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Diffstat (limited to 'stand/i386/boot0/boot0.S')
| -rw-r--r-- | stand/i386/boot0/boot0.S | 682 |
1 files changed, 682 insertions, 0 deletions
diff --git a/stand/i386/boot0/boot0.S b/stand/i386/boot0/boot0.S new file mode 100644 index 000000000000..708f0934c427 --- /dev/null +++ b/stand/i386/boot0/boot0.S @@ -0,0 +1,682 @@ +/* + * 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. + * + * $FreeBSD$ + */ + +/* build options: */ +#ifdef SIO /* use serial console on COM1. */ +#endif + +#ifdef PXE /* enable PXE/INT18 booting with F6 */ +#define SAVE_MORE_MEMORY +#endif + +#ifdef CHECK_DRIVE /* make sure we boot from a HD. */ +#endif + +#ifdef ONLY_F_KEYS /* Only F1..F6, no digits on console */ +#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 "\nF6 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 partition table that fdisk would fiddle with. + */ +partbl: .fill 0x40,0x1,0x0 # Partition table + .word MAGIC # Magic number + .org 0x200 # again, safety check +endblock: |