[BOOTSECT]

Add a simple MBR boot sector that chainloads the El Torito BIOS Boot Sector of the ReactOS ISO.
This is our equivalent to syslinux's isohdpfx.S and will be used as input for the soon to be imported "isohybrid" patching tool.

Largely untested Work In Progress towards CORE-12648

svn path=/trunk/; revision=73555

reactos/boot/freeldr/bootsect/CMakeLists.txt

@@ -20,6 +20,7 @@ if(ARCH STREQUAL "i386" OR ARCH STREQUAL "amd64")
 
     CreateBootSectorTarget(isoboot ${CMAKE_CURRENT_SOURCE_DIR}/isoboot.S ${CMAKE_CURRENT_BINARY_DIR}/isoboot.bin 7000)
     CreateBootSectorTarget(isobtrt ${CMAKE_CURRENT_SOURCE_DIR}/isobtrt.S ${CMAKE_CURRENT_BINARY_DIR}/isobtrt.bin 7000)
+    CreateBootSectorTarget(isombr ${CMAKE_CURRENT_SOURCE_DIR}/isombr.S ${CMAKE_CURRENT_BINARY_DIR}/isombr.bin 7000)
 
     add_cd_file(TARGET dosmbr DESTINATION loader NO_CAB FILE ${CMAKE_CURRENT_BINARY_DIR}/dosmbr.bin FOR bootcd regtest)
     add_cd_file(TARGET ext2 DESTINATION loader NO_CAB FILE ${CMAKE_CURRENT_BINARY_DIR}/ext2.bin FOR bootcd regtest)

reactos/boot/freeldr/bootsect/isombr.S

@@ -0,0 +1,102 @@
+/*
+ * PROJECT:     ReactOS MBR Boot Sector for ISO file system ("isohybrid mode")
+ * LICENSE:     GNU GPLv2 or any later version as published by the Free Software Foundation
+ * 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.
+
+.endcode16
+
+END