reactos/boot/freeldr/notes.txt

FreeLoader notes (for x86 PC architecture only!)
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Memory layout (WARNING: may be out-of-date)
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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
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FAT 12/16/32 Boot Sector

    The BIOS loads the boot sector at 0000:7C00. The FAT12/16 boot sector just
searches for FREELDR.SYS and loads its first 512 bytes to 0000:F800. This extra
helper code enables it to fully navigate the file allocation table. The boot
sector then jumps to FREELDR.SYS entry point at 0000:F803 and the helper code
takes the relay. It finishes loading the FREELDR.SYS image and finally jumps to
its final entry point at 0000:FA00.
    The FAT32 boot sector loads its extra sector at 0000:7E00 and looks for
FREELDR.SYS on the file system. Once found it loads FREELDR.SYS to 0000:F800
and jumps to its entry point at the same address. This allows it to jump over
the FAT12/16 extra helper code situated at this address, and go to the final
entry point at 0000:FA00.


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:F800. Finally it restores the boot drive
number in the DL register and jumps to FreeLoader's entry point at 0000:F800.


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:F800 it will start by relocating itself there
and then jumping to the relocated copy.


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