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387
boot/i386/pc/diskboot.S
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387
boot/i386/pc/diskboot.S
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/*
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* PUPA -- Preliminary Universal Programming Architecture for GRUB
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* Copyright (C) 1999,2000,2001 Free Software Foundation, Inc.
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* Copyright (C) 2002 Yoshinori K. Okuji <okuji@enbug.org>
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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*/
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#include <pupa/machine/boot.h>
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/*
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* defines for the code go here
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*/
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/* Absolute addresses
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This makes the assembler generate the address without support
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from the linker. (ELF can't relocate 16-bit addresses!) */
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#define ABS(x) (x-_start+PUPA_BOOT_MACHINE_KERNEL_ADDR)
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/* Print message string */
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#define MSG(x) movw $ABS(x), %si; call message
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.file "diskboot.S"
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.text
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/* Tell GAS to generate 16-bit instructions so that this code works
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in real mode. */
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.code16
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.globl start, _start
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start:
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_start:
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/*
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* _start is loaded at 0x2000 and is jumped to with
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* CS:IP 0:0x2000 in kernel.
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*/
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/*
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* we continue to use the stack for boot.img and assume that
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* some registers are set to correct values. See boot.S
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* for more information.
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*/
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/* save drive reference first thing! */
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pushw %dx
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/* print a notification message on the screen */
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pushw %si
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MSG(notification_string)
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popw %si
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/* this sets up for the first run through "bootloop" */
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movw $ABS(firstlist - PUPA_BOOT_MACHINE_LIST_SIZE), %di
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/* save the sector number of the second sector in %ebp */
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movl (%di), %ebp
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/* this is the loop for reading the rest of the kernel in */
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bootloop:
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/* check the number of sectors to read */
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cmpw $0, 4(%di)
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/* if zero, go to the start function */
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je bootit
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setup_sectors:
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/* check if we use LBA or CHS */
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cmpb $0, -1(%si)
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/* jump to chs_mode if zero */
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je chs_mode
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lba_mode:
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/* load logical sector start */
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movl (%di), %ebx
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/* the maximum is limited to 0x7f because of Phoenix EDD */
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xorl %eax, %eax
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movb $0x7f, %al
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/* how many do we really want to read? */
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cmpw %ax, 4(%di) /* compare against total number of sectors */
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/* which is greater? */
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jg 1f
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/* if less than, set to total */
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movw 4(%di), %ax
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1:
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/* subtract from total */
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subw %ax, 4(%di)
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/* add into logical sector start */
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addl %eax, (%di)
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/* set up disk address packet */
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/* the size and the reserved byte */
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movw $0x0010, (%si)
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/* the number of sectors */
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movw %ax, 2(%si)
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/* the absolute address (low 32 bits) */
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movl %ebx, 8(%si)
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/* the segment of buffer address */
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movw $PUPA_BOOT_MACHINE_BUFFER_SEG, 6(%si)
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/* save %ax from destruction! */
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pushw %ax
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/* zero %eax */
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xorl %eax, %eax
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/* the offset of buffer address */
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movw %ax, 4(%si)
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/* the absolute address (high 32 bits) */
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movl %eax, 12(%si)
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/*
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* BIOS call "INT 0x13 Function 0x42" to read sectors from disk into memory
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* Call with %ah = 0x42
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* %dl = drive number
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* %ds:%si = segment:offset of disk address packet
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* Return:
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* %al = 0x0 on success; err code on failure
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*/
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movb $0x42, %ah
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int $0x13
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jc read_error
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movw $PUPA_BOOT_MACHINE_BUFFER_SEG, %bx
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jmp copy_buffer
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chs_mode:
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/* load logical sector start (bottom half) */
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movl (%di), %eax
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/* zero %edx */
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xorl %edx, %edx
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/* divide by number of sectors */
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divl (%si)
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/* save sector start */
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movb %dl, 10(%si)
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xorl %edx, %edx /* zero %edx */
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divl 4(%si) /* divide by number of heads */
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/* save head start */
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movb %dl, 11(%si)
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/* save cylinder start */
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movw %ax, 12(%si)
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/* do we need too many cylinders? */
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cmpw 8(%si), %ax
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jge geometry_error
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/* determine the maximum sector length of this read */
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movw (%si), %ax /* get number of sectors per track/head */
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/* subtract sector start */
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subb 10(%si), %al
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/* how many do we really want to read? */
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cmpw %ax, 4(%di) /* compare against total number of sectors */
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/* which is greater? */
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jg 2f
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/* if less than, set to total */
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movw 4(%di), %ax
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2:
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/* subtract from total */
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subw %ax, 4(%di)
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/* add into logical sector start */
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addl %eax, (%di)
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/*
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* This is the loop for taking care of BIOS geometry translation (ugh!)
