Export the 64-bit partition offset and use it in chain.c

When used with the "fs" option to chain.c32, we need to know our own
filesystem offset.  That means knowing if we used the MBR vs GPT
partition information, as well as if we ended up using the passed-in
information or not.

Resolve this by providing an explicit pointer to the current partition
offset.  Eventually this should be replaced by some kind of statfs()
call.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 5 insertions, 4 deletions

diff --git a/doc/comboot.txt b/doc/comboot.txt
index 9e9f99a0..4b4b8803 100644
--- a/doc/comboot.txt
+++ b/doc/comboot.txt
@@ -426,8 +426,12 @@ AX=000Ah [2.00]	Get Derivative-Specific Information
 	Output:	AL	31h (SYSLINUX), 34h (EXTLINUX)
 		DL	drive number
 		CL	sector size as a power of 2 (9 = 512 bytes) [3.35]
+		CH	mode [3.73]
+			1 = CBIOS mode
+			2 = EBIOS mode
 		ES:BX	pointer to partition table entry (if DL >= 80h)
 		FS:SI	pointer to initial ES:DI value [3.53]
+		GS:DI	pointer to partition offset (QWORD) [4.00]
 
 		Note: This function was broken in EXTLINUX 3.00-3.02.
 
@@ -501,6 +505,7 @@ AX=000Ah [2.00]	Get Derivative-Specific Information
 			2 = Hybrid (hard disk), EBIOS mode
 		ES:BX	pointer to El Torito spec packet
 		FS:SI	pointer to initial ES:DI value [3.53]
+		GS:DI	pointer to partition offset (QWORD) [4.00]
 
 		Note: Some very broken El Torito implementations do
 		not provide the spec packet information.  If so, ES:BX
@@ -508,10 +513,6 @@ AX=000Ah [2.00]	Get Derivative-Specific Information
 		AX=4B01h to obtain the spec packet directly from the
 		BIOS if necessary.
 
-	This call gives information specific to a particular Syslinux
-	derivative.  The value returned in AL is the same as is
-	returned in DL by INT 22h AX=0001h.
-
 
 AX=000Bh [2.00]	Get Serial Console Configuration