int 13h 42h doesn't load anything in Bochs
I changed my bootloader from CHS to LBA, so I replaced int 13h 02h with int 13h 42h. It works correctly in QEMU, however, I have troubles running i
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LBA Disk Access Availability
Not all BIOSes support the extended disk read and write functions (although on modern hardware they almost all likely will). Not all BIOSes support extended disk reads of floppies via Int 13h/AH=42h. This is true of BOCHS as well. You can test whether extended disk functions are available on a drive via Int 13/AH=41h/BX=55AAh. This does an extended disk installation check.
If you want to test your code on BOCHS using extended disk reads and LBA you will have to create a hard disk image and modify BOCHS to boot from it instead of floppy. The minimum size hard disk image size that BOCHS supports is one with CHS = 1/16/63 which is 512*16*63 = 516096 bytes or 1008 sectors of 512 bytes each.
You can modify your
bochsrc.txtto be:megs: 32 romimage: file=/usr/share/bochs/BIOS-bochs-latest, address=0xfffe0000 vgaromimage: file=/usr/share/bochs/VGABIOS-lgpl-latest boot: c ata0: enabled=1, ioaddr1=0x1f0, ioaddr2=0x3f0, irq=14 ata0-master: type=disk, path="disk.img", mode=flat, cylinders=0, heads=0, spt=0, model="Generic 1234", biosdetect=auto, translation=auto panic: action=ask log: bochsout.txt mouse: enabled=0 keyboard: type=mf, serial_delay=200, paste_delay=100000 display_library: x, options="gui_debug"I use a disk image called
disk.img. You can create it and place the sectors you generate into it with commands like:nasm -f bin main.asm -o main.binCreate an image of 516096 bytes:
dd if=/dev/zero of=disk.img count=1008 bs=512Place the boot sectors into the beginning of
disk.imgwithout truncating the file:dd if=main.bin of=disk.img conv=notruncI have additional information on using DD to create disk images in this Stackoverflow answer.
General Code Observations
Although the disk image is part of the issue with BOCHS you do have some coding issues. You can't assume the value of CS will be set to what you think when control is transferred from the BIOS to your bootloader. See my General Bootloader Tips for more information. If you want DS and ES to be zero (which is what you'd need with
org 0x7c00), you should modify the start of your code to be something like:org 0x7c00 bits 16 boot: cli xor ax, ax ; Explicitly set DS and ES to 0 mov ds, ax mov es, axTo test out your code I added this after
bootend:bootend: ; Print MDP to upper left of screen in white on light magenta mov ax, 0xb800 mov es, ax mov word [es:0x0000], 0x57<<8 | 'M' mov word [es:0x0002], 0x57<<8 | 'D' mov word [es:0x0004], 0x57<<8 | 'P' ; Infinite loop so we don't have the CPU wander memory cli endloop: hlt jmp endloop
I'm not sure this line is a typo or you are using some type of pre-processor on your assembly files before passing them to NASM. Normally this line would be a problem with the
%symbol before1:dw %1 ; Number of sectors to be loadedNASM would only support this directly:
dw 1 ; Number of sectors to be loaded
Real Hardware / USB / Laptop Issues
If you are attempting to use USB to boot on real hardware then you may encounter another issue even if you get it working in BOCHS with the changes above. If your BIOS is set to do USB FDD emulation (and not USB HDD or something else) you may need to add a Boot Parameter Block(BPB) to the beginning of your bootloader. You can create a fake one like this:
org 0x7c00 bits 16 boot: jmp main TIMES 3-($-$$) DB 0x90 ; Support 2 or 3 byte encoded JMPs before BPB. ; Dos 4.0 EBPB 1.44MB floppy OEMname: db "mkfs.fat" ; mkfs.fat is what OEMname mkdosfs uses bytesPerSector: dw 512 sectPerCluster: db 1 reservedSectors: dw 1 numFAT: db 2 numRootDirEntries: dw 224 numSectors: dw 2880 mediaType: db 0xf0 numFATsectors: dw 9 sectorsPerTrack: dw 18 numHeads: dw 2 numHiddenSectors: dd 0 numSectorsHuge: dd 0 driveNum: db 0 reserved: db 0 signature: db 0x29 volumeID: dd 0x2d7e5a1a volumeLabel: db "NO NAME " fileSysType: db "FAT12 " main: cli xor ax, ax ; Explicitly set DS and ES to 0 mov ds, ax mov es, ax [rest of your code here]If you were to modify your code to have the layout above the Unix/Linux
filecommand may be able to dump out the BPB data that it thinks makes up your MBR in the disk image. Run the commandfile disk.imgand you may get this output:disk.img: DOS/MBR boot sector, code offset 0x3c+2, OEM-ID "mkfs.fat", root entries 224, sectors 2880 (volumes <=32 MB) , sectors/FAT 9, sectors/track 18, serial number 0x2d7e5a1a, unlabeled, FAT (12 bit)
Complete Example with Int 13h Extension Checks
The following code will check that Int 13h extensions are available in the BIOS and will also determine if the drive in DL supports Int 13h extensions. If there is a failure the code will print an appropriate error. A
