GSoC2009 Liu Notes
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[edit] Notes: Yuanhan Liu, Add EXT4 Support in EXTLINUX and Convert the Filesystem Part Code to C
[edit] Code reading notes
Here takes some notes while reading the Syslinux source code.
[edit] The js instruction problem
After seeing the following code in diskstart.inc make me reminds of the interview of Syslinux GSoC2009 in the IRC.
mov [DriveNumber],dl ; Save drive number in DL
and dl,dl ; If floppy disk (00-7F), assume no partition table
js harddisk
It's quite same as the following code asked by hpa in the interview:
and eax, eax
js .foo
call edx
.foo:
yeah, it's a simple question, but at that moment I'm not good at the flags so a wrong answer answered. In fact, the fisrt code explains it well. As we know, the hard disk drive number counts from 0x80 that means the most significant bit is always 1 make the js be true to jump the destination. while in C, we can simply implement it in the following way:
if ( drive_num >= 0x80 ) {
/* harddisk */
} else {
/* floppy */
}
[edit] The chosen of CBIOS or EBIOS by getlinsec
hpa made a trick here. It's not done in function getlinsec but in the EDD check program in a patch way; that's the trick. In the diskstart.inc, there's a EDD check program. If EDD checked, then:
;
; We have EDD support...
;
mov byte [getlinsec.jmp+1],(getlinsec_ebios-(getlinsec.jmp+2))
getlinsec function will jump to getlinsec_cbios by default; after this, it will jump to getlinsec_ebios instead.
getlinsec:
add eax,[bsHidden] ; Add partition offset
xor edx,edx ; Zero-extend LBA (eventually allow 64 bits)
.jmp: jmp strict short getlinsec_cbios
