[core] fix MBR protective message overwriting the primary GPT

* We distractedly chose to populate the message from our protective MBR
  for GPT/UEFI-only boot media into the 4KB that directly followed the
  MBR, which of course is space that is being used by the primary GPT.
* This resulted on systems having to fall back to using the secondary
  GPT, which not all appear to be designed to do.
* Alter the code to ensure the protective message is written at LBA 34,
  after the primary GPT.
* Closes #1507

res/mbr/Makefile

@@ -53,9 +53,11 @@ dis: $(TEST_TARGET).out
 # Run the MBR in a Bochs environment (append msg.txt in subsequent blocks)
 test: $(TEST_TARGET).bin
 	@test -s $(BOCHS) || { echo "Error: $(BOCHS) was not found on this system"; exit 1; }
-	@cat $(TEST_TARGET).bin msg.txt > disk.img
+	@cp $(TEST_TARGET).bin disk.img
+	@truncate -c -s 17K disk.img
+	@cat msg.txt >> disk.img
 	@truncate -c -s 10M disk.img
-	-@$(BOCHS) -f bochsrc.bxrc -q
+	@-$(BOCHS) -f bochsrc.bxrc -q
 
 %.out: %.o mbr.ld
 	@echo "[LD]  $@"

res/mbr/msg.S

@@ -31,6 +31,7 @@
 /********************************************************************************/
 MBR_ADDR      = 0x7c00
 MBR_RESERVED  = 0x1b8	# Start of the reserved section (partition table, etc.)
+MSG_SECTOR    = 0x22  # First sector of the message (must be after the GPT)
 NB_SECTORS    = 0x08	# Number of sectors to read
 PT_MAX        = 0x04	# Number of partition entries in the partition table
 PT_ENTRY_SIZE = 0x10	# Size of a partition entry in the partition table
@@ -109,10 +110,9 @@ read_sectors:			# Copy the next sectors into RAM
 ext:	# http://en.wikipedia.org/wiki/INT_13H#INT_13h_AH.3D42h:_Extended_Read_Sectors_From_Drive
 	xor  eax, eax
 	push eax		# bits 32-63 of sector address
-	inc  eax
-	push eax		# bits 0-31 of sector address
+	push MSG_SECTOR # bits 0-31 of sector address
 	push es			# destination segment
-	push 0x0000		# destination address
+	push eax		# destination address (0)
 	push NB_SECTORS		# number of sectors to be read
 	push 0x0010		# size of DAP struct
 	mov  si, sp		# DAP address (= stack)
@@ -125,7 +125,7 @@ ext:	# http://en.wikipedia.org/wiki/INT_13H#INT_13h_AH.3D42h:_Extended_Read_Sect
 
 no_ext:	# http://en.wikipedia.org/wiki/INT_13H#INT_13h_AH.3D02h:_Read_Sectors_From_Drive
 	mov  ax, 0x0200 + NB_SECTORS
-	mov  cx, 0x0002		# Sector address (starts at 1)
+	mov  cx, (MSG_SECTOR + 1)	# Sector address (starts at 1)
 	mov  dx, 0x0080		# Drive ID
 	xor  bx, bx		# Destination address in ES
 	int  0x13

res/mbr/msg.txt

@@ -10,8 +10,8 @@
              boot it in BIOS/Legacy mode. THIS WILL NOT WORK!
 
              To remove this message you need to do \02ONE\07 of the following:
-             o If this computer supports UEFI, go to the UEFI settings
-               and disable or lower the priority of \09CSM/Legacy mode\07.
+             o If this computer supports UEFI, go to your UEFI settings
+               and lower or disable the priority of \09CSM/Legacy mode\07.
              o \02OR\07 Recreate the drive in Rufus and use:
                * \09Partition scheme\07 -> \09MBR\07.
                * \09Target system\07 -> \09BIOS (...)\07

res/mbr/readme.txt

@@ -15,7 +15,8 @@ the partition table lists the disk ID for the first partition as 0x81, then it
 will be swapped for 0x80.
 
 An additional MBR (msg.S) also exists in this directory, that can be used to
-display an ASCII message contained in the sectors directly following the MBR.
+display an ASCII message contained in the sectors following the primary GPT
+(LBA sectors 34 and later).
 This can be used, for instance, to display a notice for media that cannot be
 booted in BIOS/Legacy mode.