xref: /freebsd-11-stable/stand/i386/boot2/boot1.S

//
// Copyright (c) 1998 Robert Nordier
// 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.
//

// $FreeBSD: head/sys/boot/i386/boot2/boot1.S 104683 2002-10-08 18:19:02Z jhb $

// Memory Locations
		.set MEM_REL,0x700		// Relocation address
		.set MEM_ARG,0x900		// Arguments
		.set MEM_ORG,0x7c00		// Origin
		.set MEM_BUF,0x8c00		// Load area
		.set MEM_BTX,0x9000		// BTX start
		.set MEM_JMP,0x9010		// BTX entry point
		.set MEM_USR,0xa000		// Client start
		.set BDA_BOOT,0x472		// Boot howto flag

// Partition Constants
		.set PRT_OFF,0x1be		// Partition offset
		.set PRT_NUM,0x4		// Partitions
		.set PRT_BSD,0xa5		// Partition type

// Flag Bits
		.set FL_PACKET,0x80		// Packet mode

// Misc. Constants
		.set SIZ_PAG,0x1000		// Page size
		.set SIZ_SEC,0x200		// Sector size

#ifdef UFS1_ONLY
		.set NSECT,0x10
#else
		.set NSECT,0x14
#endif
		.globl start
		.globl xread
		.code16

start:		jmp main			// Start recognizably

// This is the start of a standard BIOS Parameter Block (BPB). Most bootable
// FAT disks have this at the start of their MBR. While normal BIOS's will
// work fine without this section, IBM's El Torito emulation "fixes" up the
// BPB by writing into the memory copy of the MBR. Rather than have data
// written into our xread routine, we'll define a BPB to work around it.
// The data marked with (T) indicates a field required for a ThinkPad to
// recognize the disk and (W) indicates fields written from IBM BIOS code.
// The use of the BPB is based on what OpenBSD and NetBSD implemented in
// their boot code but the required fields were determined by trial and error.
//
// Note: If additional space is needed in boot1, one solution would be to
// move the "prompt" message data (below) to replace the OEM ID.

		.org 0x03, 0x00
oemid:		.space 0x08, 0x00	// OEM ID

		.org 0x0b, 0x00
bpb:		.word   512		// sector size (T)
		.byte	0		// sectors/clustor
		.word	0		// reserved sectors
		.byte	0		// number of FATs
		.word	0		// root entries
		.word	0		// small sectors
		.byte	0		// media type (W)
		.word	0		// sectors/fat
		.word	18		// sectors per track (T)
		.word	2		// number of heads (T)
		.long	0		// hidden sectors (W)
		.long	0		// large sectors

		.org 0x24, 0x00
ebpb:		.byte	0		// BIOS physical drive number (W)

		.org 0x25,0x90
//
// Trampoline used by boot2 to call read to read data from the disk via
// the BIOS.  Call with:
//
// %cx:%ax	- long    - LBA to read in
// %es:(%bx)	- caddr_t - buffer to read data into
// %dl		- byte    - drive to read from
// %dh		- byte    - num sectors to read
//

xread:		push %ss			// Address
		pop %ds				//  data
//
// Setup an EDD disk packet and pass it to read
//
xread.1:					// Starting
		pushl $0x0			//  absolute
		push %cx			//  block
		push %ax			//  number
		push %es			// Address of
		push %bx			//  transfer buffer
		xor %ax,%ax			// Number of
		movb %dh,%al			//  blocks to
		push %ax			//  transfer
		push $0x10			// Size of packet
		mov %sp,%bp			// Packet pointer
		callw read			// Read from disk
		lea 0x10(%bp),%sp		// Clear stack
		lret				// To far caller
//
// Load the rest of boot2 and BTX up, copy the parts to the right locations,
// and start it all up.
//

