From 1576981e386ec9180da2c124e65ee344f4cc2ffc Mon Sep 17 00:00:00 2001
From: Matthew Dillon <dillon@dragonflybsd.org>
Date: Sat, 26 Jun 2004 02:00:11 +0000
Subject: [PATCH] Stage 1/999: Rewrite boot0 to relocate to higher memory
 (beyond 64K).  This is currently a demonstration only but will become part of
 a larger project to try to make our boot0,1,2/loader code more BIOS-friendly.

---
 sys/boot/i386/bootn0/Makefile |  36 +++
 sys/boot/i386/bootn0/README   |  16 ++
 sys/boot/i386/bootn0/boot0.S  | 435 ++++++++++++++++++++++++++++++++++
 sys/boot/pc32/bootn0/Makefile |  36 +++
 sys/boot/pc32/bootn0/README   |  16 ++
 sys/boot/pc32/bootn0/boot0.S  | 435 ++++++++++++++++++++++++++++++++++
 6 files changed, 974 insertions(+)
 create mode 100644 sys/boot/i386/bootn0/Makefile
 create mode 100644 sys/boot/i386/bootn0/README
 create mode 100644 sys/boot/i386/bootn0/boot0.S
 create mode 100644 sys/boot/pc32/bootn0/Makefile
 create mode 100644 sys/boot/pc32/bootn0/README
 create mode 100644 sys/boot/pc32/bootn0/boot0.S

diff --git a/sys/boot/i386/bootn0/Makefile b/sys/boot/i386/bootn0/Makefile
new file mode 100644
index 0000000000..05506f2978
--- /dev/null
+++ b/sys/boot/i386/bootn0/Makefile
@@ -0,0 +1,36 @@
+# $FreeBSD: src/sys/boot/i386/boot0/Makefile,v 1.17 2002/09/17 01:48:54 peter Exp $
+# $DragonFly: src/sys/boot/i386/bootn0/Attic/Makefile,v 1.1 2004/06/26 02:00:11 dillon Exp $
+
+PROG=		boot0
+NOMAN=
+STRIP=
+BINDIR?=	/boot
+BINMODE=	444
+
+# The default set of flags compiled into boot0.  This enables update (writing
+# the modified boot0 back to disk after running so that the selection made is
+# saved), packet mode (detect and use the BIOS EDD extensions if we try to
+# boot past the 1024 cylinder liimt), and booting from all valid slices.
+BOOT_BOOT0_FLAGS?=	0xf
+
+# The number of timer ticks to wait for a keypress before assuming the default
+# selection.  Since there are 18.2 ticks per second, the default value of
+# 0xb6 (182d) corresponds to 10 seconds.
+BOOT_BOOT0_TICKS?=	0xb6
+
+# We relocate boot0 to this address, with the stack heading downward and 
+# the contents of boot0 heading upward.
+BOOT_BOOT0_ORG?=	0x91000
+
+boot0: boot0.o
+	${LD} -N -e start -Ttext ${BOOT_BOOT0_ORG} -o boot0.out boot0.o
+	objcopy -S -O binary boot0.out ${.TARGET}
+
+boot0.o: boot0.S
+	${CC} -E ${.IMPSRC} | \
+	${AS} ${AFLAGS} --defsym FLAGS=${BOOT_BOOT0_FLAGS} \
+		--defsym TICKS=${BOOT_BOOT0_TICKS} -o ${.TARGET}
+
+CLEANFILES+= boot0.out boot0.o
+
+.include <bsd.prog.mk>
diff --git a/sys/boot/i386/bootn0/README b/sys/boot/i386/bootn0/README
new file mode 100644
index 0000000000..743599cad2
--- /dev/null
+++ b/sys/boot/i386/bootn0/README
@@ -0,0 +1,16 @@
+
+		    BOOTN0 - DragonFly rewrite of the Boot0 code
+
+    This code is not yet hooked into the system but will eventually, once
+    tested, replace the existing boot0 code.
+
+    This is part of a personal project of mine to relocate *ALL* the boot
+    elements to high memory (in the 64KB-640KB range) and to make it possible
+    to adjust the relocation addresses without having to go through loops.
+
+    All existing FreeBSD boot code works almost exclusively in the first
+    64KB of ram, and through experimentation I have come to the conclusion
+    that this is no longer a BIOS-friendly way of doing things.
+
+						-Matt
+$DragonFly: src/sys/boot/i386/bootn0/Attic/README,v 1.1 2004/06/26 02:00:11 dillon Exp $
diff --git a/sys/boot/i386/bootn0/boot0.S b/sys/boot/i386/bootn0/boot0.S
new file mode 100644
index 0000000000..1770a94830
--- /dev/null
+++ b/sys/boot/i386/bootn0/boot0.S
@@ -0,0 +1,435 @@
+/*
+ * Copyright (c) 1998 Robert Nordier
+ * Copyright (c) 2004 by Matthew Dillon, All rights reserved.
+ *
+ * Redistribution and use in source and binary forms are freely
+ * permitted provided that the above copyright notice and this
+ * paragraph and the following disclaimer are duplicated in all
+ * such forms.
