[syslinux] [PATCH 4/8] Support sectors >512

[Frediano Ziglio]

If EBIOS is detected for this drive it tries to read sector size and
use it. If error or no EBIOS 512 is assumed.
Buffer to read sector size is always allocated into the stack.
CHS informations are not readed as not needed and save space not
restoring %dl for drive number.

Signed-off-by: Frediano Ziglio <frediano.ziglio at citrix.com>
---
 mbr/gptmbr.S |   41 +++++++++++++++++++++++++++--------------
 1 files changed, 27 insertions(+), 14 deletions(-)

diff --git a/mbr/gptmbr.S b/mbr/gptmbr.S
index b0c11b9..88d2809 100644
--- a/mbr/gptmbr.S
+++ b/mbr/gptmbr.S
@@ -77,6 +77,12 @@ next:
 
 	ADJUST_DRIVE
 	pushw	%dx		/* 0(%bp) = %dl -> drive number */
+	movw	%sp, %bp	/* %bp -> frame pointer: LEAVE UNCHANGED */
+
+	/* prepare to read sector size */
+	sub	$0x1c, %sp	/* -28(%bp) == %sp */
+	pushw	$0x1e		/* -30(%bp) == %sp */
+	movw	$0x200, -6(%bp)	/* -6(%bp) sector size */
 
 	/* Check to see if we have EBIOS */
 	pushw	%dx		/* drive number */
@@ -86,6 +92,8 @@ next:
 	xorb	%dh, %dh
 	stc
 	int	$0x13
+	popw	%dx		/* restore drive */
+	movb	$0x08, %ah	/* get CHS geometry */
 	jc	1f
 	cmpw	$0xaa55, %bx
 	jne	1f
@@ -97,22 +105,28 @@ next:
 	movl	$0xeb42b4+((read_common-read_sector_cbios-4) << 24), \
 		(read_sector_cbios)
 
-1:
-	popw	%dx
+	/*
+	 * read sector size.
+	 * Should not fail but if it does I assume that at least
+	 * previous 512 value is not overridden
+	 */
+	movb	$0x48, %ah
+	movw	%sp, %si
 
+1:
 	/* Get (C)HS geometry */
-	movb	$0x08, %ah
 	int	$0x13
+
+	/* here we computer CHS values or just do some dummy computation for EBIOS */
 	andw	$0x3f, %cx	/* Sector count */
-	movw	%sp, %bp	/* %bp -> frame pointer: LEAVE UNCHANGED */
-	pushw	%cx		/* -2(%bp) Save sectors on the stack */
+	pushw	%cx		/* -32(%bp) Save sectors on the stack */
 	movzbw	%dh, %ax	/* dh = max head */
 	incw	%ax		/* From 0-based max to count */
 	mulw	%cx		/* Heads*sectors -> sectors per cylinder */
 
 	/* Save sectors/cylinder on the stack */
-	pushw	%dx		/* -4(%bp) High word */
-	pushw	%ax		/* -6(%bp) Low word */
+	pushw	%dx		/* -34(%bp) High word */
+	pushw	%ax		/* -36(%bp) Low word */
 
 	/* Load partition table header */
 	xorl	%eax,%eax
@@ -121,15 +135,14 @@ next:
 	call	read_sector_phdr
 
 	/* Number of partition sectors */
-	/* We assume the partition table is 32K or less, and that
-	   the sector size is 512. */
+	/* We assume the partition table is 32K or less */
 	/* Note: phdr == 6(%bp) */
 	movw	(80+6)(%bp),%cx		/* NumberOfPartitionEntries */
 	movw	(84+6)(%bp),%ax		/* SizeOfPartitionEntry */
 	pushw	%ax
 	pushw	%cx
 	mulw	%cx
-	shrw	$9,%ax
+	divw	-6(%bp)	/* %dx == 0 here */
 	xchgw	%ax,%cx
 	incw	%cx
 
@@ -214,7 +227,7 @@ boot:
 	movl	(32+20)(%si),%eax
 	movl	(36+20)(%si),%edx
 	call	read_sector_phdr
-	cmpw	$0xaa55, -2(%bx)
+	cmpw	$0xaa55, (0x7c00+0x1fe)
 	jne	missing_os	/* Not a valid boot sector */
 	movw	%bp, %sp	/* driveno == bootsec-6 */
 	popw	%dx		/* dl -> drive number */
@@ -258,12 +271,12 @@ read_sector:
 	/* This chunk is skipped if we have ebios */
 	/* Do not clobber %es:%bx or %edx:%eax before this chunk! */
 read_sector_cbios:
-	divl	-6(%bp)	/* secpercyl */
+	divl	-36(%bp)	/* secpercyl */
 	shlb	$6, %ah
 	movb	%ah, %cl
 	movb	%al, %ch
 	xchgw	%dx, %ax
-	divb	-2(%bp)	/* sectors */
+	divb	-32(%bp)	/* sectors */
 	movb	%al, %dh
 	orb	%ah, %cl
 	incw	%cx	/* Sectors are 1-based */
@@ -275,7 +288,7 @@ read_common:
 	leaw	16(%si), %sp	/* Drop DAPA */
 	popal
 	jc	disk_error
-	addb	$2, %bh		/* bx += 512: point to the next buffer */
+	addb	-5(%bp), %bh		/* bx += sector size: point to the next buffer */
 
 	/* fall through and increment sector number */
 
-- 
1.7.5.4