RoBootloader

Low Memory

start	end	use
0x00000	0x004ff	BIOS stuff, don't touch
0x00500	0x06bff	free space
0x06c00	0x07bff	Bootloader stack
0x07c00	0x07dff	Bootloader
0x07e00	0x07fff	free space
0x08000	0x0ffff	ELF file (64 sectors)
0x00000	0x7ffff	free space
0x80000	0x9ffff	EDBA, partially usable
0xa0000	0xfffff	unusable

Higher Memory

start	end	use
0x00000	0xfffff	Low memory
0x100000	0x200000	Kernel space

Bootsector layout

(all addresses are offsets from 0x7C00)

start	end	use
0x0000	?	Bootloader code
?	0x0193	padding
0x0194	0x01bd	SFS header
0x01be	?	padding
?	0x01fd	data
0x01fe	0x01ff	0x55AA boot magic

Hard Drive layout

start	end	use
0x00000000	0x000001ff	Bootloader code (including SFS superblock)
0x00000200	0x000081ff	SFS reserved area (kernel ELF file)
0x00008200	???	SFS data area
???	???	SFS free area
???	0x00ffffff	SFS index area
0x01000000	end	Unused (Disk size is set to 16MiB)

GDT

Entry	Base	Limit	Flags	Access	Purpose
0x0	0x00000000	0x00000	0b0000	0b00000000	Null Segment
0x8	0x00000000	0xfffff	0b1100	0b10011110	Code Segment
0x10	0x00000000	0xfffff	0b1100	0b10010110	Data Segment

Known issues

Stack setup

As I don't quite get how segments work in real mode, there are most likely errors in the way I set up my stack. I'm going on a it works on my machine approach currently.

Kernel is not a file

I reserve 32KiB of SFS reserved area for the ELF file of the kernel. This is currently for "historic reasons" (a.k.a. I'm too lazy to load the filesystem in the bootloader). The kernel then handles the filesystem.

BSS does not get cleared

I do calculate the size of the BSS, so either I intialize every variable in a function or things like global variables that are false by default don't work. It does work in emulators because they initialize all memory to 0, but real computerss don't always do that.

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