+ *
+ * This software is provided "AS IS" and without any express or
+ * implied warranties, including, without limitation, the implied
+ * warranties of merchantability and fitness for a particular
+ * purpose.
+ *
+ * $DragonFly: src/sys/boot/i386/bootn0/Attic/boot0.S,v 1.1 2004/06/26 02:00:11 dillon Exp $
+ *
+ * A 512-byte boot manager.
+ */
+
+		.set NHRDRV,0x475	# Number of hard drives
+		.set FAKE,0x800 	# Partition entry
+		.set BIOS_LOAD,0x7c00	# BIOS Load address
+		.set ORG_ADDR,0x91000	# 'start' symbol
+		.set ORG_SEG,0x900	# leave room in segment for stk
+		.set ORG_OFF,0x1000
+
+		.set PRT_OFF,0x1be	# Partition table
+
+		.set TBL0SZ,3 		# Table 0 size
+		.set TBL1SZ,9	 	# Table 1 size
+
+		.set MAGIC,0xaa55	# Magic: bootable
+		.set B0MAGIC,0xbb66	# Identification
+
+		.set KEY_ENTER,0x1c	# Enter key scan code
+		.set KEY_F1,0x3b	# F1 key scan code
+		.set KEY_1,0x02		# #1 key scan code
+
+		#
+		# Addresses in the sector of embedded data values.  Accessed
+		# with negative offsets from the end of the relocated sector
+		# (%ebp)
+		#
+		.set _NXTDRV,-0x48	# Next drive
+		.set _OPT,-0x47 	# Default option
+		.set _SETDRV,-0x46	# Drive to force
+		.set _FLAGS,-0x45	# Flags
+		.set _TICKS,-0x44	# Timeout ticks
+		.set _FAKE,0x0		# Fake partition entry
+		.set _MNUOPT,0xc	# Menu options
+
+#define DRELOC(symbol)	(symbol-start+ORG_OFF)
+
+		.globl start		# Entry point
+		.code16			# This runs in real mode
+
+		# Initialize segments and relocate the boot sector.  Our
+		# stack starts at our target relocation address and grows
+		# downward while the copy of boot0 grows upwards.
+		#
+
+start:		cld			# String ops inc
+		movw $ORG_SEG,%ax	# Setup various segments
+		movw %ax,%es
+		movw %ax,%ss		# (coupled with next instruction)
+		movw $ORG_OFF,%sp	# stack downward from start
+
+		xorw %ax,%ax
+		movw %ax,%ds		# source is in segment 0
+		movw $BIOS_LOAD,%si	# source is the BIOS load address
+		movw %sp,%di
+		movw $0x100,%cx		# 256x2 = 512 bytes
+		rep
+		movsw
+
+		# Note: %ds points at segment 0
+		#	%es points at our relocated segment
+		#	%ss points at our relocated segment
+		#	(%ax still 0)
+		#
+		# Remember that %bp relative addresses uses the stack segment
+
+		movw %di,%bp		# address variables
+		movb $8,%cl		# clear 16 bytes
+		rep
+		stosw
+
+		# Point %ds at our relocated segment, cleanup, and
+		# jump to our relocated segment.
+		#
+		movw %ss,%ax
+		movw %ax,%ds
+		incb _FAKE+2(%bp)	# Start with sector 1
+		ljmp $ORG_SEG,$DRELOC(main)
+
+		# Check what flags were loaded with us.  Specifically, use a
+		# predefined drive.  If the BIOS gives us junk use the '0'
+		# in the block instead, as well.
+		#
+main:		testb $0x20,_FLAGS(%bp)	# Set number drive?
+		jnz main.1		# Yes
+		testb %dl,%dl		# Drive number valid?
+		js main.2		# Possibly (0x80 set)
+main.1: 	movb _SETDRV(%bp),%dl	# Drive number to use
+
+		# Save our working drive in a temporary variable
+		#
+main.2: 	movb %dl,_FAKE(%bp)	# Save drive number
+		callw putn		# To new line
+		pushw %dx		# Save drive number
+
+		# Start out with a pointer to the 4th byte of the first table
+		# entry so that after 4 iterations it is beyond the end of
+		# the sector (hits a 256 byte boundary and overflows 8 bits)
+		#
+		# Remember that the table starts 2 bytes earlier then you
+		# would expect as the bootable flag is after it in the block.
+
+		movw $DRELOC(partbl+0x4),%bx	# Partition table (+4)
+		xorw %dx,%dx		# Item number
+
+		# Loop on the partition table printing values until we 
+		# hit the 256 byte boundary (at main.5).
+		#
+main.3: 	movb %ch,-4(%bx)	# Zero active flag (ch == 0)
+		btw %dx,_FLAGS(%bp)	# Entry enabled?
+		jnc main.5		# No
+
+		# If any of the table entries match 'type' in the slice
+		# table entry, then this is an empty or non bootable partition.
+		# Skip it.
+		#
+		movb (%bx),%al		# Load type
+		movw $DRELOC(tables),%di # Lookup tables
+		movb $TBL0SZ,%cl	# Number of entries
+		repne			# Exclude partition?
