o mix_asm.asm ld -Ttext 0x100000 --oformat binary -o kernel32.bin mix_c.o mix_asm.o |
Author: | Fear [ Wed May 24, 2006 11:00 pm ] |
Post subject: | Re: From Bootloader to Kernel? |
So, how would I load my C kernel into memory? I think it's 'int 13', but I have no idea what functions to pass it, etc. |
Author: | osprogram [ Wed May 24, 2006 11:00 pm ] |
Post subject: | Re: From Bootloader to Kernel? |
The primary responsibility of the bootloader is to load the rest of the things into memory. It is indeed int 13h; for a reference on this google for Ralf Brown's interrupt list. If you are just learning os programming, I would suggest that you use a ready-made bootloader. I use John Fine's. If you need protected mode for DJGPP-generated (or other 32-bit) code, use bootf02. For real mode (16-bit), I think the correct loader (for floppy) is in bootr01.zip. Link:http://my.execpc.com/~geezer/johnfine/ |
Author: | Fear [ Wed May 24, 2006 11:00 pm ] |
Post subject: | Re: From Bootloader to Kernel? |
Wow, that was like the most useful response I could have gotten, never thought of using a prewritten bootloader. Thanks, I'll get on that now. |
Author: | osprogram [ Wed May 24, 2006 11:00 pm ] |
Post subject: | Re: From Bootloader to Kernel? |
Sure thing. You may wish to revisit the bootloader once you have some experience, but sectors and tracks are not the easiest things to deal with for a beginner. |
Author: | obandu [ Wed May 24, 2006 11:00 pm ] |
Post subject: | Re: From Bootloader to Kernel? |
Thanks to "osprogram". What I have been missing is the --oformat binary The code now looks more reasonable via ms-dos debug.exe. I will now test it with real booting. |
Author: | Fear [ Thu May 25, 2006 11:00 pm ] |
Post subject: | Re: From Bootloader to Kernel? |
Wow, I feel major stupid. I can't even do this with a premade bootsector. Either way, I don't procrastinate. Is there a resource where I can learn to load my Kernel from a binary file (no external refrences)? |
Author: | osprogram [ Thu May 25, 2006 11:00 pm ] |
Post subject: | Re: From Bootloader to Kernel? |
NASM sample bootsector from John Fine: Code: ; boot12.asm FAT12 bootstrap for real mode image or loader ; Version 1.0, Jul 5, 1999 ; Sample code ; by John S. Fine [email protected] ; I do not place any restrictions on your use of this source code ; I do not provide any warranty of the correctness of this source code ;_____________________________________________________________________________ ; ; Documentation: ; ; I) BASIC features ; II) Compiling and installing ; III) Detailed features and limits ; IV) Customization ;_____________________________________________________________________________ ; ; I) BASIC features ; ; This boot sector will load and start a real mode image from a file in the ; root directory of a FAT12 formatted floppy or partition. ; ; Inputs: ; DL = drive number ; ; Outputs: ; The boot record is left in memory at 7C00 and the drive number is patched ; into the boot record at 7C24. ; SS = DS = 0 ; BP = 7C00 ;_____________________________________________________________________________ ; ; II) Compiling and installing ; ; To compile, use NASM ; ; nasm boot12.asm -o boot12.bin ; ; Then you must copy the first three bytes of BOOT12.BIN to the first three ; bytes of the volume and copy bytes 0x3E through 0x1FF of BOOT12.BIN to ; bytes 0x3E through 0x1FF of the volume. Bytes 0x3 through 0x3D of the ; volume should be set by a FAT12 format program and should not be modified ; when copying boot12.bin to the volume. ; ; If you use my PARTCOPY program to install BOOT12.BIN on A:, the ; commands are: ; ; partcopy boot12.bin 0 3 -f0 ; partcopy boot12.bin 3e 1c2 -f0 3e ; ; PARTCOPY can also install to a partition on a hard drive. Please read ; partcopy documentation and use it carefully. Careless use could overwrite ; important parts of your hard drive. ; ; You can find PARTCOPY and links to NASM on my web page at ; http://www.erols.com/johnfine/ ;_____________________________________________________________________________ ; ; III) Detailed features and limits ; ; Most of the limits are stable characteristics of the volume. If you are ; using boot12 in a personal project, you should check the limits before ; installing boot12. If you are using boot12 in a project for general ; distribution, you should include an installation program