le0kar0ub1 / SecondBreath

x86 bootloader

SecondBreath

x86 bootloader wroten in GNU syntax assembly.

One of many 2 stages minimal bootloader (which load the Kernel in higher half).

Dependencies

• make (dev under V4.3)

• as (dev under V2.34)

• ld (dev under V2.34)

Build

make arch=$ARCH

Where $ARCH is the arch you want to target.

Available are x86 and x64 (defaulting to x86 if unset).

Tree

Path	Description
arch	Arch-specific objects
boot	The purposed bootloader
krn	A kernel entry

x86 Memory map

Binary Offset	Virtual Address	Description
0x0 - 0x200	0x7C00 - 0x7E00	DOS bootsector
0x200 - 0x8400	0x7E00 - 0x10000	Bootloader second breath
0x10000 - ...	0xC0010000 - ...	Kernel

x64 Memory map (not implemented yet)

Binary Offset	Virtual Address	Description
0x0 - 0x200	0x7C00 - 0x7E00	DOS bootsector
0x200 - 0x8400	0x7E00 - 0x10000	Bootloader second breath
0x10000 - ...	0xFFFFFFFF80010000 - ...	Kernel

Bootloader steps

• Load the stage 2 bootloader
• Enable A20 by all means
• Load the GDT
• Load the totality of the kernel at 0x10000
• Setup some informations for kernel usage
• Enable Protected Mode
• Transfer control to the kernel

About

A kernel entry is provided in krn directory.

To use this bootloader you must provide the classical symbol entry _start.

The kernel is loaded in higher half but no memory mapping is provided by the bootloader so far.

You must then use a virtToPhys operation on every absolute addressing.

Epilogue

Just for the fun.

Feel free to fork, use, improve.