diff options
Diffstat (limited to 'mbr/mbr.S')
| -rw-r--r-- | mbr/mbr.S | 299 |
1 files changed, 299 insertions, 0 deletions
diff --git a/mbr/mbr.S b/mbr/mbr.S new file mode 100644 index 00000000..81e5dd00 --- /dev/null +++ b/mbr/mbr.S @@ -0,0 +1,299 @@ +/* ----------------------------------------------------------------------- + * + * Copyright 2007 H. Peter Anvin - All Rights Reserved + * + * Permission is hereby granted, free of charge, to any person + * obtaining a copy of this software and associated documentation + * files (the "Software"), to deal in the Software without + * restriction, including without limitation the rights to use, + * copy, modify, merge, publish, distribute, sublicense, and/or + * sell copies of the Software, and to permit persons to whom + * the Software is furnished to do so, subject to the following + * conditions: + * + * The above copyright notice and this permission notice shall + * be included in all copies or substantial portions of the Software. + * + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, + * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES + * OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND + * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT + * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, + * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING + * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR + * OTHER DEALINGS IN THE SOFTWARE. + * + * ----------------------------------------------------------------------- */ + + .code16 + .text + + .globl bootsec +stack = 0x7c00 +driveno = (stack-6) +heads = (stack-8) +sectors = (stack-10) + +BIOS_page = 0x462 + + /* gas/ld has issues with doing this as absolute addresses... */ + .section ".bootsec", "a", @nobits + .globl bootsec +bootsec: + .space 512 + + .text + .globl _start +_start: + cli + xorw %ax, %ax + movw %ax, %ds + movw %ax, %ss + movw $stack, %sp + movw %sp, %si + pushw %es /* es:di -> $PnP header */ + pushw %di + pushw %dx /* dl -> drive number */ + movw %ax, %es + sti + cld + + /* Copy down to 0:0x600 */ + movw $_start, %di + movw $(512/2), %cx + rep; movsw + + ljmpw $0, $next + +next: + /* Check to see if we have EBIOS */ + pushw %dx /* drive number */ + movw $0x4100, %ax + movw $0x55aa, %bx + xorw %cx, %cx + xorb %dh, %dh + stc + int $0x13 + jc 1f + cmpw $0xaa55, %bx + jne 1f + testb $0x01, %cl + jz 1f + + /* We have EBIOS; patch in a jump to read_sector_ebios */ + movw $0xeb+((read_sector_ebios-read_sector_cbios-2)<< 8), (read_sector_cbios) + +1: + popw %dx + + /* Get (C)HS geometry */ + movb $0x08, %ah + int $0x13 + xorw %ax, %ax + movb %dh, %al /* dh = number of heads */ + incw %ax /* From 0-based to count */ + pushw %ax /* Save heads on the stack */ + andw $0x3f, %cx /* Sector count */ + pushw %cx /* Save sectors on the stack */ + + xorl %eax, %eax + pushl %eax /* Base */ + pushl %eax /* Root */ + call scan_partition_table + + /* If we get here, we have no OS */ + jmp missing_os + +/* + * read_sector: read a single sector pointed to by %eax to 0x7c00. + * CF is set on error. All registers clobbered. + */ +read_sector: +read_sector_cbios: + movl %eax, %edx + shrl $16, %edx + divw (sectors) + incw %dx + movw %dx, %cx + xorw %dx, %dx + divw (heads) + /* dx = head, ax = cylinder */ + movb %al, %ch + shrw $2, %ax + shrw %ax + andb $0xc0, %al + orb %al, %cl + movb %dl, %dh + movw $bootsec, %bx + movw $0x0201, %ax + jmp read_common +read_sector_ebios: + movw $dapa, %si + movl %eax, 8(%si) + movb $0x42, %ah +read_common: + movb (driveno), %dl + int $0x13 + ret + +/* + * read_partition_table: + * Read a partition table (pointed to by %eax), and copy + * the partition table into the ptab buffer. + * Preserve registers. + */ +ptab = _start+446 + +read_partition_table: + pushal + call read_sector + jc 20f + movw $bootsec+446, %si + movw $ptab, %di + movw $(16*4/2), %cx + rep ; movsw +20: + popal + ret + +/* + * scan_partition_table: + * Scan a partition table currently loaded in the partition table + * area. Preserve 16-bit registers. + * + * On stack: + * 18(%bp) - root (offset from MBR, or 0 for MBR) + * 22(%bp) - base (location of this partition table) + */ + +scan_partition_table: + pusha + movw %sp, %bp + + /* Search for active partitions */ + movw $ptab, %di + movw $4, %cx + xorw %ax, %ax +5: + testb $0x80, (%di) + jz 6f + incw %ax + movw %di, %si +6: + addw $16, %di + loopw 5b + + cmpw $1, %ax /* Number of active partitions found */ + je boot + ja too_many_active + + /* No active partitions found, look for extended partitions */ + movw $ptab, %di + movb $4, %cl /* cx == 0 here */ +7: + movb 4(%di), %al + cmpb $0x05, %al /* MS-DOS extended */ + je 8f + cmpb $0x0f, %al /* Win9x extended */ + je 8f + cmpb $0x85, %al /* Linux extended */ + jne 9f + + /* It is an extended partition. Read the extended partition and + try to scan it. If the scan returns, re-load the current + partition table and resume scan. */ +8: + movl 8(%di), %eax /* Partition table offset */ + movl 18(%bp), %edx /* "Root" */ + addl %edx, %eax /* Compute location of new ptab */ + andl %edx, %edx /* Is this the MBR? */ + jnz 10f + movl %eax, %edx /* Offset -> root if this was MBR */ +10: + pushl %eax /* Push new base */ + pushl %edx /* Push new root */ + call read_partition_table + jc 11f + call scan_partition_table +11: + addw $8, %sp + /* This returned, so we need to reload the current partition table */ + movl 22(%bp), %eax + call read_partition_table + + /* fall through */ +9: + /* Not an extended partition */ + addw $16, %di + loopw 7b + + /* Nothing found, return */ + popa + ret + +/* + * boot: invoke the actual bootstrap. (%si) points to the partition + * table entry, and 22(%bp) has the partition table base. + */ +boot: + movl 8(%si), %eax + addl 22(%bp), %eax + movl %eax, 8(%si) + pushw %si + call read_sector + popw %si + jc disk_error + cmpw $0xaa55, (bootsec+510) + jne missing_os /* Not a valid boot sector */ + movw $driveno, %sp + popw %dx /* dl -> drive number */ + popw %di /* es:di -> $PnP vector */ + popw %es + cli + jmp bootsec + +/* + * error messages + */ +missing_os: + movw $missing_os_msg, %si + jmp error +disk_error: + movw $disk_error_msg, %si + jmp error +too_many_active: + movw $too_many_active_msg, %si + /* jmp error */ + +error: +2: + lodsb + andb %al, %al + jz 3f + movb $0x0e, %ah + movb (BIOS_page), %bh + movb $0x07, %bl + int $0x10 + jmp 2b +3: + int $0x18 /* Boot failure */ + jmp . /* Die */ + +missing_os_msg: + .ascii "Missing operating system." + .byte 0 +disk_error_msg: + .ascii "Operating system load error." + .byte 0 +too_many_active_msg: + .ascii "Multiple active partitions." + .byte 0 + + .balign 4 +dapa: + .short 16 /* Size of packet */ + .short 1 /* Sector count */ + .short 0x7c00 /* Buffer offset */ + .short 0 /* Buffer segment */ + .long 0 /* LSW of LBA */ + .long 0 /* MSW of LBA */ |