2018-06-05 14:34:03 -04:00
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BOOTBOOT BIOS / Multiboot Implementation
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========================================
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See [BOOTBOOT Protocol](https://gitlab.com/bztsrc/bootboot) for common details.
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On [BIOS](http://www.scs.stanford.edu/05au-cs240c/lab/specsbbs101.pdf) based systems, the same image can be loaded via
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[Multiboot](https://www.gnu.org/software/grub/manual/multiboot/multiboot.html),
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2018-07-22 02:34:56 -04:00
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chainload from MBR or VBR (GPT hybrid booting via __boot.bin__) or run as a BIOS Expansion ROM
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2018-06-05 14:34:03 -04:00
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(so not only the ramdisk can be in ROM, but the loader as well).
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Machine state
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-------------
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IRQs masked. GDT unspecified, but valid, IDT unset. Code is running in supervisor mode in ring 0.
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Installation
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------------
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1. *BIOS disk*: copy __bootboot.bin__ to **_FS0:\BOOTBOOT\LOADER_**. You can place it inside your INITRD partition
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or outside of partition area as well (with `dd conv=notrunc oseek=x`). Finally install __boot.bin__ in the
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master boot record (or in volume boot record if you have a boot manager), saving bootboot.bin's first sector's
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LBA number in a dword at 0x1B0. The [mkboot](https://gitlab.com/bztsrc/bootboot/blob/master/x86_64-bios/mkboot.c)
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utility will do that for you.
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2. *BIOS ROM*: install __bootboot.bin__ in a **_BIOS Expansion ROM_**.
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2018-07-22 02:34:56 -04:00
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3. *GRUB*: specify __bootboot.bin__ as a Multiboot "kernel" in grub.cfg, or you can also chainload __boot.bin__. You can load
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the initrd and the environment file as modules, if not given, they will be loaded from disk as usual. Example:
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```
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menuentry "MyKernel" {
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multiboot /bootboot/loader # bootboot.bin
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module /bootboot/initrd # first module is the initrd (optional)
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module /bootboot/config # second module is the environment file (optional)
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boot
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}
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```
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2018-06-05 14:34:03 -04:00
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Limitations
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-----------
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- As it boots in protected mode, it only maps the first 4G of RAM.
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- Compressed initrd in ROM is limited to ~96k.
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- The CMOS nvram does not store timezone, so always GMT+0 returned in bootboot.timezone.
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