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*/
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/* get high bits of cylinder */
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movb 13(%si), %dl
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shlb $6, %dl /* shift left by 6 bits */
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movb 10(%si), %cl /* get sector */
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incb %cl /* normalize sector (sectors go
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from 1-N, not 0-(N-1) ) */
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orb %dl, %cl /* composite together */
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movb 12(%si), %ch /* sector+hcyl in cl, cylinder in ch */
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/* restore %dx */
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popw %dx
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pushw %dx
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/* head number */
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movb 11(%si), %dh
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pushw %ax /* save %ax from destruction! */
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/*
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* BIOS call "INT 0x13 Function 0x2" to read sectors from disk into memory
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* Call with %ah = 0x2
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* %al = number of sectors
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* %ch = cylinder
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* %cl = sector (bits 6-7 are high bits of "cylinder")
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* %dh = head
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* %dl = drive (0x80 for hard disk, 0x0 for floppy disk)
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* %es:%bx = segment:offset of buffer
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* Return:
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* %al = 0x0 on success; err code on failure
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*/
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movw $PUPA_BOOT_MACHINE_BUFFER_SEG, %bx
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movw %bx, %es /* load %es segment with disk buffer */
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xorw %bx, %bx /* %bx = 0, put it at 0 in the segment */
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movb $0x2, %ah /* function 2 */
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int $0x13
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jc read_error
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/* save source segment */
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movw %es, %bx
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copy_buffer:
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/* load addresses for copy from disk buffer to destination */
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movw 6(%di), %es /* load destination segment */
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/* restore %ax */
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popw %ax
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/* determine the next possible destination address (presuming
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512 byte sectors!) */
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shlw $5, %ax /* shift %ax five bits to the left */
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addw %ax, 6(%di) /* add the corrected value to the destination
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address for next time */
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/* save addressing regs */
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pusha
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pushw %ds
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/* get the copy length */
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shlw $3, %ax
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movw %ax, %cx
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xorw %di, %di /* zero offset of destination addresses */
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xorw %si, %si /* zero offset of source addresses */
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movw %bx, %ds /* restore the source segment */
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cld /* sets the copy direction to forward */
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/* perform copy */
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rep /* sets a repeat */
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movsw /* this runs the actual copy */
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/* restore addressing regs and print a dot with correct DS
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(MSG modifies SI, which is saved, and unused AX and BX) */
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popw %ds
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MSG(notification_step)
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popa
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/* check if finished with this dataset */
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cmpw $0, 4(%di)
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jne setup_sectors
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/* update position to load from */
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subw $PUPA_BOOT_MACHINE_LIST_SIZE, %di
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/* jump to bootloop */
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jmp bootloop
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/* END OF MAIN LOOP */
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bootit:
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/* print a newline */
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MSG(notification_done)
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popw %dx /* this makes sure %dl is our "boot" drive */
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ljmp $0, $(PUPA_BOOT_MACHINE_KERNEL_ADDR + 0x200)
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/*
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* BIOS Geometry translation error (past the end of the disk geometry!).
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*/
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geometry_error:
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MSG(geometry_error_string)
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jmp general_error
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/*
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* Read error on the disk.
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*/
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read_error:
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MSG(read_error_string)
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general_error:
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MSG(general_error_string)
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/* go here when you need to stop the machine hard after an error condition */
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stop: jmp stop
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notification_string: .string "Loading kernel"
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notification_step: .string "."
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notification_done: .string "\r\n"
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geometry_error_string: .string "Geom"
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read_error_string: .string "Read"
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general_error_string: .string " Error"
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/*
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* message: write the string pointed to by %si
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*
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* WARNING: trashes %si, %ax, and %bx
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*/
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/*
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* Use BIOS "int 10H Function 0Eh" to write character in teletype mode
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* %ah = 0xe %al = character
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* %bh = page %bl = foreground color (graphics modes)
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*/
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1:
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movw $0x0001, %bx
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movb $0xe, %ah
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int $0x10 /* display a byte */
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incw %si
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message:
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movb (%si), %al
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cmpb $0, %al
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jne 1b /* if not end of string, jmp to display */
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ret
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lastlist:
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/*
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* This area is an empty space between the main body of code below which
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* grows up (fixed after compilation, but between releases it may change
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* in size easily), and the lists of sectors to read, which grows down
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* from a fixed top location.
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*/
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.word 0
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.word 0
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. = _start + 0x200 - PUPA_BOOT_MACHINE_LIST_SIZE
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/* fill the first data listing with the default */
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blocklist_default_start:
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/* this is the sector start parameter, in logical sectors from
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the start of the disk, sector 0 */
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.long 2
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blocklist_default_len:
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/* this is the number of sectors to read the command "install"
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will fill this up */
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.word 0
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blocklist_default_seg:
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/* this is the segment of the starting address to load the data into */
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.word (PUPA_BOOT_MACHINE_KERNEL_SEG + 0x20)
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firstlist: /* this label has to be after the list data!!! */
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