//
// Setup the segment registers to flat addressing (segment 0) and setup the
// stack to end just below the start of our code.
//
main:		cld				// String ops inc
		xor %cx,%cx			// Zero
		mov %cx,%es			// Address
		mov %cx,%ds			//  data
		mov %cx,%ss			// Set up
		mov $start,%sp			//  stack
//
// Relocate ourself to MEM_REL.  Since %cx == 0, the inc %ch sets
// %cx == 0x100.
//
		mov %sp,%si			// Source
		mov $MEM_REL,%di		// Destination
		incb %ch			// Word count
		rep				// Copy
		movsw				//  code
//
// If we are on a hard drive, then load the MBR and look for the first
// FreeBSD slice.  We use the fake partition entry below that points to
// the MBR when we call nread.  The first pass looks for the first active
// FreeBSD slice.  The second pass looks for the first non-active FreeBSD
// slice if the first one fails.
//
		mov $part4,%si			// Partition
		cmpb $0x80,%dl			// Hard drive?
		jb main.4			// No
		movb $0x1,%dh			// Block count
		callw nread			// Read MBR
		mov $0x1,%cx	 		// Two passes
main.1: 	mov $MEM_BUF+PRT_OFF,%si	// Partition table
		movb $0x1,%dh			// Partition
main.2: 	cmpb $PRT_BSD,0x4(%si)		// Our partition type?
		jne main.3			// No
		jcxz main.5			// If second pass
		testb $0x80,(%si)		// Active?
		jnz main.5			// Yes
main.3: 	add $0x10,%si	 		// Next entry
		incb %dh			// Partition
		cmpb $0x1+PRT_NUM,%dh		// In table?
		jb main.2			// Yes
		dec %cx				// Do two
		jcxz main.1			//  passes
//
// If we get here, we didn't find any FreeBSD slices at all, so print an
// error message and die.
//
		mov $msg_part,%si		// Message
		jmp error			// Error
//
// Floppies use partition 0 of drive 0.
//
main.4: 	xor %dx,%dx			// Partition:drive
//
// Ok, we have a slice and drive in %dx now, so use that to locate and load
// boot2.  %si references the start of the slice we are looking for, so go
// ahead and load up the first 16 sectors (boot1 + boot2) from that.  When
// we read it in, we conveniently use 0x8c00 as our transfer buffer.  Thus,
// boot1 ends up at 0x8c00, and boot2 starts at 0x8c00 + 0x200 = 0x8e00.
// The first part of boot2 is the disklabel, which is 0x200 bytes long.
// The second part is BTX, which is thus loaded into 0x9000, which is where
// it also runs from.  The boot2.bin binary starts right after the end of
// BTX, so we have to figure out where the start of it is and then move the
// binary to 0xc000.  Normally, BTX clients start at MEM_USR, or 0xa000, but
// when we use btxld to create boot2, we use an entry point of 0x2000.  That
// entry point is relative to MEM_USR; thus boot2.bin starts at 0xc000.
//
main.5: 	mov %dx,MEM_ARG			// Save args
		movb $NSECT,%dh			// Sector count
		callw nread			// Read disk
		mov $MEM_BTX,%bx		// BTX
		mov 0xa(%bx),%si		// Get BTX length and set
		add %bx,%si			//  %si to start of boot2.bin
		mov $MEM_USR+SIZ_PAG*2,%di	// Client page 2
		mov $MEM_BTX+(NSECT-2)*SIZ_SEC,%cx // Byte
		sub %si,%cx			//  count
		rep				// Relocate
		movsb				//  client
		sub %di,%cx			// Byte count
		xorb %al,%al			// Zero assumed bss from
		rep				//  the end of boot2.bin
		stosb				//  up to 0x10000
		callw seta20			// Enable A20
		jmp start+MEM_JMP-MEM_ORG	// Start BTX
//
// Enable A20 so we can access memory above 1 meg.
//
seta20: 	cli				// Disable interrupts
seta20.1:	inb $0x64,%al			// Get status
		testb $0x2,%al			// Busy?
		jnz seta20.1			// Yes
		movb $0xd1,%al			// Command: Write
		outb %al,$0x64			//  output port
seta20.2:	inb $0x64,%al			// Get status
		testb $0x2,%al			// Busy?
		jnz seta20.2			// Yes
		movb $0xdf,%al			// Enable
		outb %al,$0x60			//  A20
		sti				// Enable interrupts
		retw				// To caller
//
// Trampoline used to call read from within boot1.
//
nread:		mov $MEM_BUF,%bx		// Transfer buffer
		mov 0x8(%si),%ax		// Get
		mov 0xa(%si),%cx		//  LBA
		push %cs			// Read from
		callw xread.1	 		//  disk
		jnc return			// If success, return
		mov $msg_read,%si		// Otherwise, set the error
						//  message and fall through to
						//  the error routine
//
// Print out the error message pointed to by %ds:(%si) followed
// by a prompt, wait for a keypress, and then reboot the machine.
//
error:		callw putstr			// Display message
		mov $prompt,%si			// Display
		callw putstr			//  prompt
		xorb %ah,%ah			// BIOS: Get
		int $0x16			//  keypress
		movw $0x1234, BDA_BOOT		// Do a warm boot
		ljmp $0xffff,$0x0		// reboot the machine
//
// Display a null-terminated string using the BIOS output.
//
putstr.0:	mov $0x7,%bx	 		// Page:attribute
		movb $0xe,%ah			// BIOS: Display
		int $0x10			//  character
putstr: 	lodsb				// Get char
		testb %al,%al			// End of string?
		jne putstr.0			// No

