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106 lines
3.2 KiB
ArmAsm
106 lines
3.2 KiB
ArmAsm
/*
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* PROJECT: ReactOS MBR Boot Sector for ISO file system ("isohybrid mode")
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* LICENSE: GPL-2.0+ (https://spdx.org/licenses/GPL-2.0+)
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* PURPOSE: Providing an MBR Boot Sector that enables an ISO to be booted from a disk
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* COPYRIGHT: Copyright 2017 Colin Finck (colin@reactos.org)
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*/
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#include <asm.inc>
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.code16
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// The "isohybrid" tool patching our ISO will write the LBA of the El Torito BIOS Boot Sector to this address.
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pBootSectorLBA = start+432
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// Our BIOS Boot Sector contains this signature at the specified position and our third-party imported "isohybrid" patching tool checks for it.
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// This is also why it's the same signature ISOLINUX uses.
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HybridSignature = HEX(7078C0FB)
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pBootSectorHybridSignature = HEX(7C00)+64
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pBootSectorHybridEntryPoint = HEX(7C00)+64+4
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start:
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// The MBR needs to start with 33h, because some dumb BIOSes check for that (cf. syslinux commit d0f275981c9289dc4b8df64e72cd9902bf85aebe).
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// The following line encodes a "xor ax, ax" (but it's not the only way to encode it, so we can't just write that instruction here).
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.byte HEX(33), HEX(C0)
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// Set up our stack and a flat addressing model.
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cli
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mov ss, ax
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mov sp, offset start
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mov ds, ax
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mov es, ax
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mov fs, ax
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mov gs, ax
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sti
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// Our boot sector has been loaded to address 0x7C00.
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// Relocate our 512 bytes boot sector to the given base address (should be 0x7000).
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cld
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mov cx, 512 / 4
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mov si, HEX(7C00)
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mov di, offset start
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rep movsd
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// Jump to the relocated code.
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ljmp16 0, relocated
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relocated:
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// Prepare the Disk Access Packet (DAP) for INT 13h, Function 42h: Extended Read Sectors from Drive.
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// Read 4 sectors to address 7C00h, which is exactly our 2K-sized BIOS Boot Sector.
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push 0
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push 0
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push dword ptr es:[pBootSectorLBA]
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push es
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push HEX(7C00)
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push 4
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push HEX(10)
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// Call the BIOS function. Note that we haven't clobbered DL up to this point, so the Drive Number passed by the BIOS is still there.
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// Read errors are indicated by the Carry Flag.
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mov ah, HEX(42)
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mov si, sp
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int HEX(13)
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jc read_error
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// Verify the Hybrid Signature.
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cmp dword ptr es:[pBootSectorHybridSignature], HybridSignature
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jne invalid_signature
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// Signature is valid, so jump to the entry point for the hybrid code.
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ljmp16 0, pBootSectorHybridEntryPoint
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read_error:
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call die_with_error
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.ascii "ISOMBR: Read Error!", CR, LF
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invalid_signature:
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call die_with_error
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.ascii "ISOMBR: Invalid Boot Sector Hybrid Signature!", CR, LF
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die_with_error:
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// Fetch the message to output stored at the return address on the stack.
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pop si
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// Call BIOS INT 10h, Function 0Eh to output a single character.
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// Do this in a loop and stop after we have printed the newline LF character.
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next_character:
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lodsb
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mov ah, HEX(0E)
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xor bx, bx
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int HEX(10)
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cmp al, 10
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jne next_character
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// Die gracefully, that means in an infinite HLT loop to not put any stress on the CPU.
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die:
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hlt
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jmp die
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// The "isohybrid" tool will add the remaining information, including the infamous 0xAA55 MBR signature.
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.org 431
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.byte 0
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.endcode16
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END
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