+		scasb			# (note: uses %es by default)
+		je main.5		# Yes
+
+		#
+		# Now scan the table of known types
+		#
+		movb $TBL1SZ,%cl	# Number of entries
+		repne			# Known type ?
+		scasb
+		jne main.4		# No
+
+		/*
+		 * If it matches get the matching element in the next
+		 * array.  If it doesn't we are already pointing at the
+		 * first element, which points to a "?".
+		 */
+		addw $TBL1SZ,%di	# Adjust
+main.4: 	movb (%di),%cl		# Partition description
+		addw %cx,%di
+		callw putx		# Display it
+main.5: 	incw %dx		# Next item 
+		addb $0x10,%bl		# Next entry
+		jnc main.3		# Till done
+
+		# End of loop.  Add one to the drive number and check validity.
+		# 
+		popw %ax		# Drive number
+		subb $0x80-0x1,%al	# Does next drive exist? (BIOS)
+		cmpb NHRDRV,%al
+		jb main.6		# Yes
+
+		# If %ax is 1 there is only one drive in the system and we
+		# skip the drive selection code.  Note that %ax becomes an
+		# index.
+		#
+		decw %ax		# Already drive 0?
+		jz main.7		# Yes
+
+		# No, loop on drive selection starting with 0
+		#
+		xorb %al,%al		# Drive 0
+main.6: 	addb $'0'|0x80,%al	# Save next drive number
+		movb %al,_NXTDRV(%bp)
+		movw $DRELOC(drive),%di	# Display item
+		callw putx
+
+		# Prompt and setup for input but do not yet initiate the
+		# input.
+		#
+main.7: 	movw $DRELOC(prompt),%si # Display prompt
+		callw putstr
+		movb _OPT(%bp),%dl	# Display default key
+		decw %si
+		callw putkey
+		xorb %ah,%ah		# BIOS: Get system time
+		int $0x1a
+		movw %dx,%di		# Figure out tick value of timeout
+		addw _TICKS(%bp),%di
+
+		# Initiate input
+		#
+main.8: 	movb $0x1,%ah		# BIOS: Check for keypress
+		int $0x16
+		jnz main.11
+		xorb %ah,%ah		# BIOS: Get system time
+		int $0x1a
+		cmpw %di,%dx		# loop if no timeout
+		jb main.8
+
+		# A timeout or default comes to here
+		#
+main.9: 	movb _OPT(%bp),%al	# Load default
+		jmp main.12		# Join common code
+
+		/* 
+		 * If the user's last try was bad, beep.  Since nothing was
+		 * printed, continue as if the user hadn't done anything.
+		 */
+main.10:	movb $0x7,%al		# Signal error
+		callw putchr
+
+		# Obtain the keystroke we previously polled successfully for
+		#
+main.11:	xorb %ah,%ah		# BIOS: Get keypress
+		int $0x16
+		movb %ah,%al		# Scan code
+
+		# An ENTER is equivalent to a timeout
+		#
+		cmpb $KEY_ENTER,%al	# Enter pressed?
+		je main.9		# Yes
+
+		# Look for a legal keystroke, jump to main.10 (beep) and
+		# retry if it is not legal.
+		#
+		subb $KEY_F1,%al	# Less F1 scan code
+		cmpb $0x4,%al		# F1..F5?
+		jna main.12
+		subb $(KEY_1-KEY_F1),%al # Less #1 scan code
+		cmpb $0x4,%al		# #1..#5?
+		ja main.10
+
+		# Check the selection, complain if its bad.  Only options
+		# that were printed (MNUOPT bit set) are valid.
+		#
+main.12:	cbtw			# Option enabled?
+		btw %ax,_MNUOPT(%bp)
+		jnc main.10		# No
+
+		/*
+		 * We're good.  Save the data for later rewriting back to disk
+		 */
+		movb %al,_OPT(%bp)	# Save for future action
+		leaw _FAKE(%bp),%si	# Partition for write
+		movb (%si),%dl		# Drive number
+		movw %si,%bx		# Partition for read
+		cmpb $0x4,%al		# F5 pressed?
+		pushf			# Save result of check
+		je main.13		# Yes
+		shlb $0x4,%al		# Point to selected partition
+		addw $DRELOC(partbl),%ax
+		xchgw %bx,%ax
+		movb $0x80,(%bx)	# Flag active
+
+		# Only writeback if we were told to in _FLAGS
+		#
+main.13:	pushw %bx		# Save
+		testb $0x40,_FLAGS(%bp)	# No updates?
+		jnz main.14		# Yes
+		movw $DRELOC(start),%bx	# Data to write (relative to %es)
+		movb $0x3,%ah		# Write sector to disk
+		callw intx13
+main.14:	popw %si		# Restore %si for next intx13
+
+		# Retrieve the perviously saved F5 (next drive) request
+		# status and move us to the next drive if necessary
+		#
+		popf
+		jne main.15
+		movb _NXTDRV(%bp),%dl
+		subb $'0',%dl
+
+		/*
+		 * Load the selected bootsector to the LOAD location in RAM.
+		 * We are already relocated so it will not overwrite us
+		 * in particular.  If it fails to read or isn't marked bootable,
+		 * treat it was a bad selection.
+		 *
+		 * note: drive number in %dl
+		 */
+main.15:	movw $BIOS_LOAD,%bx	# Address for read
+		pushw $0
+		popw %es
+		movb $0x2,%ah		# Read sector from disk
+		callw intx13
+		movw %es:0x1fe(%bx),%ax # (these do not effect carry)
+		pushw %ss
+		popw %es
+		jc main.10		# error on carry
+		subw $MAGIC,%ax		# Bootable?
+		jnz main.10
+		pushw %ax		# seg: setup for lret (note %ax is 0)
+		pushw %bx		# offset: setup for lret
+		pushw %ax		# (so we can clear %ds)
+		callw putn
+		popw %ds
+		lret
+
+		# SUPPORT ROUTINES
+		#
+		# Display functions.  %bx is not modified by any of these
+		# calls.
+		#
+putkey: 	movb $'F',%al		# Display 'F'
+		callw putchr
+		movb $'1',%al		# Display digit and remainder of prompt
+		addb %dl,%al
+		jmp putstr.1
+
+		# Display the option and note that it is a valid option
+		# by setting the appropriate bit in _MNUOPT.
+		#
+putx:		btsw %dx,_MNUOPT(%bp)	# Enable menu option
+		movw $DRELOC(item),%si	# Display key
+		callw putkey
+		movw %di,%si		# Display the option string
+puts:		callw putstr		# Display string in %si
+
+putn:		movw $DRELOC(crlf),%si	# Output a CR+LF
+		lodsb
+putstr.1:	callw putchr		# Output %al + string at (%si)
+putstr:		lodsb			# Output string at (%si)
+		testb %al,%al
+		jns   putstr.1
+		andb $0x7f,%al
+putchr: 	pushw %bx		# %bx is not modified by this call
+		movw $0x7,%bx	 	# Page:attribute
+		movb $0xe,%ah		# BIOS: Display character
+		int $0x10
+		popw %bx
+		retw
+
+		# One-sector disk I/O routine
+		#
+		# note: drive number in %dl, %si points to local variable
+		# area holding (driveno, head, w:cylinder/sector)
+		#
+intx13: 	movb 0x1(%si),%dh	# Load head
+		movw 0x2(%si),%cx	# Load cylinder:sector
+		movb $0x1,%al		# Sector count
+		movw %sp,%di
+		testb $0x80,_FLAGS(%bp)	# Use packet interface?
+		jz intx13.1
+		pushl $0x0		# Set the LBA Address
+		pushl 0x8(%si)
+		pushw %es		# Set the Transfer Buffer Address
+		pushw %bx
+		push  $0x1		# Block count
+		push  $0x10		# Packet size
+		movw %sp,%si		# Packet pointer (%es?)
+		decw %ax		# Verify off
+		orb $0x40,%ah		# Use disk packet
+intx13.1:	int $0x13		# BIOS: Disk I/O
+		movw %di,%sp		# Restore
+		retw			# WARNING: carry must be preserved
+
+		/*
+		 * MENU STRINGS
+		 */
+
+item:		.ascii "  "
+		.byte ' '|0x80
+prompt: 	.ascii "\nDefault:"
+		.byte ' '|0x80
+crlf:		.ascii "\r"
+		.byte '\n'|0x80
+
+		/*
+		 * Partition table types
+		 */
+tables:
+		/*
+		 * These entries identify invalid or NON BOOT types and
+		 * partitions.
+		 */
+		.byte 0x0, 0x5, 0xf
+
+		/*
+		 * These values indicate bootable types we know the names of
+		 */
+		.byte 0xb, 0xc, 0xe, 0x83
+		.byte 0xa5, 0xa6, 0xa9
+
+		/*
+		 * These are offsets that match the known names above and
+		 * point to the strings that will be printed.
+		 */
+		.byte os_misc-. 		# Unknown
+		.byte os_dos-.			# Windows
+		.byte os_dos-.			# Windows
+		.byte os_dos-.			# Windows
+		.byte os_linux-.		# Linux
+		.byte os_bsd-.			# FreeBSD/DragonFly
+		.byte os_bsd-.			# OpenBSD
+		.byte os_bsd-.			# NetBSD
+
+		/*
+		 * And here are the strings themselves. 0x80 or'd into a 
+		 * byte indicates the end of the string. (not so great
+		 * for Russians but...)
+		 */
+os_misc:	.ascii "?";    .byte '?'|0x80
+os_dos: 	.ascii "DO";   .byte 'S'|0x80
+os_linux:	.ascii "Linu"; .byte 'x'|0x80
+os_bsd: 	.ascii "BS";   .byte 'D'|0x80
+
+		.org PRT_OFF-0xe,0x90
+
+		/*
+		 * 14 bytes then partition table.  Some of these values may
+		 * be adjusted by the above code and then written back to the
+		 * drive.  Be especially careful in that nxtdrv: must come 
+		 * after drive:, as it is part of the same string.
