--- /dev/null
+/* -----------------------------------------------------------------------
+ *
+ * Copyright 2007-2009 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)
+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:
+
+ /* Escape for the user: if Ctrl is pressed, assume drive hd0 */
+ testb $0x04, BIOS_kbdflags /* Ctrl pressed */
+ jz 1f
+ movb $0x80, %dl
+1:
+ 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
+
+ /* Scan the primary partition table */
+ movw $ptab, %si
+ movw $4, %cx
+ push %si
+ push %cx
+
+ /* Is it a primary partition? */
+ andl %edx, %edx
+ jnz 6f
+
+5:
+ decb (partition)
+ jz boot
+ addw $16, %bx
+ loopw 5b
+
+6:
+ /* No active partitions found, look for extended partitions */
+ popw %cx /* %cx <- 4 */
+ popw %si /* %si <- ptab */
+7:
+ movl 20(%bp), %edx /* "Root" */
+
+ movb 4(%si), %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(%si), %eax /* Partition table offset */
+ 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 */
+ andb %al, %al
+ jz 12f /* Not a valid partition */
+ andl %edx, %edx /* Are we inside an extended part? */
+ /* If so, this is a logical partition */
+ decb (partition)
+ je boot
+12:
+ addw $16, %si
+ loopw 7b
+
+ /* Nothing found, return */
+ popal
+ ret
+
+/*
+ * boot: invoke the actual bootstrap. (%si) points to the partition
+ * table entry, and 28(%bp) has the partition table base.
+ */
+boot:
+ cmpb $0, 4(%si)
+ je missing_os
+ 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
+
+/* Location of the partition configuration byte */
+partition = _start + 439
projects / platform / upstream / syslinux.git / commitdiff