which checks the ; limits automatically. ; ; CPU: Supports any 8088+ CPU. ; ; Volume format: Supports only FAT12. ; ; Sector size: Supports only 512 bytes per sector. ; ; Drive/Partition: Supports whole drive or any partition of any drive number ; supported by INT 13h. ; ; Diskette parameter table: This code does not patch the diskette parameter ; table. If you boot this code from a diskette that has more sectors per ; track than the default initialized by the BIOS then the failure to patch ; that table may be a problem. Because this code splits at track boundaries ; a diskette with fewer sectors per track should not be a problem. ; ; File position: The file name may be anywhere in the root directory and the ; file may be any collection of clusters on the volume. There are no ; contiguity requirements. (But see track limit). ; ; Track boundaries: Transfers are split on track boundaries. Many BIOS's ; require that the caller split floppy transfers on track boundaries. ; ; 64Kb boundaries: Transfers are split on 64Kb boundaries. Many BIOS's ; require that the caller split floppy transfers on track boundaries. ; ; Cluster boundaries: Transfers are merged across cluster boundaries whenever ; possible. On some systems, this significantly reduces load time. ; ; Cluster 2 limit: Cluster 2 must start before sector 65536 of the volume. ; This is very likely because only the reserved sectors (usually 1) and ; the FAT's (two of up to 12 sectors each) and the root directory (usually ; either 15 or 32 sectors) precede cluster 2. ; ; Track limit: The entire image file must reside before track 32768 of the ; entire volume. This is true on most media up to 1GB in size. If it is a ; problem it is easy to fix (see boot16.asm). I didn't expect many people ; to put FAT12 partitions beyond the first GB of a large hard drive. ; ; Memory boundaries: The FAT, Root directory, and Image must all be loaded ; starting at addresses that are multiples of 512 bytes (32 paragraphs). ; ; Memory use: The FAT and Root directory must each fit entirely in the ; first 64Kb of RAM. They may overlap. ; ; Root directory size: As released, it supports up to 928 entries in the ; root directory. If ROOT_SEG were changed to 0x7E0 it would support up ; to 1040. Most FAT12 volumes have either 240 or 512 root directory ; entries. ;_____________________________________________________________________________ ; ; IV) Customization ; ; The memory usage can be customized by changing the _SEG variables (see ; directly below). ; ; The file name to be loaded and the message displayed in case of error ; may be customized (see end of this file). ; ; The ouput values may be customized. For example, many loaders expect the ; bootsector to leave the drive number in DL. You could add "mov dl,[drive]" ; at the label "eof:". ; ; Some limits (like maximum track) may be removed. See boot16.asm for ; comparison. ; ; Change whatever else you like. The above are just likely possibilities. ;_____________________________________________________________________________ ; Change the _SEG values to customize memory use during the boot. ; When planning memory use, remember: ; ; *) Each of ROOT_SEG, FAT_SEG, and IMAGE_SEG must be divisible by 0x20 ; ; *) None of ROOT, FAT or IMAGE should overlap the boot code itself, or ; its stack. That means: avoid paragraphs 0x7B0 to 0x7DF. ; ; *) The FAT area must not overlap the IMAGE area. Either may overlap ; the ROOT area; But, if they do then the root will not remain in ; memory for possible reuse by the next stage. ; ; *) The FAT area and the root area must each fit within the first 64Kb ; excluding BIOS area (paragraphs 0x60 to 0xFFF). ; ; *) A FAT12 FAT can be up to 6Kb (0x180 paragraphs). ; ; *) A FAT12 root directory is typically either 0x1E0 or 0x400 paragraphs ; long, but larger sizes are possible. ; ; *) The code will be two bytes shorter when FAT_SEG is 0x800 than when it ; is another value. (If you reach the point of caring about two bytes). ; %define ROOT_SEG 0x60 %define FAT_SEG 0x800 %define IMAGE_SEG 0x1000 %if ROOT_SEG & 31 %error "ROOT_SEG must be divisible by 0x20" %endif %if ROOT_SEG > 0xC00 %error "Root directory must fit within first 64Kb" %endif %if FAT_SEG & 31 %error "FAT_SEG must be divisible by 0x20" %endif %if FAT_SEG > 0xE80 %error "FAT must fit within first 64Kb" %endif %if IMAGE_SEG & 31 %error "IMAGE_SEG must be divisible by 0x20" %endif ; The following %define directives declare the parts of the FAT12 "DOS BOOT ; RECORD" that are used by this code, based on BP being set to 7C00. ; %define sc_p_clu bp+0Dh ;byte Sectors per cluster %define sc_b4_fat bp+0Eh ;word Sectors (in partition) before FAT %define fats bp+10h ;byte Number of FATs %define dir_ent bp+11h ;word Number of root directory entries %define sc_p_fat bp+16h ;word Sectors per FAT %define sc_p_trk bp+18h ;word Sectors per track %define heads bp+1Ah ;word Number of heads %define sc_b4_prt bp+1Ch ;dword Sectors before partition %define drive bp+24h ;byte Drive number org 0x7C00 entry: jmp short begin ; -------------------------------------------------- ; data portion of the "DOS BOOT RECORD" ; ---------------------------------------------------------------------- brINT13Flag DB 90H ; 0002h - 0EH for INT13 AH=42 READ brOEM DB 'MSDOS5.0' ; 0003h - OEM ID - Windows 95B brBPS DW 512 ; 000Bh - Bytes per sector brSPC DB 1 ; 000Dh - Sector per cluster brSc_b4_fat DW 1 ; 000Eh - Reserved sectors brFATs DB 2 ; 0010h - FAT copies brRootEntries DW 0E0H ; 0011h - Root directory entries brSectorCount DW 2880 ; 0013h - Sectors in volume, < 32MB brMedia DB 240 ; 0015h - Media descriptor brSPF DW 9 ; 0016h - Sectors per FAT brSc_p_trk DW 18 ; 0018h - Sectors per head/track brHPC DW 2 ; 001Ah - Heads per cylinder brSc_b4_prt DD 0 ; 001Ch - Hidden sectors brSectors DD 0 ; 0020h - Total number of sectors brDrive DB 0 ; 0024h - Physical drive no. DB 0 ; 0025h - Reserved (FAT32) DB 29H ; 0026h - Extended boot record sig (FAT32) brSerialNum DD 404418EAH ; 0027h - Volume serial number brLabel DB 'Joels disk ' ; 002Bh - Volume label brFSID DB 'FAT12 ' ; 0036h - File System ID ;------------------------------------------------------------------------ begin: xor ax, ax mov ds, ax mov ss, ax mov sp, 0x7C00 mov bp, sp mov [drive], dl ;Drive number mov al, [fats] ;Number of FATs mul word [sc_p_fat] ; * Sectors per FAT add ax, [sc_b4_fat] ; + Sectors before FAT ;AX = Sector of Root directory mov si, [dir_ent] ;Max root directory entries mov cl, 4 dec si shr si, cl inc si ;SI = Length of root in sectors mov di, ROOT_SEG/32 ;Buffer (paragraph / 32) call read_16 ;Read root directory push ax ;Sector of cluster two %define sc_clu2 bp-2 ;Later access to the word just pushed is via bp mov dx, [dir_ent] ;Number of directory entries push ds pop es mov di, ROOT_SEG*16 search: dec dx ;Any more directory entries? js error ;No mov si, filename ;Name we are searching for mov cx, 11 ;11 characters long lea ax, [di+0x20] ;Precompute next entry address push ax repe cmpsb ;Compare pop di jnz search ;Repeat until match push word [di-6] ;Starting cluster number mov ax, [sc_b4_fat] ;Sector number of FAT mov si, [sc_p_fat] ;Length of FAT mov di, FAT_SEG/32 ;Buffer (paragraph / 32) call read_16 ;Read FAT next: pop bx ;Cluster number mov si, bx ;First cluster in this sequence mov ax, bx ;Last cluster in this sequence .0: cmp bx, 0xFF8 ;End of file? jae .2 ; Yes inc ax ;Last cluster plus one in sequence ;Look in FAT for next cluster mov di, bx ;Cluster number rcr bx, 1 ;1.5 byte entry per cluster ;bx = 0x8000 + cluster/2 ;c-bit set for odd clusters mov bx, [bx+di+FAT_SEG*16-0x8000] jnc .1 shr bx, 1 shr bx, 1 shr bx, 1 shr bx, 1 .1: and bh, 0xF cmp ax, bx ;Is the next one contiguous? je .0 ;Yes: look further ahead .2: sub ax, si ;How many contiguous in this sequence? jz eof ;None, must be done. push bx ;Save next (eof or discontiguous) cluster mov bl, [sc_p_clu] ;Sectors per cluster mov bh, 0 ; as a word mul bx ;Length of sequence in sectors .3: mov di, IMAGE_SEG/32 ;Destination (paragraph / 32) add [.3+1], ax ;Precompute next destination xchg ax, si ;AX = starting cluster ;SI = length in sectors dec ax dec ax ;Starting cluster minus two mul bx ; * sectors per cluster add ax, [sc_clu2] ; + sector number of cluster two adc dl, dh ;Allow 24-bit result call read_32 ;Read it jmp short next ;Look for more eof: jmp IMAGE_SEG:0 error: mov si, errmsg ;Same message for all detected errors mov ax, 0xE0D ;Start message with CR mov bx, 7 .1: int 10h lodsb test al, al jnz .1 xor ah, ah int 16h ;Wait for a key int 19h ;Try to reboot read_16: xor dx, dx read_32: ; ; Input: ; dx:ax = sector within partition ; si = sector count ; di = destination segment / 32 ; ; The sector number is