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().