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linux-bootstrap-1: Clearify the boot sector origin example

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Following abf4f68 - removes all references to the deleted [ORG 0x7c00],
and generally mention that the BIOS copies the boot sector into that
fixed memory location
This commit is contained in:
Roy Kupershmid 2020-09-26 20:48:40 +03:00
parent 738fefc3c3
commit f57c653b00
1 changed files with 4 additions and 3 deletions
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7
Booting/linux-bootstrap-1.md
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@ -103,7 +103,7 @@ SECTIONS {
}
```
Now the BIOS starts. After initializing and checking the hardware, the BIOS needs to find a bootable device. A boot order is stored in the BIOS configuration, controlling which devices the BIOS attempts to boot from. When attempting to boot from a hard drive, the BIOS tries to find a boot sector. On hard drives partitioned with an [MBR partition layout](https://en.wikipedia.org/wiki/Master_boot_record), the boot sector is stored in the first `446` bytes of the first sector, where each sector is `512` bytes. The final two bytes of the first sector are `0x55` and `0xaa`, which designates to the BIOS that this device is bootable.
Now the BIOS starts. After initializing and checking the hardware, the BIOS needs to find a bootable device. A boot order is stored in the BIOS configuration, controlling which devices the BIOS attempts to boot from. When attempting to boot from a hard drive, the BIOS tries to find a boot sector. On hard drives partitioned with an [MBR partition layout](https://en.wikipedia.org/wiki/Master_boot_record), the boot sector is stored in the first `446` bytes of the first sector, where each sector is `512` bytes. The final two bytes of the first sector are `0x55` and `0xaa`, which designates to the BIOS that this device is bootable. Once the BIOS finds the boot sector, it copies it into a fixed memory location at 0x7c00, jumps to there and start executing it.
For example:
@ -134,13 +134,14 @@ Build and run this with:
nasm -f bin boot.nasm && qemu-system-x86_64 boot
```
This will instruct [QEMU](https://www.qemu.org/) to use the `boot` binary that we just built as a disk image. Since the binary generated by the assembly code above fulfills the requirements of the boot sector (the origin is set to `0x7c00` and we end it with the magic sequence), QEMU will treat the binary as the master boot record (MBR) of a disk image.
This will instruct [QEMU](https://www.qemu.org/) to use the `boot` binary that we just built as a disk image. Since the binary generated by the assembly code above fulfills the requirements of the boot sector (we end it with the magic sequence), QEMU will treat the binary as the master boot record (MBR) of a disk image. Note that when providing a boot binary image to QEMU, setting the origin to 0x7c00 (using `[ORG 0x7c00]`)
is unneeded.
You will see:
![Simple bootloader which prints only `!`](images/simple_bootloader.png)
In this example, we can see that the code will be executed in `16-bit` real mode and will start at `0x7c00` in memory. After starting, it calls the [0x10](http://www.ctyme.com/intr/rb-0106.htm) interrupt, which just prints the `!` symbol. It fills the remaining `510` bytes with zeros and finishes with the two magic bytes `0xaa` and `0x55`.
In this example, we can see that the code will be executed in `16-bit` real mode. After starting, it calls the [0x10](http://www.ctyme.com/intr/rb-0106.htm) interrupt, which just prints the `!` symbol. It fills the remaining `510` bytes with zeros and finishes with the two magic bytes `0xaa` and `0x55`.
You can see a binary dump of this using the `objdump` utility: