/* * Copyright (c) 2008 Luigi Rizzo (mostly documentation) * Copyright (c) 2002 Bruce M. Simpson * 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. */ /* build options: */ #ifdef PXE /* enable PXE/INT18 booting with F6 */ #define SAVE_MORE_MEMORY #endif #ifdef VOLUME_SERIAL /* support Volume serial number */ #define B0_BASE 0x1ae /* move the internal data area */ #define SAVE_MEMORY #else #define B0_BASE 0x1b2 #endif #ifdef TEST /* enable some test code */ #define SAVE_MEMORY #define SAVE_MORE_MEMORY #endif /* * Note - this code uses many tricks to save space and fit in one sector. * This includes using side effects of certain instructions, reusing * register values from previous operations, etc. * Be extremely careful when changing the code, even for simple things. */ /* * BOOT BLOCK STRUCTURE * * This code implements a Master Boot Record (MBR) for an Intel/PC disk. * It is 512 bytes long and it is normally loaded by the BIOS (or another * bootloader) at 0:0x7c00. This code depends on %cs:%ip being 0:0x7c00 * * The initial chunk of instructions is used as a signature by external * tools (e.g. boot0cfg) which can manipulate the block itself. * * The area at offset 0x1b2 contains a magic string ('Drive '), also * used as a signature to detect the block, and some variables that can * be updated by boot0cfg (and optionally written back to the disk). * These variables control the operation of the bootloader itself, * e.g. which partitions to enable, the timeout, the use of LBA * (called 'packet') or CHS mode, whether to force a drive number, * and whether to write back the user's selection back to disk. * * As in every Master Boot Record, the partition table is at 0x1be, * made of four 16-byte entries each containing: * * OFF SIZE DESCRIPTION * 0 1 status (0x80: bootable, 0: non bootable) * 1 3 start sector CHS * 8:head, 6:sector, 2:cyl bit 9..8, 8:cyl bit 7..0 * 4 1 partition type * 5 3 end sector CHS * 8 4 LBA of first sector * 12 4 partition size in sectors * * and followed by the two bytes 0x55, 0xAA (MBR signature). */ /* * BOOT BLOCK OPERATION * * On entry, the registers contain the following values: * * %cs:%ip 0:0x7c00 * %dl drive number (0x80, 0x81, ... ) * %si pointer to the partition table from which we were loaded. * Some boot code (e.g. syslinux) use this info to relocate * themselves, so we want to pass a valid one to the next stage. * NOTE: the use of %si is not a standard. * * This boot block first relocates itself at a different address (0:0x600), * to free the space at 0:0x7c00 for the next stage boot block. * * It then initializes some memory at 0:0x800 and above (pointed by %bp) * to store the original drive number (%dl) passed to us, and to construct a * fake partition entry. The latter is used by the disk I/O routine and, * in some cases, passed in %si to the next stage boot code. * * The variables at 0x1b2 are accessed as negative offsets from %bp. * * After the relocation, the code scans the partition table printing * out enabled partition or disks, and waits for user input. * * When a partition is selected, or a timeout expires, the currently * selected partition is used to load the next stage boot code, * %dl and %si are set appropriately as when we were called, and * control is transferred to the newly loaded code at 0:0x7c00. */ /* * CONSTANTS * * NHRDRV is the address in segment 0 where the BIOS writes the * total number of hard disks in the system. * LOAD is the original load address and cannot be changed. * ORIGIN is the relocation address. If you change it, you also need * to change the value passed to the linker in the Makefile * PRT_OFF is the location of the partition table (from the MBR standard). * B0_OFF is the location of the data area, known to boot0cfg so * it cannot be changed. Computed as a negative offset from 0x200 * MAGIC is the signature of a boot block. */ .set NHRDRV,0x475 # Number of hard drives .set ORIGIN,0x600 # Execution address .set LOAD,0x7c00 # Load address .set PRT_OFF,0x1be # Partition table .set B0_OFF,(B0_BASE-0x200) # Offset of boot0 data .set MAGIC,0xaa55 # Magic: bootable .set KEY_ENTER,0x1c # Enter key scan code .set KEY_F1,0x3b # F1 key scan code .set KEY_1,0x02 # #1 key scan code .set ASCII_BEL,'#' # ASCII code for <BEL> .set ASCII_CR,0x0D # ASCII code for <CR> /* * Offsets of variables in the block at B0_OFF, and in the volatile * data area, computed as displacement from %bp. * We need to define them as constant as the assembler cannot * compute them in its single pass. */ .set _NXTDRV, B0_OFF+6 # Next drive .set _OPT, B0_OFF+7 # Default option .set _SETDRV, B0_OFF+8 # Drive to force .set _FLAGS, B0_OFF+9 # Flags .set SETDRV, 0x20 # the 'setdrv' flag .set NOUPDATE, 0x40 # the 'noupdate' flag .set USEPACKET, 0x80 # the 'packet' flag /* ticks is at a fixed position */ .set _TICKS, (PRT_OFF - 0x200 - 2) # Timeout ticks .set _MNUOPT, 0x10 # Saved menu entries .set TLEN, (desc_ofs - bootable_ids) # size of bootable ids .globl start # Entry point .code16 # This runs in real mode /* * MAIN ENTRY POINT * Initialise segments and registers to known values. * segments start at 0. * The stack is immediately below the address we were loaded to. * NOTE: the initial section of the code (up to movw $LOAD,%sp) * is used by boot0cfg, together with the 'Drive ' string and * the 0x55, 0xaa at the end, as an identifier for version 1.0 * of the boot code. Do not change it. * In version 1.0 the parameter table (_NEXTDRV etc) is at 0x1b9 */ start: cld # String ops inc xorw %ax,%ax # Zero movw %ax,%es # Address movw %ax,%ds # data movw %ax,%ss # Set up movw $LOAD,%sp # stack /* * Copy this code to the address it was linked for, 0x600 by default. */ movw %sp,%si # Source movw $start,%di # Destination movw $0x100,%cx # Word count rep # Relocate movsw # code /* * After the code, (i.e. at %di+0, 0x800) create a partition entry, * initialized to LBA 0 / CHS 0:0:1. * Set %bp to point to the partition and also, with negative offsets, * to the variables embedded in the bootblock (nextdrv and so on). */ movw %di,%bp # Address variables movb $0x8,%cl # Words to clear rep # Zero stosw # them incb -0xe(%di) # Set the S field to 1 jmp main-LOAD+ORIGIN # Jump to relocated code main: #if defined(SIO) && COMSPEED != 0 /* * Init the serial port. bioscom preserves the driver number in DX. */ movw $COMSPEED,%ax # defined by Makefile callw bioscom #endif /* * If the 'setdrv' flag is set in the boot sector, use the drive * number from the boot sector at 'setdrv_num'. * Optionally, do the same if the BIOS gives us an invalid number * (note though that the override prevents booting from a floppy * or a ZIP/flash drive in floppy emulation). * The test costs 4 bytes of code so it is disabled by default. */ testb $SETDRV,_FLAGS(%bp) # Set drive number? #ifndef CHECK_DRIVE /* disable drive checks */ jz save_curdrive # no, use the default #else jnz disable_update # Yes testb %dl,%dl # Drive number valid? js save_curdrive # Possibly (0x80 set) #endif /* * Disable updates if the drive number is forced. */ disable_update: orb $NOUPDATE,_FLAGS(%bp) # Disable updates movb _SETDRV(%bp),%dl # Use stored drive number /* * Whatever drive we decided to use, store it at (%bp). The byte * is normally used for the state of the partition (0x80 or 0x00), * but we abuse it as it is very convenient to access at offset 0. * The value is read back after 'check_selection' */ save_curdrive: movb %dl, (%bp) # Save drive number pushw %dx # Also in the stack #ifdef TEST /* test code, print internal bios drive */ rolb $1, %dl movw $drive, %si call putkey #endif callw putn # Print a newline /* * Start out with a pointer to the 4th byte of the first table entry * so that after 4 iterations it's