reactos/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: Unused
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 I386 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.


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().