diff options
Diffstat (limited to 'contrib/syslinux-4.02/mbr/mbr.S')
| -rw-r--r-- | contrib/syslinux-4.02/mbr/mbr.S | 299 |
1 files changed, 299 insertions, 0 deletions
diff --git a/contrib/syslinux-4.02/mbr/mbr.S b/contrib/syslinux-4.02/mbr/mbr.S new file mode 100644 index 0000000..7caf4fc --- /dev/null +++ b/contrib/syslinux-4.02/mbr/mbr.S @@ -0,0 +1,299 @@ +/* ----------------------------------------------------------------------- + * + * Copyright 2007-2009 H. Peter Anvin - All Rights Reserved + * Copyright 2009 Intel Corporation; author: H. Peter Anvin + * + * 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. + * + * ----------------------------------------------------------------------- */ + +#include "adjust.h" + + .code16 + .text + + .globl bootsec +stack = 0x7c00 +driveno = (stack-6) +sectors = (stack-8) +secpercyl = (stack-12) + +BIOS_kbdflags = 0x417 +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 + movw %ax, %es + sti + cld + + /* Copy down to 0:0x600 */ + movw $_start, %di + movw $(512/2), %cx + rep; movsw + + ljmpw $0, $next +next: + + ADJUST_DRIVE + pushw %dx /* dl -> drive number */ + + /* Check to see if we have EBIOS */ + pushw %dx /* drive number */ + movb $0x41, %ah /* %al == 0 already */ + movw $0x55aa, %bx + xorw %cx, %cx + xorb %dh, %dh + stc + int $0x13 + jc 1f + cmpw $0xaa55, %bx + jne 1f + shrw %cx /* Bit 0 = fixed disk subset */ + jnc 1f + + /* We have EBIOS; patch in the following code at + read_sector_cbios: movb $0x42, %ah ; jmp read_common */ + movl $0xeb42b4+((read_common-read_sector_cbios-4) << 24), \ + (read_sector_cbios) + +1: + popw %dx + + /* Get (C)HS geometry */ + movb $0x08, %ah + int $0x13 + andw $0x3f, %cx /* Sector count */ + pushw %cx /* Save sectors on the stack */ + movzbw %dh, %ax /* dh = max head */ + incw %ax /* From 0-based max to count */ + mulw %cx /* Heads*sectors -> sectors per cylinder */ + + /* Save sectors/cylinder on the stack */ + pushw %dx /* High word */ + pushw %ax /* Low word */ + + xorl %eax, %eax /* Base */ + cdq /* Root (%edx <- 0) */ + call scan_partition_table + + /* If we get here, we have no OS */ +missing_os: + call error + .ascii "Missing operating system.\r\n" + +/* + * read_sector: read a single sector pointed to by %eax to 0x7c00. + * CF is set on error. All registers saved. + */ +read_sector: + pushal + xorl %edx, %edx + movw $bootsec, %bx + pushl %edx /* MSW of LBA */ + pushl %eax /* LSW of LBA */ + pushw %es /* Buffer segment */ + pushw %bx /* Buffer offset */ + pushw $1 /* Sector count */ + pushw $16 /* Size of packet */ + movw %sp, %si + + /* This chunk is skipped if we have ebios */ + /* Do not clobber %eax before this chunk! */ + /* This also relies on %bx and %edx as set up above. */ +read_sector_cbios: + divl (secpercyl) + shlb $6, %ah + movb %ah, %cl + movb %al, %ch + xchgw %dx, %ax + divb (sectors) + movb %al, %dh + orb %ah, %cl + incw %cx /* Sectors are 1-based */ + movw $0x0201, %ax + +read_common: + movb (driveno), %dl + int $0x13 + leaw 16(%si), %sp /* Drop DAPA */ + popal + ret + +/* + * read_partition_table: + * Read a partition table (pointed to by %eax), and copy + * the partition table into the ptab buffer. + * + * Clobbers %si, %di, and %cx, other registers preserved. + * %cx = 0 on exit. + * + * On error, CF is set and ptab is overwritten with junk. + */ +ptab = _start+446 + +read_partition_table: + call read_sector + movw $bootsec+446, %si + movw $ptab, %di + movw $(16*4/2), %cx + rep ; movsw + ret + +/* + * scan_partition_table: + * Scan a partition table currently loaded in the partition table + * area. Preserve all registers. + * + * On entry: + * %eax - base (location of this partition table) + * %edx - root (offset from MBR, or 0 for MBR) + * + * These get pushed into stack slots: + * 28(%bp) - %eax - base + * 20(%bp) - %edx - root + */ + +scan_partition_table: + pushal + movw %sp, %bp + + /* Search for active partitions */ + movw $ptab, %bx + movw $4, %cx + xorw %ax, %ax + push %bx + push %cx +5: + testb $0x80, (%bx) + jz 6f + incw %ax + movw %bx, %si +6: + addw $16, %bx + loopw 5b + + decw %ax /* Number of active partitions found */ + jz boot + jns too_many_active + + /* No active partitions found, look for extended partitions */ + popw %cx /* %cx <- 4 */ + popw %bx /* %bx <- ptab */ +7: + movb 4(%bx), %al + cmpb $0x0f, %al /* 0x0f = Win9x extended */ + je 8f + andb $~0x80, %al /* 0x85 = Linux extended */ + cmpb $0x05, %al /* 0x05 = MS-DOS 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(%bx), %eax /* Partition table offset */ + movl 20(%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: + call read_partition_table + jc 11f + call scan_partition_table +11: + /* This returned, so we need to reload the current partition table */ + movl 28(%bp), %eax /* "Base" */ + call read_partition_table + + /* fall through */ +9: + /* Not an extended partition */ + addw $16, %bx + loopw 7b + + /* Nothing found, return */ + popal + ret + +too_many_active: + call error + .ascii "Multiple active partitions.\r\n" + +/* + * boot: invoke the actual bootstrap. (%si) points to the partition + * table entry, and 28(%bp) has the partition table base. + */ +boot: + movl 8(%si), %eax + addl 28(%bp), %eax + movl %eax, 8(%si) /* Adjust in-memory partition table entry */ + call read_sector + jc disk_error + cmpw $0xaa55, (bootsec+510) + jne missing_os /* Not a valid boot sector */ + movw $driveno, %sp /* driveno == bootsec-6 */ + popw %dx /* dl -> drive number */ + popw %di /* es:di -> $PnP vector */ + popw %es + cli + jmpw *%sp /* %sp == bootsec */ + +disk_error: + call error + .ascii "Operating system load error.\r\n" + +/* + * Print error messages. This is invoked with "call", with the + * error message at the return address. + */ +error: + popw %si +2: + lodsb + movb $0x0e, %ah + movb (BIOS_page), %bh + movb $0x07, %bl + int $0x10 /* May destroy %bp */ + cmpb $10, %al /* Newline? */ + jne 2b + + int $0x18 /* Boot failure */ +die: + hlt + jmp die |