beyond the end of the sector * and beyond a 256 byte boundary. We use the latter trick to check for * end of the loop without using an extra register (see start.5). */ movw $(partbl+0x4),%bx # Partition table (+4) xorw %dx,%dx # Item number /* * Loop around on the partition table, printing values until we * pass a 256 byte boundary. */ read_entry: movb %ch,-0x4(%bx) # Zero active flag (ch == 0) btw %dx,_FLAGS(%bp) # Entry enabled? jnc next_entry # No movb (%bx),%al # Load type test %al, %al # skip empty partition jz next_entry /* * Scan the table of bootable ids, which starts at %di and has * length TLEN. On a match, %di points to the element following the * match; the corresponding offset to the description is $(TLEN-1) * bytes ahead. We use a count of TLEN+1 so if we don't find a match * within the first TLEN entries, we hit the 'unknown' entry. */ movw $bootable_ids,%di # Lookup tables movb $(TLEN+1),%cl # Number of entries repne # Locate scasb # type /* * Get the matching element in the next array. * The byte at $(TLEN-1)(%di) contains the offset of the description * string from %di, so we add the number and print the string. */ addw $(TLEN-1), %di # Adjust movb (%di),%cl # Partition addw %cx,%di # description callw putx # Display it next_entry: incw %dx # Next item addb $0x10,%bl # Next entry jnc read_entry # Till done /* * We are past a 256 byte boundary: the partition table is finished. * Add one to the drive number and check it is valid. * Note that if we started from a floppy, %dl was 0 so we still * get an entry for the next drive, which is the first Hard Disk. */ popw %ax # Drive number subb $0x80-0x1,%al # Does next cmpb NHRDRV,%al # drive exist? (from BIOS?) jb print_drive # Yes /* * If this is the only drive, don't display it as an option. */ decw %ax # Already drive 0? jz print_prompt # Yes /* * If it was illegal or we cycled through them, go back to drive 0. */ xorb %al,%al # Drive 0 /* * Whatever drive we selected, make it an ascii digit and save it * back to the "nxtdrv" location in case we want to save it to disk. * This digit is also part of the printed drive string, so add 0x80 * to indicate end of string. */ print_drive: addb $'0'|0x80,%al # Save next movb %al,_NXTDRV(%bp) # drive number movw $drive,%di # Display callw putx # item /* * Menu is complete, display a prompt followed by current selection. * 'decw %si' makes the register point to the space after 'Boot: ' * so we do not see an extra CRLF on the screen. */ print_prompt: movw $prompt,%si # Display callw putstr # prompt movb _OPT(%bp),%dl # Display decw %si # default callw putkey # key jmp start_input # Skip beep /* * Here we have the code waiting for user input or a timeout. */ beep: movb $ASCII_BEL,%al # Input error, print or beep callw putchr start_input: /* * Actual Start of input loop. Take note of time */ xorb %ah,%ah # BIOS: Get int $0x1a # system time movw %dx,%di # Ticks when addw _TICKS(%bp),%di # timeout read_key: /* * Busy loop, looking for keystrokes but keeping one eye on the time. */ #ifndef SIO movb $0x1,%ah # BIOS: Check int $0x16 # for keypress #else /* SIO */ movb $0x03,%ah # BIOS: Read COM call bioscom testb $0x01,%ah # Check line status # (bit 1 indicates input) #endif /* SIO */ jnz got_key # Have input xorb %ah,%ah # BIOS: int 0x1a, 00 int $0x1a # get system time cmpw %di,%dx # Timeout? jb read_key # No /* * Timed out or default selection */ use_default: movb _OPT(%bp),%al # Load default orb $NOUPDATE,_FLAGS(%bp) # Disable updates jmp check_selection # Join common code /* * Get the keystroke. * ENTER or CR confirm the current selection (same as a timeout). * Otherwise convert F1..F6 (or '1'..'6') to 0..5 and check if the * selection is valid. * The SIO code uses ascii chars, the console code uses scancodes. */ got_key: #ifndef SIO xorb %ah,%ah # BIOS: int 0x16, 00 int $0x16 # get keypress movb %ah,%al # move scan code to %al cmpb $KEY_ENTER,%al #else movb $0x02,%ah # BIOS: Receive call bioscom cmpb $ASCII_CR,%al #endif je use_default # enter -> default /* * Check if the key is acceptable, and loop back if not. * The console (non-SIO) code looks at scancodes and accepts * both F1..F6 and 1..6 (the latter costs 6 bytes of code), * relying on the fact that F1..F6 have higher scancodes than 1..6 * The SIO code only takes 1..6 */ #ifdef SIO /* SIO mode, use ascii values */ subb $'1',%al # Subtract '1' ascii code #else /* console mode -- use scancodes */ subb $KEY_F1,%al /* Subtract F1 scan code */ #if !defined(ONLY_F_KEYS) cmpb $0x5,%al # F1..F6 jna 3f # Yes subb $(KEY_1 - KEY_F1),%al # Less #1 scan code 3: #endif /* ONLY_F_KEYS */ #endif /* SIO */ check_selection: cmpb $0x5,%al # F1..F6 or 1..6 ? #ifdef PXE /* enable PXE/INT18 using F6 */ jne 1f; int $0x18 # found F6, try INT18 1: #endif /* PXE */ jae beep # Not in F1..F5, beep /* * We have a selection. If it's a bad selection go back to complain. * The bits in MNUOPT were set when the options were printed. * Anything not printed is not an option. */ cbtw # Extend (%ah=0 used later) btw %ax,_MNUOPT(%bp) # Option enabled? jnc beep # No /* * Save the info in the original tables * for rewriting to the disk. */ movb %al,_OPT(%bp) # Save option /* * Make %si and %bx point to the fake partition at LBA 0 (CHS 0:0:1). * Because the correct address is already in %bp, just use it. * Set %dl with the drive number saved in byte 0. * If we have pressed F5 or 5, then this is a good, fake value * to present to the next stage boot code. */ movw %bp,%si # Partition for write movb (%si),%dl # Drive number, saved above movw %si,%bx # Partition for read cmpb $0x4,%al # F5/#5 pressed? pushf # Save results for later je 1f # Yes, F5 /* * F1..F4 was pressed, so make %bx point to the currently * selected partition, and leave the drive number unchanged. */ shlb $0x4,%al # Point to addw $partbl,%ax # selected xchgw %bx,%ax # partition movb $0x80,(%bx) # Flag active /* * If not asked to do a write-back (flags 0x40) don't do one. * Around the call, save the partition pointer to %bx and * restore to %si which is where the next stage expects it. */ 1: pushw %bx # Save testb $NOUPDATE,_FLAGS(%bp) # No updates? jnz 2f # skip update movw $start,%bx # Data to write movb $0x3,%ah # Write sector callw intx13 # to disk 2: popw %si # Restore /* * If going to next drive, replace drive with selected one. * Remember to un-ascii it. Hey 0x80 is already set, cool! */ popf # Restore %al test results jne 3f # If not F5/#5 movb _NXTDRV(%bp),%dl # Next drive subb $'0',%dl # number /* * Load selected bootsector to the LOAD location in RAM. If read * fails or there is no 0x55aa marker, treat it as a bad selection. */ 3: movw $LOAD,%bx # Address for read movb $0x2,%ah # Read sector callw intx13 # from disk jc beep # If error cmpw $MAGIC,0x1fe(%bx) # Bootable? jne beep # No pushw %si # Save ptr to selected part. callw putn # Leave some space popw %si # Restore, next stage uses it jmp *%bx # Invoke bootstrap /* * Display routines * putkey prints the option selected in %dl (F1..F5 or 1..5) followed by * the string at %si * putx: print the option in %dl followed by the string at %di * also record the drive as valid. * putn: print a crlf * putstr: print the string at %si * putchr: print the char in al */ /* * Display the option and record the drive as valid in the options. * That last point is done using the btsw instruction which does * a test and set. We don't care for the test part. */ putx: btsw %dx,_MNUOPT(%bp) # Enable menu option movw $item,%si # Display callw putkey # key movw %di,%si # Display the rest callw putstr # Display string putn: movw $crlf,%si # To next line jmp putstr putkey: #ifndef SIO movb $'F',%al # Display callw putchr # 'F' #endif movb $'1',%al # Prepare addb %dl,%al # digit