+		 */
+		.word B0MAGIC			# Magic number
+
+drive:		.ascii "Drive "
+nxtdrv: 	.byte 0x0			# Next drive number
+opt:		.byte 0x0			# Option
+setdrv: 	.byte 0x80			# Drive to force
+flags:		.byte FLAGS			# Flags
+ticks:		.word TICKS			# Delay
+
+		/*
+		 * here is the 64 byte partition table that fdisk would
+		 * fiddle with.
+		 */
+partbl: 	.fill 0x40,0x1,0x0		# Partition table
+		.word MAGIC			# Magic number
diff --git a/sys/boot/pc32/bootn0/Makefile b/sys/boot/pc32/bootn0/Makefile
new file mode 100644
index 0000000000..644077ebc4
--- /dev/null
+++ b/sys/boot/pc32/bootn0/Makefile
@@ -0,0 +1,36 @@
+# $FreeBSD: src/sys/boot/i386/boot0/Makefile,v 1.17 2002/09/17 01:48:54 peter Exp $
+# $DragonFly: src/sys/boot/pc32/bootn0/Attic/Makefile,v 1.1 2004/06/26 02:00:11 dillon Exp $
+
+PROG=		boot0
+NOMAN=
+STRIP=
+BINDIR?=	/boot
+BINMODE=	444
+
+# The default set of flags compiled into boot0.  This enables update (writing
+# the modified boot0 back to disk after running so that the selection made is
+# saved), packet mode (detect and use the BIOS EDD extensions if we try to
+# boot past the 1024 cylinder liimt), and booting from all valid slices.
+BOOT_BOOT0_FLAGS?=	0xf
+
+# The number of timer ticks to wait for a keypress before assuming the default
+# selection.  Since there are 18.2 ticks per second, the default value of
+# 0xb6 (182d) corresponds to 10 seconds.
+BOOT_BOOT0_TICKS?=	0xb6
+
+# We relocate boot0 to this address, with the stack heading downward and 
+# the contents of boot0 heading upward.
+BOOT_BOOT0_ORG?=	0x91000
+
+boot0: boot0.o
+	${LD} -N -e start -Ttext ${BOOT_BOOT0_ORG} -o boot0.out boot0.o
+	objcopy -S -O binary boot0.out ${.TARGET}
+
+boot0.o: boot0.S
+	${CC} -E ${.IMPSRC} | \
+	${AS} ${AFLAGS} --defsym FLAGS=${BOOT_BOOT0_FLAGS} \
+		--defsym TICKS=${BOOT_BOOT0_TICKS} -o ${.TARGET}
+
+CLEANFILES+= boot0.out boot0.o
+
+.include <bsd.prog.mk>
diff --git a/sys/boot/pc32/bootn0/README b/sys/boot/pc32/bootn0/README
new file mode 100644
index 0000000000..86f9050a59
--- /dev/null
+++ b/sys/boot/pc32/bootn0/README
@@ -0,0 +1,16 @@
+
+		    BOOTN0 - DragonFly rewrite of the Boot0 code
+
+    This code is not yet hooked into the system but will eventually, once
+    tested, replace the existing boot0 code.
+
+    This is part of a personal project of mine to relocate *ALL* the boot
+    elements to high memory (in the 64KB-640KB range) and to make it possible
+    to adjust the relocation addresses without having to go through loops.
+
+    All existing FreeBSD boot code works almost exclusively in the first
+    64KB of ram, and through experimentation I have come to the conclusion
+    that this is no longer a BIOS-friendly way of doing things.
+
+						-Matt
+$DragonFly: src/sys/boot/pc32/bootn0/Attic/README,v 1.1 2004/06/26 02:00:11 dillon Exp $
diff --git a/sys/boot/pc32/bootn0/boot0.S b/sys/boot/pc32/bootn0/boot0.S
new file mode 100644
index 0000000000..ce9c37a3c7
--- /dev/null
+++ b/sys/boot/pc32/bootn0/boot0.S
@@ -0,0 +1,435 @@
+/*
+ * Copyright (c) 1998 Robert Nordier
+ * Copyright (c) 2004 by Matthew Dillon, All rights reserved.
+ *
+ * Redistribution and use in source and binary forms are freely
+ * permitted provided that the above copyright notice and this
+ * paragraph and the following disclaimer are duplicated in all
+ * such forms.
+ *
+ * This software is provided "AS IS" and without any express or
+ * implied warranties, including, without limitation, the implied
+ * warranties of merchantability and fitness for a particular
+ * purpose.
+ *
+ * $DragonFly: src/sys/boot/pc32/bootn0/Attic/boot0.S,v 1.1 2004/06/26 02:00:11 dillon Exp $
+ *
+ * A 512-byte boot manager.
