[syslinux:master] chain.c32: add grldr= command for Grub4dos

[syslinux-bot for Kim Mik]

Commit-ID:  91334030e93d5d0a60a178e214140054dfc5d69c
Gitweb:     http://syslinux.zytor.com/commit/91334030e93d5d0a60a178e214140054dfc5d69c
Author:     Kim Mik <kimmik999999 at yahoo.co.uk>
AuthorDate: Sun, 10 Jan 2010 14:57:40 +0000
Committer:  H. Peter Anvin <hpa at zytor.com>
CommitDate: Sun, 10 Jan 2010 12:33:16 -0800

chain.c32: add grldr= command for Grub4dos

grldr of Grub4dos wants the partition number in DH:
	0xff: whole drive
	0-3:  primary partitions
	4-*:  logical partitions

Hmmm... there really isn't a huge reason not to do this
unconditionally, at least unless it's known to cause problems.  It
would be better, of course, if grldr used the standard DS:SI, but it
doesn't, so oh well.

Some info of a Grub4dos developer (Tinybit):
  GRLDR can be loaded at any address with alignment 16(i.e., a possible
  segment base address). Generally you want to load it at 0000:7C00, or at
  2000:0000. Of course you never load it at 0000:0000 or similar.

  Before jumping to the entry point at the very beginning of GRLDR, you
  should setup DL=(BIOS drive) and DH=(partition number). For partition
  numbers, 0 - 3 are primary, 4 - 0xFE are logical. (DH=0xFF) stands for
  whole drive(unpartitioned). DH will later be passed to
  install_partition(the third byte, from bit 16 to bit 23).

http://www.boot-land.net/forums/index.php?showtopic=8457&st=20&start=20 post #22


---
 com32/modules/chain.c |   13 ++++++++++++-
 1 files changed, 12 insertions(+), 1 deletions(-)

diff --git a/com32/modules/chain.c b/com32/modules/chain.c
index da64707..3d192d7 100644
--- a/com32/modules/chain.c
+++ b/com32/modules/chain.c
@@ -782,9 +782,20 @@ int main(int argc, char *argv[])
 
     whichpart = 0;		/* Default */
 
-    if (partition)
+    /* grldr of Grub4dos wants the partition number in DH:
+	 0xff: whole drive (default)
+	 0-3:  primary partitions
+	 4-*:  logical partitions
+     */
+    regs.edx.b[1] = 0xff;
+
+    if (partition) {
 	whichpart = strtoul(partition, NULL, 0);
 
+	/* grldr of Grub4dos wants the partiton number in DH. */
+	regs.edx.b[1] = whichpart -1;
+    }
+
     if (!(drive & 0x80) && whichpart) {
 	error("Warning: Partitions of floppy devices may not work\n");
     }