From 6c2def279ea92f1bfad331aeda0b2c7edd4703f0 Mon Sep 17 00:00:00 2001
From: bde <bde@FreeBSD.org>
Date: Fri, 8 Mar 1996 05:15:54 +0000
Subject: [PATCH] Fixed restoring segment descriptors in prot_to_real().  The
 descriptors must have limit 0xffff and attribute G = 0 (byte granularity) as
 well as other properties that they already had (see e.g., the Intel i486
 manual section 22.5).  Not restoring them broke Ctrl-Alt-Del in the bootstrap
 for my ASUS P55TP4XE system, probably because the Award BIOS does
 anti-tracing stuff involving inaccessible %esp's.

asm.S:
Don't use lret in prot_to_real().  This reduces the risk of using an
incompletely intialized stack segment and saves space.

Submitted by:	"K.Higashino" <a00303@cc.hc.keio.ac.jp> (on 13 Jan 1995!)
		reworked by me
---
 sys/i386/boot/biosboot/asm.S   | 21 +++++++++++----------
 sys/i386/boot/biosboot/table.c |  4 ++--
 2 files changed, 13 insertions(+), 12 deletions(-)

diff --git a/sys/i386/boot/biosboot/asm.S b/sys/i386/boot/biosboot/asm.S
index ac59d5bb3bcb..82844a7aab7e 100644
--- a/sys/i386/boot/biosboot/asm.S
+++ b/sys/i386/boot/biosboot/asm.S
@@ -24,7 +24,7 @@
  * the rights to redistribute these changes.
  *
  *	from: Mach, Revision 2.2  92/04/04  11:34:13  rpd
- *	$Id: asm.S,v 1.5 1994/10/06 09:41:01 rgrimes Exp $
+ *	$Id: asm.S,v 1.6 1995/01/25 21:37:36 bde Exp $
  */
 
 
@@ -116,16 +116,17 @@ xprot:
 
 ENTRY(prot_to_real)
 
-	/* set up a dummy stack frame for the second seg change. */
-	movw 	$BOOTSEG, %ax
-	pushw	%ax
-	movw	$xreal, %ax	/* gas botches pushw $xreal, extra bytes 0, 0 */
-	pushw	%ax		/* decode to add %al, (%eax) (%al usually 0) */
+	/* Prepare %ax while we're still in a mode that gas understands. */
+	movw	$0x30, %ax
 
 	/* Change to use16 mode. */
 	ljmp	$0x28, $x16
-
 x16:
+
+	mov	%ax, %ds
+	mov	%ax, %ss
+	mov	%ax, %es
+
 	/* clear the PE bit of CR0 */
 	mov	%cr0, %eax
 	data32
@@ -134,12 +135,12 @@ x16:
 
 	/*
 	 * make intersegment jmp to flush the processor pipeline
-	 * using the fake stack frame set up earlier
 	 * and reload CS register
 	 */
-	lret
-
+	data32
+	ljmp	$BOOTSEG, $xreal
 xreal:
+
 	/*
 	 * we are in real mode now
 	 * set up the real mode segment registers : DS, SS, ES
diff --git a/sys/i386/boot/biosboot/table.c b/sys/i386/boot/biosboot/table.c
index aa9778911b97..7faa9a8930bf 100644
--- a/sys/i386/boot/biosboot/table.c
+++ b/sys/i386/boot/biosboot/table.c
@@ -24,7 +24,7 @@
  * the rights to redistribute these changes.
  *
  *	from: Mach, Revision 2.2  92/04/04  11:36:43  rpd
- *	$Id: table.c,v 1.7 1995/05/30 07:58:36 rgrimes Exp $
+ *	$Id: table.c,v 1.8 1995/06/25 14:02:57 joerg Exp $
  */
 
 /*
@@ -86,9 +86,9 @@ struct seg_desc	Gdt[] = {
 	{0xFFFF, RUN, RUN, 0x9E, 0x40, 0x0},	/* 0x18 : boot code */
 	{0xFFFF, RUN, RUN, 0x92, 0x40, 0x0},	/* 0x20 : boot data */
 	{0xFFFF, RUN, RUN, 0x9E, 0x0, 0x0},	/* 0x28 : boot code, 16 bits */
+	{0xFFFF, 0x0, 0x0, 0x92, 0x0, 0x0},	/* 0x30 : boot data, 16 bits */
 #ifdef BDE_DEBUGGER
 	/* More for bdb. */
-	{},					/* BIOS_CS_INDEX = 6 : null */
 	{},					/* BIOS_TMP_INDEX = 7 : null */
 	{},					/* TSS_INDEX = 8 : null */
 	{0xFFFF, 0x0, 0x0, 0xB2, 0x40, 0x0},	/* DS_286_INDEX = 9 */