[FREELDR] Introduce a MSVC "linker script" file that centralizes the commands for...
[reactos.git] / boot / freeldr / notes.txt
1 FreeLoader notes (for x86 PC architecture only!)
2 ================================================
4 Memory layout (WARNING: may be out-of-date)
5 -~-~-~-~-~-~-
7 0000:0000 - 0000:0FFF: Interrupt vector table & BIOS data
8 0000:1000 - 0000:6FFF: Real mode stack area
9 0000:7000 - 0000:7FFF: Cmdline (multiboot)
10 0000:8000 - xxxx:xxxx: FreeLoader program & data area
11 xxxx:xxxx - 7000:7FFF: Random memory allocation heap
12 7000:8000 - 7000:FFFF: Protected mode stack area
13 8000:0000 - 8000:FFFF: File system read buffer
14 9000:0000 - 9000:FFFF: Disk read buffer for BIOS Int 13h
15 A000:0000 - FFFF:FFFF: reserved
18 FreeLoader Boot Process
19 -~-~-~-~-~-~-~-~-~-~-~-
21 FAT 12/16/32 Boot Sector
23 The BIOS loads the boot sector at 0000:7C00. The FAT12/16 boot sector just
24 searches for FREELDR.SYS and loads its first 512 bytes to 0000:F800. This extra
25 helper code enables it to fully navigate the file allocation table. The boot
26 sector then jumps to FREELDR.SYS entry point at 0000:F803 and the helper code
27 takes the relay. It finishes loading the FREELDR.SYS image and finally jumps to
28 its final entry point at 0000:FA00.
29 The FAT32 boot sector loads its extra sector at 0000:7E00 and looks for
30 FREELDR.SYS on the file system. Once found it loads FREELDR.SYS to 0000:F800
31 and jumps to its entry point at the same address. This allows it to jump over
32 the FAT12/16 extra helper code situated at this address, and go to the final
33 entry point at 0000:FA00.
36 ISO-9660 (CD-ROM) Boot Sector
38 The BIOS loads the boot sector (2048 bytes) at 0000:7C00. First, the
39 boot sector relocates itself to 0000:7000 (up to 0000:7800). Then it looks
40 for the LOADER directory and makes it the current directory. Next it looks
41 for FREELDR.SYS and loads it at 0000:F800. Finally it restores the boot drive
42 number in the DL register and jumps to FreeLoader's entry point at 0000:F800.
45 Multiboot
47 Freeldr contains a multiboot signature and can itself be loaded by a
48 multiboot-compliant loader (like GRUB). The multiboot header instructs the
49 primary loader to load FREELDR.SYS at 0x200000 (needs to be above 1MB). Control
50 is then transferred to the multiboot entry point. Since FREELDR.SYS expects to
51 be loaded at a base address 0000:F800 it will start by relocating itself there
52 and then jumping to the relocated copy.
55 FreeLoader Initialization
56 -~-~-~-~-~-~-~-~-~-~-~-~-
58 When FreeLoader gets control it saves the boot drive and partition, passed
59 to it respectively in the DL and DH registers, and sets up the stack, enables
60 protected mode, and calls BootMain().