+ */
+
+		.set NHRDRV,0x475	# Number of hard drives
+		.set FAKE,0x800 	# Partition entry
+		.set BIOS_LOAD,0x7c00	# BIOS Load address
+		.set ORG_ADDR,0x91000	# 'start' symbol
+		.set ORG_SEG,0x900	# leave room in segment for stk
+		.set ORG_OFF,0x1000
+
+		.set PRT_OFF,0x1be	# Partition table
+
+		.set TBL0SZ,3 		# Table 0 size
+		.set TBL1SZ,9	 	# Table 1 size
+
+		.set MAGIC,0xaa55	# Magic: bootable
+		.set B0MAGIC,0xbb66	# Identification
+
+		.set KEY_ENTER,0x1c	# Enter key scan code
+		.set KEY_F1,0x3b	# F1 key scan code
+		.set KEY_1,0x02		# #1 key scan code
+
+		#
+		# Addresses in the sector of embedded data values.  Accessed
+		# with negative offsets from the end of the relocated sector
+		# (%ebp)
+		#
+		.set _NXTDRV,-0x48	# Next drive
+		.set _OPT,-0x47 	# Default option
+		.set _SETDRV,-0x46	# Drive to force
+		.set _FLAGS,-0x45	# Flags
+		.set _TICKS,-0x44	# Timeout ticks
+		.set _FAKE,0x0		# Fake partition entry
+		.set _MNUOPT,0xc	# Menu options
+
+#define DRELOC(symbol)	(symbol-start+ORG_OFF)
+
+		.globl start		# Entry point
+		.code16			# This runs in real mode
+
+		# Initialize segments and relocate the boot sector.  Our
+		# stack starts at our target relocation address and grows
+		# downward while the copy of boot0 grows upwards.
+		#
+
+start:		cld			# String ops inc
+		movw $ORG_SEG,%ax	# Setup various segments
+		movw %ax,%es
+		movw %ax,%ss		# (coupled with next instruction)
+		movw $ORG_OFF,%sp	# stack downward from start
+
+		xorw %ax,%ax
+		movw %ax,%ds		# source is in segment 0
+		movw $BIOS_LOAD,%si	# source is the BIOS load address
+		movw %sp,%di
+		movw $0x100,%cx		# 256x2 = 512 bytes
+		rep
+		movsw
+
+		# Note: %ds points at segment 0
+		#	%es points at our relocated segment
+		#	%ss points at our relocated segment
+		#	(%ax still 0)
+		#
+		# Remember that %bp relative addresses uses the stack segment
+
+		movw %di,%bp		# address variables
+		movb $8,%cl		# clear 16 bytes
+		rep
+		stosw
+
+		# Point %ds at our relocated segment, cleanup, and
+		# jump to our relocated segment.
+		#
+		movw %ss,%ax
+		movw %ax,%ds
+		incb _FAKE+2(%bp)	# Start with sector 1
+		ljmp $ORG_SEG,$DRELOC(main)
+
+		# Check what flags were loaded with us.  Specifically, use a
+		# predefined drive.  If the BIOS gives us junk use the '0'
+		# in the block instead, as well.
+		#
+main:		testb $0x20,_FLAGS(%bp)	# Set number drive?
+		jnz main.1		# Yes
+		testb %dl,%dl		# Drive number valid?
+		js main.2		# Possibly (0x80 set)
+main.1: 	movb _SETDRV(%bp),%dl	# Drive number to use
+
+		# Save our working drive in a temporary variable
+		#
+main.2: 	movb %dl,_FAKE(%bp)	# Save drive number
+		callw putn		# To new line
+		pushw %dx		# Save drive number
+
+		# Start out with a pointer to the 4th byte of the first table
+		# entry so that after 4 iterations it is beyond the end of
+		# the sector (hits a 256 byte boundary and overflows 8 bits)
+		#
+		# Remember that the table starts 2 bytes earlier then you
+		# would expect as the bootable flag is after it in the block.
+
+		movw $DRELOC(partbl+0x4),%bx	# Partition table (+4)
+		xorw %dx,%dx		# Item number
+
+		# Loop on the partition table printing values until we 
+		# hit the 256 byte boundary (at main.5).
+		#
+main.3: 	movb %ch,-4(%bx)	# Zero active flag (ch == 0)
+		btw %dx,_FLAGS(%bp)	# Entry enabled?
+		jnc main.5		# No
+
+		# If any of the table entries match 'type' in the slice
+		# table entry, then this is an empty or non bootable partition.
+		# Skip it.
+		#
+		movb (%bx),%al		# Load type
+		movw $DRELOC(tables),%di # Lookup tables
+		movb $TBL0SZ,%cl	# Number of entries
+		repne			# Exclude partition?
+		scasb			# (note: uses %es by default)
+		je main.5		# Yes
+
+		#
+		# Now scan the table of known types
+		#
+		movb $TBL1SZ,%cl	# Number of entries
+		repne			# Known type ?
+		scasb
+		jne main.4		# No
+
+		/*
+		 * If it matches get the matching element in the next
+		 * array.  If it doesn't we are already pointing at the
+		 * first element, which points to a "?".
