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[FREELDR]: Update the FreeLdr's startup notes.

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svn path=/trunk/; revision=73859
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Hermès Bélusca-Maïto 2017-02-20 15:45:01 +00:00
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reactos/boot/freeldr/notes.txt
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@ -1,6 +1,8 @@
FreeLoader notes
FreeLoader notes (for x86 PC architecture only!)
================================================
Memory layout:
Memory layout (WARNING: may be out-of-date)
-~-~-~-~-~-~-
0000:0000 - 0000:0FFF: Interrupt vector table & BIOS data
0000:1000 - 0000:6FFF: Real mode stack area
@ -14,44 +16,45 @@ 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.
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
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.
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 0000:8000 it will start by relocating itself there
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
-~-~-~-~-~-~-~-~-~-~-~-~-
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().
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().