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58 lines
2.3 KiB
Plaintext
58 lines
2.3 KiB
Plaintext
FreeLoader notes
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Memory layout:
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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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FAT 12/16/32 Boot Sector
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The BIOS loads the boot sector at 0000:7C00. The FAT32 boot sector
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relocates itself higher in memory at 9000:0000 and loads it's extra sector
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at 9000:0200 and then looks for freeldr.sys on the file system. Once found
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it loads freeldr.sys to 0000:7E00 and then jumps to it's entry point at
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0000:8000. The FAT12/16 boot sector does no relocation, it just searches for
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the freeldr.sys and loads the first 512 bytes to 0000:7E00. This extra code
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enables it to fully navigate the file allocation table. Then it loads
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freeldr.sys to 0000:7E00 and jumps to it's entry point at 0000:8000. Before
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FreeLoader gets control the boot sector saves the screen contents to a buffer
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at 9000:8000 and the cursor x & y position to bytes at 9000:8FA0 & 9000:8FA1
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respectively.
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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 for
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FREELDR.SYS and loads it at 0000:8000. Finally it restores the boot drive
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number in the DL register and jumps to FreeLoader's entry point at 0000:8000.
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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 0000:8000 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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When FreeLoader gets control it saves the boot drive, passed to it in
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the DL register, and sets up the stack, enables protected mode, and calls
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BootMain().
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