+		 */
+		addw $TBL1SZ,%di	# Adjust
+main.4: 	movb (%di),%cl		# Partition description
+		addw %cx,%di
+		callw putx		# Display it
+main.5: 	incw %dx		# Next item 
+		addb $0x10,%bl		# Next entry
+		jnc main.3		# Till done
+
+		# End of loop.  Add one to the drive number and check validity.
+		# 
+		popw %ax		# Drive number
+		subb $0x80-0x1,%al	# Does next drive exist? (BIOS)
+		cmpb NHRDRV,%al
+		jb main.6		# Yes
+
+		# If %ax is 1 there is only one drive in the system and we
+		# skip the drive selection code.  Note that %ax becomes an
+		# index.
+		#
+		decw %ax		# Already drive 0?
+		jz main.7		# Yes
+
+		# No, loop on drive selection starting with 0
+		#
+		xorb %al,%al		# Drive 0
+main.6: 	addb $'0'|0x80,%al	# Save next drive number
+		movb %al,_NXTDRV(%bp)
+		movw $DRELOC(drive),%di	# Display item
+		callw putx
+
+		# Prompt and setup for input but do not yet initiate the
+		# input.
+		#
+main.7: 	movw $DRELOC(prompt),%si # Display prompt
+		callw putstr
+		movb _OPT(%bp),%dl	# Display default key
+		decw %si
+		callw putkey
+		xorb %ah,%ah		# BIOS: Get system time
+		int $0x1a
+		movw %dx,%di		# Figure out tick value of timeout
+		addw _TICKS(%bp),%di
+
+		# Initiate input
+		#
+main.8: 	movb $0x1,%ah		# BIOS: Check for keypress
+		int $0x16
+		jnz main.11
+		xorb %ah,%ah		# BIOS: Get system time
+		int $0x1a
+		cmpw %di,%dx		# loop if no timeout
+		jb main.8
+
+		# A timeout or default comes to here
+		#
+main.9: 	movb _OPT(%bp),%al	# Load default
+		jmp main.12		# Join common code
+
+		/* 
+		 * If the user's last try was bad, beep.  Since nothing was
+		 * printed, continue as if the user hadn't done anything.
+		 */
+main.10:	movb $0x7,%al		# Signal error
+		callw putchr
+
+		# Obtain the keystroke we previously polled successfully for
+		#
+main.11:	xorb %ah,%ah		# BIOS: Get keypress
+		int $0x16
+		movb %ah,%al		# Scan code
+
+		# An ENTER is equivalent to a timeout
+		#
+		cmpb $KEY_ENTER,%al	# Enter pressed?
+		je main.9		# Yes
+
+		# Look for a legal keystroke, jump to main.10 (beep) and
+		# retry if it is not legal.
+		#
+		subb $KEY_F1,%al	# Less F1 scan code
+		cmpb $0x4,%al		# F1..F5?
+		jna main.12
+		subb $(KEY_1-KEY_F1),%al # Less #1 scan code
+		cmpb $0x4,%al		# #1..#5?
+		ja main.10
+
+		# Check the selection, complain if its bad.  Only options
+		# that were printed (MNUOPT bit set) are valid.
+		#
+main.12:	cbtw			# Option enabled?
+		btw %ax,_MNUOPT(%bp)
+		jnc main.10		# No
+
+		/*
+		 * We're good.  Save the data for later rewriting back to disk
+		 */
+		movb %al,_OPT(%bp)	# Save for future action
+		leaw _FAKE(%bp),%si	# Partition for write
+		movb (%si),%dl		# Drive number
+		movw %si,%bx		# Partition for read
+		cmpb $0x4,%al		# F5 pressed?
+		pushf			# Save result of check
+		je main.13		# Yes
+		shlb $0x4,%al		# Point to selected partition
+		addw $DRELOC(partbl),%ax
+		xchgw %bx,%ax
+		movb $0x80,(%bx)	# Flag active
+
+		# Only writeback if we were told to in _FLAGS
+		#
+main.13:	pushw %bx		# Save
+		testb $0x40,_FLAGS(%bp)	# No updates?
+		jnz main.14		# Yes
+		movw $DRELOC(start),%bx	# Data to write (relative to %es)
+		movb $0x3,%ah		# Write sector to disk
+		callw intx13
+main.14:	popw %si		# Restore %si for next intx13
+
+		# Retrieve the perviously saved F5 (next drive) request
+		# status and move us to the next drive if necessary
+		#
+		popf
+		jne main.15
+		movb _NXTDRV(%bp),%dl
+		subb $'0',%dl
+
+		/*
+		 * Load the selected bootsector to the LOAD location in RAM.
+		 * We are already relocated so it will not overwrite us
+		 * in particular.  If it fails to read or isn't marked bootable,
+		 * treat it was a bad selection.
+		 *
+		 * note: drive number in %dl
+		 */
+main.15:	movw $BIOS_LOAD,%bx	# Address for read
+		pushw $0
+		popw %es
+		movb $0x2,%ah		# Read sector from disk
+		callw intx13
+		movw %es:0x1fe(%bx),%ax # (these do not effect carry)
+		pushw %ss
+		popw %es
+		jc main.10		# error on carry
+		subw $MAGIC,%ax		# Bootable?
