Don't be too smart, just try packet mode of INT13 first.

This way of operation is more robust than the "AI" used
before.

Add flags to mbr accessible from make.conf as BOOT_MBR_FLAGS.
Only one flag is defined now, "allow using packet mode", which
is 0x80 in accord with the rest of i386 boot code.  The "packet"
flag is on by default.

PR:             i386/70241
Submitted by:   Valentin Nechayev <netch <@> netch.kiev.ua> (inital version)
Discussed with: jhb (by Valentin Nechayev)
Tested on:      bochs (with EDD turned on or off by patching the BIOS), PCs

sys/boot/i386/mbr/Makefile

@@ -6,8 +6,12 @@ BINMODE=${NOBINMODE}
 NOMAN=
 SRCS=	${PROG}.s
 
+# MBR flags: 0x80 -- try packet interface (also known as EDD or LBA)
+BOOT_MBR_FLAGS?=	0x80
+
 ORG=	0x600
 
+AFLAGS+=--defsym FLAGS=${BOOT_MBR_FLAGS}
 LDFLAGS=-N -e start -Ttext ${ORG} -Wl,-S,--oformat,binary
 
 .include <bsd.prog.mk>

sys/boot/i386/mbr/mbr.s

@@ -21,6 +21,7 @@
 		.set EXEC,0x600 		# Execution address
 		.set PT_OFF,0x1be		# Partition table
 		.set MAGIC,0xaa55		# Magic: bootable
+		.set FL_PACKET,0x80		# Flag: try EDD
 
 		.set NHRDRV,0x475		# Number of hard drives
 
@@ -88,10 +89,8 @@ main.5:		movw %sp,%di			# Save stack pointer
 		movb 0x1(%si),%dh		# Load head
 		movw 0x2(%si),%cx		# Load cylinder:sector
 		movw $LOAD,%bx			# Transfer buffer
-		cmpb $0xff,%dh			# Might we need to use LBA?
-		jnz main.7			# No.
-		cmpw $0xffff,%cx		# Do we need to use LBA?
-		jnz main.7			# No.
+		testb $FL_PACKET,flags		# Try EDD?
+		jz main.7			# No.
 		pushw %cx			# Save %cx
 		pushw %bx			# Save %bx
 		movw $0x55aa,%bx		# Magic
@@ -151,7 +150,8 @@ msg_pt: 	.asciz "Invalid partition table"
 msg_rd: 	.asciz "Error loading operating system"
 msg_os: 	.asciz "Missing operating system"
 
-		.org PT_OFF
+		.org PT_OFF-1,0x90
+flags:		.byte FLAGS			# Flags
 
 partbl: 	.fill 0x10,0x4,0x0		# Partition table
 		.word MAGIC			# Magic number