reactos/boot/freeldr/bootsect/isombr.S

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