bochs：无法使用软盘映像加载引导加载程序

Question

I have a simple (probably the simplest) bootloader. Very similar to this and this . The code is shown below

[org  0x7c00]

mov ah, 0x0e
mov al, 'h'

int 0x10

jmp  $

times  510-($-$$)  db  0

dw  0xaa55

The code is taken from the video series tutorial from here .

I have tried everything to compile and load this bootloader using bochs. But whatever I do, I get a black screen like the following

Nothing comes on the screen and after a bit I get that bochs is not responding.

My present approach to load the image is -

1. nasm boot_simple.asm -f bin -o boot.bin

2. dd if=boot.bin of=boot.img bs=512

3. bochs -f /dev/null -q 'display_library: sdl2' 'boot:a' 'floppya: 1_44=boot.img, status=inserted'

I tried many other approaches (slight variations of the above) (even tried to compile and load a separate bootloader from here ) But nothing worked. Always the same black screen and does not show any other information.

I am becoming mad. What am I missing?

Ubuntu 22.04

Bochs version : Bochs x86 Emulator 2.7 (installed via sudo apt install )

I am super new to OS development and I want to learn it as a side project. Please help me getting started. Could not find suitable information anywhere.

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Checking the log I see the following

00000000000i[FLOPPY] fd0: 'boot.img' ro=0, h=2,t=80,spt=18
00000000000i[FLOPPY] Using boot sequence floppy, none, none
00000000000i[FLOPPY] Floppy boot signature check is enabled

Not sure how to interpret this (if this is normal or some kind of error / mistake is hiding in them)

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Tried to follow these steps as per here and here which I believe should create a simple 1.44 MB image, place the bin in the first sector and leave it to it. When done in this way (as well not padding it) in both cases qemu loads the image and shows the h each time without fault. Bochs does not load anything. Whatever I do. Always the same black screen and afterwards Not responding

I uninstalled, purged, and then reinstalled bochs. I do not know what else I can do. I can't understand why it fails to load the simple bootloader.

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Here are the steps

1. nasm boot_simple.asm -f bin -o boot.bin

2. dd if=/dev/zero of=floppy.img bs=1024 count=1440

3. dd if=boot.bin of=floppy.img seek=0 count=1 conv=notrunc

4. mkfs.vfat -F 12 ./floppy.img (This step makes the floppy un-bootable in qemu but really does not change anything for bochs)

5. bochs -f /dev/null -q 'display_library: x' 'boot:floppy' 'floppya: 1_44=floppy.img, status=inserted' 'romimage: file=/usr/share/bochs/BIOS-bochs-latest' 'error: action=report'

But nothing happens. Bochs boots to a black screen.

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Ok I have a slight idea. It seems that for some reason bochs halts at the first line of the code.

Printing something like this -

Next at t=0
(0) [0x0000fffffff0] f000:fff0 (unk. ctxt): jmpf 0xf000:e05b          ; ea5be000f0
<bochs:1>

Then waiting at the prompt <bochs:1> . If I do step there it does step through some instructions. Is there something I am missing here? Is there a bug in my code?

Answer 1

BOCHS will always stop at the first instruction in the BIOS when launched. The first instruction is at 0xf000:0xfff0 which is what you see in the output. This gives you a chance to set breakpoints ahead of time. For example b 0x7c00 would break at the first instruction of the bootloader if you wished. To continue running just use the command c to continue.