683 lines
21 KiB
ArmAsm
683 lines
21 KiB
ArmAsm
/*
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* Copyright (c) 2008 Luigi Rizzo (mostly documentation)
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* Copyright (c) 2002 Bruce M. Simpson
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* Copyright (c) 1998 Robert Nordier
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* All rights reserved.
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*
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* Redistribution and use in source and binary forms are freely
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* permitted provided that the above copyright notice and this
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* paragraph and the following disclaimer are duplicated in all
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* such forms.
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*
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* This software is provided "AS IS" and without any express or
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* implied warranties, including, without limitation, the implied
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* warranties of merchantability and fitness for a particular
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* purpose.
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*
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* $FreeBSD$
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*/
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/* build options: */
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#ifdef SIO /* use serial console on COM1. */
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#endif
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#ifdef PXE /* enable PXE/INT18 booting with F6 */
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#define SAVE_MORE_MEMORY
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#endif
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#ifdef CHECK_DRIVE /* make sure we boot from a HD. */
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#endif
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#ifdef ONLY_F_KEYS /* Only F1..F6, no digits on console */
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#endif
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#ifdef VOLUME_SERIAL /* support Volume serial number */
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#define B0_BASE 0x1ae /* move the internal data area */
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#define SAVE_MEMORY
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#else
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#define B0_BASE 0x1b2
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#endif
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#ifdef TEST /* enable some test code */
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#define SAVE_MEMORY
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#define SAVE_MORE_MEMORY
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#endif
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/*
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* Note - this code uses many tricks to save space and fit in one sector.
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* This includes using side effects of certain instructions, reusing
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* register values from previous operations, etc.
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* Be extremely careful when changing the code, even for simple things.
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*/
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/*
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* BOOT BLOCK STRUCTURE
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*
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* This code implements a Master Boot Record (MBR) for an Intel/PC disk.
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* It is 512 bytes long and it is normally loaded by the BIOS (or another
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* bootloader) at 0:0x7c00. This code depends on %cs:%ip being 0:0x7c00
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*
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* The initial chunk of instructions is used as a signature by external
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* tools (e.g. boot0cfg) which can manipulate the block itself.
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*
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* The area at offset 0x1b2 contains a magic string ('Drive '), also
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* used as a signature to detect the block, and some variables that can
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* be updated by boot0cfg (and optionally written back to the disk).
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* These variables control the operation of the bootloader itself,
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* e.g. which partitions to enable, the timeout, the use of LBA
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* (called 'packet') or CHS mode, whether to force a drive number,
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* and whether to write back the user's selection back to disk.
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*
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* As in every Master Boot Record, the partition table is at 0x1be,
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* made of four 16-byte entries each containing:
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*
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* OFF SIZE DESCRIPTION
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* 0 1 status (0x80: bootable, 0: non bootable)
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* 1 3 start sector CHS
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* 8:head, 6:sector, 2:cyl bit 9..8, 8:cyl bit 7..0
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* 4 1 partition type
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* 5 3 end sector CHS
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* 8 4 LBA of first sector
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* 12 4 partition size in sectors
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*
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* and followed by the two bytes 0x55, 0xAA (MBR signature).
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*/
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/*
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* BOOT BLOCK OPERATION
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*
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* On entry, the registers contain the following values:
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*
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* %cs:%ip 0:0x7c00
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* %dl drive number (0x80, 0x81, ... )
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* %si pointer to the partition table from which we were loaded.
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* Some boot code (e.g. syslinux) use this info to relocate
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* themselves, so we want to pass a valid one to the next stage.
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* NOTE: the use of %si is not a standard.
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*
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* This boot block first relocates itself at a different address (0:0x600),
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* to free the space at 0:0x7c00 for the next stage boot block.
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*
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* It then initializes some memory at 0:0x800 and above (pointed by %bp)
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* to store the original drive number (%dl) passed to us, and to construct a
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* fake partition entry. The latter is used by the disk I/O routine and,
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* in some cases, passed in %si to the next stage boot code.
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*
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* The variables at 0x1b2 are accessed as negative offsets from %bp.
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*
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* After the relocation, the code scans the partition table printing
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* out enabled partition or disks, and waits for user input.
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*
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* When a partition is selected, or a timeout expires, the currently
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* selected partition is used to load the next stage boot code,
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* %dl and %si are set appropriately as when we were called, and
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* control is transferred to the newly loaded code at 0:0x7c00.
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*/
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/*
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* CONSTANTS
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*
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* NHRDRV is the address in segment 0 where the BIOS writes the
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* total number of hard disks in the system.
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* LOAD is the original load address and cannot be changed.
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* ORIGIN is the relocation address. If you change it, you also need
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* to change the value passed to the linker in the Makefile
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* PRT_OFF is the location of the partition table (from the MBR standard).