//
// Overused return code.  ereturn is used to return an error from the
// read function.  Since we assume putstr succeeds, we (ab)use the
// same code when we return from putstr.
//
ereturn:	movb $0x1,%ah			// Invalid
		stc				//  argument
return: 	retw				// To caller
//
// Reads sectors from the disk.  If EDD is enabled, then check if it is
// installed and use it if it is.  If it is not installed or not enabled, then
// fall back to using CHS.  Since we use a LBA, if we are using CHS, we have to
// fetch the drive parameters from the BIOS and divide it out ourselves.
// Call with:
//
// %dl	- byte     - drive number
// stack - 10 bytes - EDD Packet
//
read:	 	push %dx			// Save
		movb $0x8,%ah			// BIOS: Get drive
		int $0x13			//  parameters
		movb %dh,%ch			// Max head number
		pop %dx				// Restore
		jc return			// If error
		andb $0x3f,%cl			// Sectors per track
		jz ereturn			// If zero
		cli				// Disable interrupts
		mov 0x8(%bp),%eax		// Get LBA
		push %dx			// Save
		movzbl %cl,%ebx			// Divide by
		xor %edx,%edx			//  sectors
		div %ebx			//  per track
		movb %ch,%bl			// Max head number
		movb %dl,%ch			// Sector number
		inc %bx				// Divide by
		xorb %dl,%dl			//  number
		div %ebx			//  of heads
		movb %dl,%bh			// Head number
		pop %dx				// Restore
		cmpl $0x3ff,%eax		// Cylinder number supportable?
		sti				// Enable interrupts
		ja read.7			// No, try EDD
		xchgb %al,%ah			// Set up cylinder
		rorb $0x2,%al			//  number
		orb %ch,%al			// Merge
		inc %ax				//  sector
		xchg %ax,%cx	 		//  number
		movb %bh,%dh			// Head number
		subb %ah,%al			// Sectors this track
		mov 0x2(%bp),%ah		// Blocks to read
		cmpb %ah,%al			// To read
		jb read.2			//  this
		movb %ah,%al			//  track
read.2: 	mov $0x5,%di	 		// Try count
read.3: 	les 0x4(%bp),%bx		// Transfer buffer
		push %ax			// Save
		movb $0x2,%ah			// BIOS: Read
		int $0x13			//  from disk
		pop %bx				// Restore
		jnc read.4			// If success
		dec %di				// Retry?
		jz read.6			// No
		xorb %ah,%ah			// BIOS: Reset
		int $0x13			//  disk system
		xchg %bx,%ax	 		// Block count
		jmp read.3			// Continue
read.4: 	movzbw %bl,%ax	 		// Sectors read
		add %ax,0x8(%bp)		// Adjust
		jnc read.5			//  LBA,
		incw 0xa(%bp)	 		//  transfer
read.5: 	shlb %bl			//  buffer
		add %bl,0x5(%bp)		//  pointer,
		sub %al,0x2(%bp)		//  block count
		ja read				// If not done
read.6: 	retw				// To caller
read.7:		testb $FL_PACKET,%cs:MEM_REL+flags-start // LBA support enabled?
		jz ereturn			// No, so return an error
		mov $0x55aa,%bx			// Magic
		push %dx			// Save
		movb $0x41,%ah			// BIOS: Check
		int $0x13			//  extensions present
		pop %dx				// Restore
		jc return			// If error, return an error
		cmp $0xaa55,%bx			// Magic?
		jne ereturn			// No, so return an error
		testb $0x1,%cl			// Packet interface?
		jz ereturn			// No, so return an error
		mov %bp,%si			// Disk packet
		movb $0x42,%ah			// BIOS: Extended
		int $0x13			//  read
		retw				// To caller

// Messages

msg_read:	.asciz "Read"
msg_part:	.asciz "Boot"

prompt: 	.asciz " error\r\n"

flags:		.byte FLAGS			// Flags

		.org PRT_OFF,0x90

// Partition table

		.fill 0x30,0x1,0x0
part4:		.byte 0x80, 0x00, 0x01, 0x00
		.byte 0xa5, 0xfe, 0xff, 0xff
		.byte 0x00, 0x00, 0x00, 0x00
		.byte 0x50, 0xc3, 0x00, 0x00	// 50000 sectors long, bleh

		.word 0xaa55			// Magic number