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times 512-($-$$) db 0 |
times 512-($-$$) db 0 |
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</source> |
</source> |
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The CPU starts in real mode and the BIOS loads this code at address 0000:7c00. |
The CPU starts in real mode and the [[BIOS]] loads this code at address 0000:7c00. "times 512-($-$$) db 0" is NASM's way of saying fill up 512 bytes with zeros. And partcopy is going to expect that (200 in Hex = 512 in Decimal). Change it and you'll see partcopy choke. |
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Often, you will see a |
Often, you will see a boot signature (0xAA55) at the end. Older versions of BIOSes looked for this in order to identify a boot sector on a disk. It is evidently unnecessary nowadays. If it's needed, the last line would be replaced with (or some version of it): |
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<source lang="asm"> |
<source lang="asm"> |
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; boot.asm |
; boot.asm |
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</source> |
</source> |
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Once you've booted, and the cursor is happily blinking on a blank screen, you might notice two things. One is that the floppy motor will turn off and the other is that you can press Ctrl-Alt-Del to reboot. The point is that [[interrupts]] as still being generated. |
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Try clearing the interrupts flag: |
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<source lang="asm"> |
<source lang="asm"> |
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;boot.asm |
;boot.asm |
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==Creating disk image== |
==Creating disk image== |
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The code is assembled in [[NASM]] and copied to floppy using partcopy,dd,or debug. Then you simply boot from the floppy. |
The code is assembled in [[NASM]] and copied to floppy using partcopy, dd, or debug. Then you simply boot from the floppy. |
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===Windows=== |
===Windows=== |
Revision as of 00:17, 1 November 2017
Difficulty level |
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![]() Beginner |
Babystep1: Your first boot sector | |
Tutorial | |
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Babystep2 |
Your first boot sector.
Code
The following code is the smallest possible example of booting code from a floppy.
; boot.asm
hang:
jmp hang
times 512-($-$$) db 0
The CPU starts in real mode and the BIOS loads this code at address 0000:7c00. "times 512-($-$$) db 0" is NASM's way of saying fill up 512 bytes with zeros. And partcopy is going to expect that (200 in Hex = 512 in Decimal). Change it and you'll see partcopy choke.
Often, you will see a boot signature (0xAA55) at the end. Older versions of BIOSes looked for this in order to identify a boot sector on a disk. It is evidently unnecessary nowadays. If it's needed, the last line would be replaced with (or some version of it):
; boot.asm
hang:
jmp hang
times 510-($-$$) db 0 ; 2 bytes less now
db 0x55
db 0xAA
Once you've booted, and the cursor is happily blinking on a blank screen, you might notice two things. One is that the floppy motor will turn off and the other is that you can press Ctrl-Alt-Del to reboot. The point is that interrupts as still being generated.
Try clearing the interrupts flag:
;boot.asm
cli
hang:
jmp hang
times 510-($-$$) db 0
db 0x55
db 0xAA
You may notice that the floppy motor doesn't turn off and you can't reboot with Ctrl-Alt-Del.
If you try to reduce this even more by removing the loop and merely pad out the sector with zeros, the BIOS will have something to say about it. On my machine, it was "Operating System Not Found". I have yet to try filling the sector with zeros except for adding a boot signature.
Not exactly something you would show your friends, but I wanted to show just what the bare minimum is before I elaborate. Unless I'm irritating anyone, in which case I'll desist.
Creating disk image
The code is assembled in NASM and copied to floppy using partcopy, dd, or debug. Then you simply boot from the floppy.
Windows
nasmw boot.asm -f bin -o boot.bin
partcopy boot.bin 0 200 -f0
OR
debug boot.bin
-W 100 0 0 1
-Q
Unix
nasm boot.asm -f bin -o boot.bin
dd if=boot.bin of=/dev/fd0