print_stringfunction is used to display strings to the console using BIOS TTY output and aprint_hex_wordfunction is provided to display the boot drive number in hexadecimal.org 0x7c00 bits 16 section .text boot: jmp main TIMES 3-($-$$) DB 0x90 ; Support 2 or 3 byte encoded JMPs before BPB. ; Dos 4.0 EBPB 1.44MB floppy OEMname: db "mkfs.fat" ; mkfs.fat is what OEMname mkdosfs uses bytesPerSector: dw 512 sectPerCluster: db 1 reservedSectors: dw 1 numFAT: db 2 numRootDirEntries: dw 224 numSectors: dw 2880 mediaType: db 0xf0 numFATsectors: dw 9 sectorsPerTrack: dw 18 numHeads: dw 2 numHiddenSectors: dd 0 numSectorsHuge: dd 0 driveNum: db 0 reserved: db 0 signature: db 0x29 volumeID: dd 0x2d7e5a1a volumeLabel: db "NO NAME " fileSysType: db "FAT12 " main: cli cld ; String instructions forward movement xor ax, ax mov ds, ax mov es, ax ; Setup 4K stack before this bootloader mov ss, ax mov sp, 0x7c00 ; Display a banner to know our bootloader is executing mov si, msg_booting call print_string ; Check that Int 13h Extensions are available ; http://www.ctyme.com/intr/rb-0706.htm mov ah, 0x41 ; Int 13h/AH=41h: Check if extensions present mov bx, 0x55aa int 0x13 jc ext_drv_none ; CF set - no extensions available for drive cmp bx, 0xaa55 ; Is BX 0xaa55? jnz ext_none ; If not, int 13h extensions not supported ; by BIOS at all. ; Int 13h extensions supported by BIOS and drive at this point ; Load next sectors mov si, DAP mov ah, 42h ; DL didn't changed int 13h ; Start jmp bootend ; Error: BIOS doesn't support Int 13h extensions ext_none: mov si, err_no_extensions call print_string jmp error_end ; Error: BIOS supports Int 13h extensions but not for drive in DL ext_drv_none: mov si, err_no_drv_ext_support call print_string ; Print the boot drive number in hex xor dh, dh ; Zero extended drive number to all of DX push word 0x00 ; Attribute and page number to write to push dx ; The value to write as hex call print_hex_word error_end: cli .loop: hlt jmp .loop ; Print 16 bit value passed on stack as first parameter ; in hexadecimal. Use page number and foreground color ; passed in second parameter. This routine will work on 8086+ ; processors. This code takes advantage of packed BCD to ; determine the ASCII values to print. This code could have ; used compare and branch to do the same or a translation table. print_hex_word: push bp mov bp, sp ; BP=SP, on 8086 can't use sp in memory operand push dx ; Save all registers we clobber push cx push bx push ax mov cx, 0x0404 ; CH = number of nibbles to process = 4 (4*4=16 bits) ; CL = Number of bits to rotate each iteration = 4 (a nibble) mov dx, [bp+4] ; DX = word parameter on stack at [bp+4] to print mov bx, [bp+6] ; BX = page / foreground attr is at [bp+6] .loop: rol dx, cl ; Roll 4 bits left. Lower nibble is value to print mov ax, 0x0e0f ; AH=0E (BIOS tty print),AL=mask to get lower nibble and al, dl ; AL=copy of lower nibble add al, 0x90 ; Work as if we are packed BCD daa ; Decimal adjust after add. ; If nibble in AL was between 0 and 9, then CF=0 and ; AL=0x90 to 0x99 ; If nibble in AL was between A and F, then CF=1 and ; AL=0x00 to 0x05 adc al, 0x40 ; AL=0xD0 to 0xD9 ; or AL=0x41 to 0x46 daa ; AL=0x30 to 0x39 (ASCII '0' to '9') ; or AL=0x41 to 0x46 (ASCII 'A' to 'F') int 0x10 ; Print ASCII character in AL dec ch jnz .loop ; Go back if more nibbles to process pop ax ; Restore registers pop bx pop cx pop dx pop bp ret ; Print string pointed to by DS:SI using ; BIOS TTY output via int 10h/AH=0eh print_string: push ax push si mov ah, 0Eh ; int 10h 'print char' function .repeat: lodsb ; Get character from string test al, al je .done ; If char is zero, end of string int 10h ; Otherwise, print it jmp .repeat .done: pop si pop ax ret ; Disk address packet DAP: db 10h, 0 dw 1 ; Number of sectors to be loaded dd bootend dq 1 msg_booting: db "Booting... ", 0x00 err_no_extensions: db "Int 13h extensions not supported by BIOS", 0x00 err_no_drv_ext_support: db "Int 13h Extensions not supported on drive 0x", 0x00 ; Fill the rest of bootsector with zeroes and end it times 510 - ($ - boot) db 0 dw 0xAA55 bootend: mov ax, 0xb800 mov es, ax mov word [es:0x0000], 0x57<<8 | 'M' mov word [es:0x0002], 0x57<<8 | 'D' mov word [es:0x0004], 0x57<<8 | 'P' cli hltUsing the information to make this a hard disk image above, when I run it in BOCHS I get this output:
If I boot this same image as a floppy disk (disk A:) in BOCHS the error message now tells me that Int 13h extensions aren't available for drive 0x0000:
If you find that you need to read a device that doesn't support Int 13h extensions you will have to fall back to the standard Int 13h disk subfunctions for reading (AH=02)/writing (AH=03).
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