putstr.1: callw putchr # Display char putstr: lodsb # Get byte testb $0x80,%al # End of string? jz putstr.1 # No andb $~0x80,%al # Clear MSB then print last putchr: #ifndef SIO pushw %bx # Save movw $0x7,%bx # Page:attribute movb $0xe,%ah # BIOS: Display int $0x10 # character popw %bx # Restore #else /* SIO */ movb $0x01,%ah # BIOS: Send character bioscom: pushw %dx # Save xorw %dx,%dx # Use COM1 int $0x14 # BIOS: Serial I/O popw %dx # Restore #endif /* SIO */ retw # To caller /* One-sector disk I/O routine */ /* * %dl: drive, %si partition entry, %es:%bx transfer buffer. * Load the CHS values and possibly the LBA address from the block * at %si, and use the appropriate method to load the sector. * Don't use packet mode for a floppy. */ intx13: # Prepare CHS parameters movb 0x1(%si),%dh # Load head movw 0x2(%si),%cx # Load cylinder:sector movb $0x1,%al # Sector count pushw %si # Save movw %sp,%di # Save #ifndef CHECK_DRIVE /* floppy support */ testb %dl, %dl # is this a floppy ? jz 1f # Yes, use CHS mode #endif testb $USEPACKET,_FLAGS(%bp) # Use packet interface? jz 1f # No pushl $0x0 # Set the pushl 0x8(%si) # LBA address pushw %es # Set the transfer pushw %bx # buffer address push $0x1 # Block count push $0x10 # Packet size movw %sp,%si # Packet pointer decw %ax # Verify off orb $0x40,%ah # Use disk packet 1: int $0x13 # BIOS: Disk I/O movw %di,%sp # Restore popw %si # Restore retw # To caller /* * Various menu strings. 'item' goes after 'prompt' to save space. * Also use shorter versions to make room for the PXE/INT18 code. */ prompt: #ifdef PXE .ascii "F6 PXE\r" #endif .ascii "\nBoot:" item: .ascii " "; .byte ' '|0x80 crlf: .ascii "\r"; .byte '\n'|0x80 /* Partition type tables */ bootable_ids: /* * These values indicate bootable types we know about. * Corresponding descriptions are at desc_ofs: * Entries don't need to be sorted. */ .byte 0x83, 0xa5, 0xa6, 0xa9, 0x06, 0x07, 0x0b #ifndef SAVE_MORE_MEMORY .byte 0x05 # extended partition #endif #ifndef SAVE_MEMORY /* other DOS partitions */ .byte 0x01 # FAT12 .byte 0x04 # FAT16 < 32M #endif desc_ofs: /* * Offsets that match the known types above, used to point to the * actual partition name. The last entry must point to os_misc, * which is used for non-matching names. */ .byte os_linux-. # 131, Linux .byte os_freebsd-. # 165, FreeBSD .byte os_bsd-. # 166, OpenBSD .byte os_bsd-. # 169, NetBSD .byte os_dos-. # 6, FAT16 >= 32M .byte os_win-. # 7, NTFS .byte os_win-. # 11, FAT32 #ifndef SAVE_MORE_MEMORY .byte os_ext-. # 5, DOS Ext #endif #ifndef SAVE_MEMORY .byte os_dos-. # 1, FAT12 DOS .byte os_dos-. # 4, FAT16 <32M #endif .byte os_misc-. # Unknown /* * And here are the strings themselves. The last byte of * the string has bit 7 set. */ os_misc: .byte '?'|0x80 os_dos: #ifndef SAVE_MORE_MEMORY /* 'DOS' remapped to 'WIN' if no room */ .ascii "DO"; .byte 'S'|0x80 #endif os_win: .ascii "Wi"; .byte 'n'|0x80 os_linux: .ascii "Linu"; .byte 'x'|0x80 os_freebsd: .ascii "Free" os_bsd: .ascii "BS"; .byte 'D'|0x80 #ifndef SAVE_MORE_MEMORY os_ext: .ascii "EX"; .byte 'T'|0x80 #endif .org (0x200 + B0_OFF),0x90 /* * The boot0 version 1.0 parameter table. * Do not move it nor change the "Drive " string, boot0cfg * uses its offset and content to identify the boot sector. * The other fields are sometimes changed before writing back to the drive * Be especially careful that nxtdrv: must come after drive:, as it * is part of the same string. */ drive: .ascii "Drive " nxtdrv: .byte 0x0 # Next drive number opt: .byte 0x0 # Option setdrv_num: .byte 0x80 # Drive to force flags: .byte FLAGS # Flags #ifdef VOLUME_SERIAL .byte 0xa8,0xa8,0xa8,0xa8 # Volume Serial Number #endif ticks: .word TICKS # Delay .org PRT_OFF /* * Here is the 64 byte MBR partition table. */ partbl: .fill 0x40,0x1,0x0 # Partition table .word MAGIC # Magic number .org 0x200 # again, safety check endblock:
