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Xtex

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Revision as of 14:18, 19 March 2020 (view source) osdev>Bzt m (→‎Creating disk image) ← Older edit		Latest revision as of 22:55, 15 June 2024 (view source) Xtex (talk \| contribs) mNo edit summary
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Line 12: ==Code== The following code is the smallest possible example of booting code from a ~~floppy~~disk. <~~source~~syntaxhighlight lang="asm"> ; boot.asm hang: Line 20: times 512-($-$$) db 0 </syntaxhighlight> ~~</source>~~ The CPU starts in real mode and the [[BIOS]] loads this code at address 0000:7c00. "<code>times 512-($-$$) db 0"</code> is NASM's way of saying fill up 512 bytes with zeroes, and partcopy is going to expect that (200 in Hex = 512 in Decimal). Change it and partcopy will likely fail. There is often a boot signature (0xAA55) at the end. Older versions of some BIOSes looked for this in order to identify a boot sector on a disk. It is evidently unnecessary nowadays, unless you're running the code on a legacy BIOS, or in QEMU. If it's needed, the last line would be replaced with (or some version of it): <~~source~~syntaxhighlight lang="asm"> ; boot.asm hang: Line 32: db 0x55 db 0xAA </syntaxhighlight> ~~</source>~~ Once you've booted, and the cursor is happily blinking on a blank screen, ~~The~~the ~~floppy~~disk's motor will turn off and you can now press Ctrl-Alt-Del to reboot. This is because [[interrupts]] are still being generated. Try clearing the interrupts flag: <~~source~~syntaxhighlight lang="asm"> ;boot.asm cli Line 46: db 0x55 db 0xAA </syntaxhighlight> ~~</source>~~ You may notice that the ~~floppy~~ motor doesn't turn off and you can't reboot with Ctrl-Alt-Del. Removing the loop and merely padding out the sector with zeroes will usually cause the BIOS to throw an error on boot. On most machines, it will say "Operating System Not Found". ==Creating disk image== The code is assembled in [[NASM]] and copied to floppy (outdated), disk or USB-sticks using partcopy, dd, or debug. Then you simply boot from ~~the~~that ~~floppy~~disk. For a more detailed description, see the [[Bootable Disk]] page. ===Windows=== <~~source~~syntaxhighlight lang="bash"> nasmw boot.asm -f bin -o boot.bin partcopy boot.bin 0 200 -f0 Line 63: debug boot.bin -W 100 0 0 1 -Q</~~source~~syntaxhighlight> ===Unix=== <~~source~~syntaxhighlight lang="bash"> nasm boot.asm -f bin -o boot.bin dd if=boot.bin of=/dev/fd0 </syntaxhighlight> ~~</source>~~ To write to a hard drive or USB-stick, use <syntaxhighlight lang="bash"> nasm boot.asm -f bin -o boot.bin dd if=boot.bin of=/dev/sda </syntaxhighlight> where replace "sda" with sdb, sdc etc. according to your configuration. ==Running the binary in QEMU== If you don't have an old machine with floppy drive you can emulate one using QEMU (note "'''f'''da"). <~~source~~syntaxhighlight lang="bash"> qemu-system-i386 -fda boot.bin </syntaxhighlight> ~~</source>~~ But it is advisable to forget about floppies altogether, and focus on USB-sticks instead. Also if you're afraid to test your code on your development machine (that would be wise), you can use QEMU (note "'''h'''da"). <syntaxhighlight lang="bash"> qemu-system-i386 -hda boot.bin </syntaxhighlight> Use the QEMU monitor command to send Ctrl-Alt-Del to the VM: <~~source~~syntaxhighlight lang="bash"> sendkey ctrl-alt-delete </syntaxhighlight> ~~</source>~~ Because of how fast emulation has become, you might need to slow down emulation speed to 1% to notice the reboots.