reactos/boot/freeldr/notes.txt

FreeLoader notes

Memory layout:

0000:0000 - 0000:0FFF: Interrupt vector table & BIOS data
0000:1000 - 0000:6FFF: Real mode stack area
0000:7000 - 0000:7FFF: Cmdline (multiboot)
0000:8000 - xxxx:xxxx: FreeLoader program & data area
xxxx:xxxx - 7000:7FFF: Random memory allocation heap
7000:8000 - 7000:FFFF: Protected mode stack area
8000:0000 - 8000:FFFF: File system read buffer
9000:0000 - 9000:FFFF: Disk read buffer for BIOS Int 13h
A000:0000 - FFFF:FFFF: reserved


FreeLoader Boot Process

FAT 12/16/32 Boot Sector

	The BIOS loads the boot sector at 0000:7C00. The FAT32 boot sector
relocates itself higher in memory at 9000:0000 and loads it's extra sector
at 9000:0200 and then looks for freeldr.sys on the file system. Once found
it loads freeldr.sys to 0000:7E00 and then jumps to it's entry point at
0000:8000. The FAT12/16 boot sector does no relocation, it just searches for
the freeldr.sys and loads the first 512 bytes to 0000:7E00. This extra code
enables it to fully navigate the file allocation table. Then it loads
freeldr.sys to 0000:7E00 and jumps to it's entry point at 0000:8000. Before
FreeLoader gets control the boot sector saves the screen contents to a buffer
at 9000:8000 and the cursor x & y position to bytes at 9000:8FA0 & 9000:8FA1
respectively.


ISO-9660 (CD-ROM) Boot Sector

	The BIOS loads the boot sector (2048 bytes) at 0000:7C00. First, the
boot sector relocates itself to 0000:7000 (up to 0000:7800). Then it looks
for the LOADER directory and makes it the current directory. Next it looks for
FREELDR.SYS and loads it at 0000:8000. Finally it restores the boot drive
number in the DL register and jumps to FreeLoader's entry point at 0000:8000.


Multiboot

	Freeldr contains a multiboot signature and can itself be loaded by a
multiboot-compliant loader (like Grub). The multiboot header instructs the
primary loader to load freeldr.sys at 0x200000 (needs to be above 1MB). Control
is then transferred to the multiboot entry point. Since freeldr.sys expects to
be loaded at a base address 0000:8000 it will start by relocating itself there
and then jumping to the relocated copy.



FreeLoader Initialization

	When FreeLoader gets control it saves the boot drive, passed to it in
the DL register, and sets up the stack, enables protected mode, and calls
BootMain().