+		jnz main.10
+		pushw %ax		# seg: setup for lret (note %ax is 0)
+		pushw %bx		# offset: setup for lret
+		pushw %ax		# (so we can clear %ds)
+		callw putn
+		popw %ds
+		lret
+
+		# SUPPORT ROUTINES
+		#
+		# Display functions.  %bx is not modified by any of these
+		# calls.
+		#
+putkey: 	movb $'F',%al		# Display 'F'
+		callw putchr
+		movb $'1',%al		# Display digit and remainder of prompt
+		addb %dl,%al
+		jmp putstr.1
+
+		# Display the option and note that it is a valid option
+		# by setting the appropriate bit in _MNUOPT.
+		#
+putx:		btsw %dx,_MNUOPT(%bp)	# Enable menu option
+		movw $DRELOC(item),%si	# Display key
+		callw putkey
+		movw %di,%si		# Display the option string
+puts:		callw putstr		# Display string in %si
+
+putn:		movw $DRELOC(crlf),%si	# Output a CR+LF
+		lodsb
+putstr.1:	callw putchr		# Output %al + string at (%si)
+putstr:		lodsb			# Output string at (%si)
+		testb %al,%al
+		jns   putstr.1
+		andb $0x7f,%al
+putchr: 	pushw %bx		# %bx is not modified by this call
+		movw $0x7,%bx	 	# Page:attribute
+		movb $0xe,%ah		# BIOS: Display character
+		int $0x10
+		popw %bx
+		retw
+
+		# One-sector disk I/O routine
+		#
+		# note: drive number in %dl, %si points to local variable
+		# area holding (driveno, head, w:cylinder/sector)
+		#
+intx13: 	movb 0x1(%si),%dh	# Load head
+		movw 0x2(%si),%cx	# Load cylinder:sector
+		movb $0x1,%al		# Sector count
+		movw %sp,%di
+		testb $0x80,_FLAGS(%bp)	# Use packet interface?
+		jz intx13.1
+		pushl $0x0		# Set the LBA Address
+		pushl 0x8(%si)
+		pushw %es		# Set the Transfer Buffer Address
+		pushw %bx
+		push  $0x1		# Block count
+		push  $0x10		# Packet size
+		movw %sp,%si		# Packet pointer (%es?)
+		decw %ax		# Verify off
+		orb $0x40,%ah		# Use disk packet
+intx13.1:	int $0x13		# BIOS: Disk I/O
+		movw %di,%sp		# Restore
+		retw			# WARNING: carry must be preserved
+
+		/*
+		 * MENU STRINGS
+		 */
+
+item:		.ascii "  "
+		.byte ' '|0x80
+prompt: 	.ascii "\nDefault:"
+		.byte ' '|0x80
+crlf:		.ascii "\r"
+		.byte '\n'|0x80
+
+		/*
+		 * Partition table types
+		 */
+tables:
+		/*
+		 * These entries identify invalid or NON BOOT types and
+		 * partitions.
+		 */
+		.byte 0x0, 0x5, 0xf
+
+		/*
+		 * These values indicate bootable types we know the names of
+		 */
+		.byte 0xb, 0xc, 0xe, 0x83
+		.byte 0xa5, 0xa6, 0xa9
+
+		/*
+		 * These are offsets that match the known names above and
+		 * point to the strings that will be printed.
+		 */
+		.byte os_misc-. 		# Unknown
+		.byte os_dos-.			# Windows
+		.byte os_dos-.			# Windows
+		.byte os_dos-.			# Windows
+		.byte os_linux-.		# Linux
+		.byte os_bsd-.			# FreeBSD/DragonFly
+		.byte os_bsd-.			# OpenBSD
+		.byte os_bsd-.			# NetBSD
+
+		/*
+		 * And here are the strings themselves. 0x80 or'd into a 
+		 * byte indicates the end of the string. (not so great
+		 * for Russians but...)
+		 */
+os_misc:	.ascii "?";    .byte '?'|0x80
+os_dos: 	.ascii "DO";   .byte 'S'|0x80
+os_linux:	.ascii "Linu"; .byte 'x'|0x80
+os_bsd: 	.ascii "BS";   .byte 'D'|0x80
+
+		.org PRT_OFF-0xe,0x90
+
+		/*
+		 * 14 bytes then partition table.  Some of these values may
+		 * be adjusted by the above code and then written back to the
+		 * drive.  Be especially careful in that nxtdrv: must come 
+		 * after drive:, as it is part of the same string.
+		 */
+		.word B0MAGIC			# Magic number
+
+drive:		.ascii "Drive "
+nxtdrv: 	.byte 0x0			# Next drive number
+opt:		.byte 0x0			# Option
+setdrv: 	.byte 0x80			# Drive to force
+flags:		.byte FLAGS			# Flags
+ticks:		.word TICKS			# Delay
+
+		/*
+		 * here is the 64 byte partition table that fdisk would
+		 * fiddle with.
+		 */
+partbl: 	.fill 0x40,0x1,0x0		# Partition table
+		.word MAGIC			# Magic number
-- 
2.41.0