/*
 * 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.
 */

/* 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

                .set NSECT,0x10
                .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.  Note that boot1 does not use this relocated copy
 * of itself while loading boot2; however, BTX reclaims the memory
 * used by boot1 during its initialization.  As a result, boot2 uses
 * xread from the relocated copy.
 */
                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-1)*SIZ_SEC,%cx # Byte
                sub %si,%cx                     #  count
                rep                             # Relocate
                movsb                           #  client

/*
 * Enable A20 so we can access memory above 1 meg.
 * Use the zero-valued %cx as a timeout for embedded hardware which do not
 * have a keyboard controller.
 */
seta20:         cli                             # Disable interrupts
seta20.1:       dec %cx                         # Timeout?
                jz seta20.3                     # Yes
                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
seta20.3:       sti                             # Enable interrupts

                jmp start+MEM_JMP-MEM_ORG       # Start BTX


/*
 * 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 $0xf000,$0xfff0            # 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:           testb $FL_PACKET,%cs:MEM_REL+flags-start # LBA support enabled?
                jz read.1                       # No, use CHS
                cmpb $0x80,%dl                  # Hard drive?
                jb read.1                       # No, use CHS
                mov $0x55aa,%bx                 # Magic
                push %dx                        # Save
                movb $0x41,%ah                  # BIOS: Check
                int $0x13                       #  extensions present
                pop %dx                         # Restore
                jc read.1                       # If error, use CHS
                cmp $0xaa55,%bx                 # Magic?
                jne read.1                      # No, so use CHS
                testb $0x1,%cl                  # Packet interface?
                jz read.1                       # No, so use CHS
                mov %bp,%si                     # Disk packet
                movb $0x42,%ah                  # BIOS: Extended
                int $0x13                       #  read
                retw                            # To caller
read.1:         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 ereturn                      # No, return an error
                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
#ifdef  TRACK_AT_A_TIME
                movb %ah,%al                    #  track
#else
                movb $1,%al                     #  one sector
#endif
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.1                       # If not done
read.6:         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