converted from a partition-relative to a whole-disk ; (LBN) value, and then converted to CHS form, and then the sectors are read ; into (di*32):0. ; ; Output: ; dx:ax updated (sector count added) ; di updated (sector count added) ; si = 0 ; bp, ds preserved ; bx, cx, es modified .1: push dx ;(high) relative sector push ax ;(low) relative sector add ax, [sc_b4_prt] ;Convert to LBN adc dx, [sc_b4_prt+2] mov bx, [sc_p_trk] ;Sectors per track div bx ;AX = track ;DX = sector-1 sub bx, dx ;Sectors remaining, this track cmp bx, si ;More than we want? jbe .2 ;No mov bx, si ;Yes: Transfer just what we want .2: inc dx ;Sector number mov cx, dx ;CL = sector ;CH = 0 cwd ;(This supports up to 32767 tracks div word [heads] ;Track number / Number of heads mov dh, dl ;DH = head xchg ch, al ;CH = (low) cylinder ;AL=0 ror ah, 1 ;rotate (high) cylinder ror ah, 1 add cl, ah ;CL = combine: sector, (high) cylinder sub ax, di and ax, byte 0x7F ;AX = sectors to next 64Kb boundary jz .3 ;On a 64Kb boundary already cmp ax, bx ;More than we want? jbe .4 ;No .3: xchg ax, bx ;Yes: Transfer just what we want .4: push ax ;Save length mov bx, di ;Compute destination seg push cx mov cl, 5 shl bx, cl pop cx mov es, bx xor bx, bx ;ES:BX = address mov dl, [drive] ;DL = Drive number mov ah, 2 ;AH = Read command int 13h ;Do it jc error pop bx ;Length pop ax ;(low) relative sector pop dx ;(high) relative sector add ax, bx ;Update relative sector adc dl, dh add di, bx ;Update destination sub si, bx ;Update count jnz .1 ;Read some more ret errmsg db 10,"Error Executing FAT12 bootsector",13 db 10,"Press any key to reboot",13,10,0 size equ $ - entry %if size+11+2 > 512 %error "code is too large for boot sector" %endif times (512 - size - 11 - 2) db 0 filename db "LOADER BIN" ;11 byte name db 0x55, 0xAA ;2 byte boot signature This should do it for you. Compile it using "nasm -o boot12.bin boot12.asm". Next, execute "debug". At the modified prompt, type the following sequence to put the bootsector on the floppy( you are doing things on a floppy, right?) : n boot12.bin (return) l 0 (return) w 0 0 0 1 (return) This code will load a 16-bit binary in "loader.bin" on the floppy drive and execute it. Put whatever you want into a binary file (i.e. your kernel) and the bootsector will load it for you. The best way for you to learn to wirte a bootsector is to look at those that others have written. I'd find a good guide on floppy disk geometry and FAT12 structure, and use Ralf Brown for the guide to int 13h. Although I do think you'd be better off getting some experience in assembly and other OS programming before dabbling in bootsectors. |
Author: | Fear [ Thu May 25, 2006 11:00 pm ] |
Post subject: | Re: From Bootloader to Kernel? |
What do you mean by, 'Put whatever you want into a binary file'? Do you mean, put whatever I want into a binary file, and then do the above sequence to move it to the floppy, or what? |
Author: | azcoder [ Fri May 26, 2006 11:00 pm ] |
Post subject: | Re: From Bootloader to Kernel? |
GRUB - Grand Unified Bootloader is also a good one to use. By the way, a great way to setup a test environment is to create a .iso containing your kernel image as well as the eltorito grub stage. Then, you can point a vmware image to boot your .iso. This enables you to quickly boot and test changes. VMWare provides an entire virtual machine, bios and all, as an app on your desktop. The commands to boot your kernel once grub is loaded are: root (cd) kernel = /MyKernel.bin boot |
Author: | earlz [ Sat May 27, 2006 11:00 pm ] |
Post subject: | Re: From Bootloader to Kernel? |
if I'm correct then that source code don't work with the most recent version of nasm (don't remeber if it is 32bit or 16bit that don't work) when i had my 16bit OS i used something called boot12.bin maybe try googling for it |
Author: | osprogram [ Mon May 29, 2006 11:00 pm ] |
Post subject: | Re: From Bootloader to Kernel? |
hckr: That is the source code for boot12.bin. I don't know about the compilation issues; I'll upload a working copy tonight. Fear: By "whatever you want", I mean put your kernel (in binary format) there. |
Author: | Da_Maestro [ Mon May 29, 2006 11:00 pm ] |
Post subject: | Re: From Bootloader to Kernel? |
if you want to be nasty you can format an MS-DOS system disk and rewrite msdos.sys hehe |
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