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61 lines
2.5 KiB
Plaintext
61 lines
2.5 KiB
Plaintext
FreeLoader notes (for x86 PC architecture only!)
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================================================
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Memory layout (WARNING: may be out-of-date)
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-~-~-~-~-~-~-
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0000:0000 - 0000:0FFF: Interrupt vector table & BIOS data
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0000:1000 - 0000:6FFF: Real mode stack area
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0000:7000 - 0000:7FFF: Cmdline (multiboot)
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0000:8000 - xxxx:xxxx: FreeLoader program & data area
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xxxx:xxxx - 7000:7FFF: Random memory allocation heap
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7000:8000 - 7000:FFFF: Protected mode stack area
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8000:0000 - 8000:FFFF: File system read buffer
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9000:0000 - 9000:FFFF: Disk read buffer for BIOS Int 13h
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A000:0000 - FFFF:FFFF: reserved
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FreeLoader Boot Process
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-~-~-~-~-~-~-~-~-~-~-~-
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FAT 12/16/32 Boot Sector
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The BIOS loads the boot sector at 0000:7C00. The FAT12/16 boot sector just
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searches for FREELDR.SYS and loads its first 512 bytes to 0000:F800. This extra
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helper code enables it to fully navigate the file allocation table. The boot
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sector then jumps to FREELDR.SYS entry point at 0000:F803 and the helper code
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takes the relay. It finishes loading the FREELDR.SYS image and finally jumps to
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its final entry point at 0000:FA00.
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The FAT32 boot sector loads its extra sector at 0000:7E00 and looks for
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FREELDR.SYS on the file system. Once found it loads FREELDR.SYS to 0000:F800
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and jumps to its entry point at the same address. This allows it to jump over
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the FAT12/16 extra helper code situated at this address, and go to the final
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entry point at 0000:FA00.
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ISO-9660 (CD-ROM) Boot Sector
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The BIOS loads the boot sector (2048 bytes) at 0000:7C00. First, the
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boot sector relocates itself to 0000:7000 (up to 0000:7800). Then it looks
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for the LOADER directory and makes it the current directory. Next it looks
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for FREELDR.SYS and loads it at 0000:F800. Finally it restores the boot drive
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number in the DL register and jumps to FreeLoader's entry point at 0000:F800.
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Multiboot
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Freeldr contains a multiboot signature and can itself be loaded by a
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multiboot-compliant loader (like GRUB). The multiboot header instructs the
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primary loader to load FREELDR.SYS at 0x200000 (needs to be above 1MB). Control
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is then transferred to the multiboot entry point. Since FREELDR.SYS expects to
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be loaded at a base address 0000:F800 it will start by relocating itself there
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and then jumping to the relocated copy.
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FreeLoader Initialization
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-~-~-~-~-~-~-~-~-~-~-~-~-
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When FreeLoader gets control it saves the boot drive and partition, passed
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to it respectively in the DL and DH registers, and sets up the stack, enables
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protected mode, and calls BootMain().
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