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confused by the several lines of source code in mbr.S
First, I am confused by the following lines of code from the mbr.S in
i386/stand/mbr directory.
---------------------------------- snip
--------------------------------------------------------
216 movw $lba_info, %si
217 movb $0x42, %ah
218 pop %dx /* recover drive # */
219 push %dx /* save drive */
220 int $0x13
221 jc wait_key /* abort menu on read fail */
222 cmpw $MBR_MAGIC, LOADADDR + MBR_MAGIC_OFFSET
223 movw $nametab - LOADADDR + BOOTADDR, %bx
224 je next_extended
---------------------------------- snip
--------------------------------------------------------
Moreover, the BOOTADDR and LOADADDR are defined as below:
---------------------------------- snip
--------------------------------------------------------
65 #define BOOTADDR 0x7c00
66 #define LOADADDR 0x0600 /* address were are linked to */
---------------------------------- snip
--------------------------------------------------------
When we call INT 0x13, BIOS will transfer the PBR code to the address
BOOTADDR included in the lba_info struct, and then we should check
whether the PBR code is valid.
The address LOADADDR + MBR_MAGIC_OFFSET in line 222 is 0x0600 +
MBR_MAGIC_OFFSET, which is just the magic number's address of the mbr
code but not the PBR code, and no process will modify the mbr's magic
number, so, there is alwasy a jump to next_extended.
So, is there any other purpos or just a bug because of incaution?
Thanks for any tips or comments.
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