reactos/freeldr/notes.txt

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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().