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* B0_OFF is the location of the data area, known to boot0cfg so
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* it cannot be changed. Computed as a negative offset from 0x200
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* MAGIC is the signature of a boot block.
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*/
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.set NHRDRV,0x475 # Number of hard drives
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.set ORIGIN,0x600 # Execution address
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.set LOAD,0x7c00 # Load address
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.set PRT_OFF,0x1be # Partition table
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.set B0_OFF,(B0_BASE-0x200) # Offset of boot0 data
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.set MAGIC,0xaa55 # Magic: bootable
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.set KEY_ENTER,0x1c # Enter key scan code
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.set KEY_F1,0x3b # F1 key scan code
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.set KEY_1,0x02 # #1 key scan code
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.set ASCII_BEL,'#' # ASCII code for <BEL>
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.set ASCII_CR,0x0D # ASCII code for <CR>
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/*
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* Offsets of variables in the block at B0_OFF, and in the volatile
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* data area, computed as displacement from %bp.
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* We need to define them as constant as the assembler cannot
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* compute them in its single pass.
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*/
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.set _NXTDRV, B0_OFF+6 # Next drive
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.set _OPT, B0_OFF+7 # Default option
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.set _SETDRV, B0_OFF+8 # Drive to force
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.set _FLAGS, B0_OFF+9 # Flags
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.set SETDRV, 0x20 # the 'setdrv' flag
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.set NOUPDATE, 0x40 # the 'noupdate' flag
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.set USEPACKET, 0x80 # the 'packet' flag
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/* ticks is at a fixed position */
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.set _TICKS, (PRT_OFF - 0x200 - 2) # Timeout ticks
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.set _MNUOPT, 0x10 # Saved menu entries
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.set TLEN, (desc_ofs - bootable_ids) # size of bootable ids
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.globl start # Entry point
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.code16 # This runs in real mode
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/*
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* MAIN ENTRY POINT
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* Initialise segments and registers to known values.
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* segments start at 0.
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* The stack is immediately below the address we were loaded to.
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* NOTE: the initial section of the code (up to movw $LOAD,%sp)
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* is used by boot0cfg, together with the 'Drive ' string and
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* the 0x55, 0xaa at the end, as an identifier for version 1.0
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* of the boot code. Do not change it.
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* In version 1.0 the parameter table (_NEXTDRV etc) is at 0x1b9
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*/
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start: cld # String ops inc
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xorw %ax,%ax # Zero
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movw %ax,%es # Address
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movw %ax,%ds # data
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movw %ax,%ss # Set up
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movw $LOAD,%sp # stack
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/*
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* Copy this code to the address it was linked for, 0x600 by default.
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*/
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movw %sp,%si # Source
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movw $start,%di # Destination
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movw $0x100,%cx # Word count
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rep # Relocate
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movsw # code
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/*
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* After the code, (i.e. at %di+0, 0x800) create a partition entry,
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* initialized to LBA 0 / CHS 0:0:1.
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* Set %bp to point to the partition and also, with negative offsets,
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* to the variables embedded in the bootblock (nextdrv and so on).
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*/
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movw %di,%bp # Address variables
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movb $0x8,%cl # Words to clear
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rep # Zero
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stosw # them
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incb -0xe(%di) # Set the S field to 1
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jmp main-LOAD+ORIGIN # Jump to relocated code
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main:
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#if defined(SIO) && COMSPEED != 0
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/*
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* Init the serial port. bioscom preserves the driver number in DX.
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*/
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movw $COMSPEED,%ax # defined by Makefile
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callw bioscom
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#endif
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/*
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* If the 'setdrv' flag is set in the boot sector, use the drive
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* number from the boot sector at 'setdrv_num'.
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* Optionally, do the same if the BIOS gives us an invalid number
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* (note though that the override prevents booting from a floppy
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* or a ZIP/flash drive in floppy emulation).
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* The test costs 4 bytes of code so it is disabled by default.
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*/
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testb $SETDRV,_FLAGS(%bp) # Set drive number?
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#ifndef CHECK_DRIVE /* disable drive checks */
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jz save_curdrive # no, use the default
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#else
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jnz disable_update # Yes
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testb %dl,%dl # Drive number valid?
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js save_curdrive # Possibly (0x80 set)
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#endif
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/*
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* Disable updates if the drive number is forced.
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*/
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disable_update: orb $NOUPDATE,_FLAGS(%bp) # Disable updates
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movb _SETDRV(%bp),%dl # Use stored drive number
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/*
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* Whatever drive we decided to use, store it at (%bp). The byte
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* is normally used for the state of the partition (0x80 or 0x00),
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* but we abuse it as it is very convenient to access at offset 0.
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* The value is read back after 'check_selection'
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*/
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save_curdrive: movb %dl, (%bp) # Save drive number
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pushw %dx # Also in the stack
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#ifdef TEST /* test code, print internal bios drive */
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rolb $1, %dl
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movw $drive, %si
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call putkey
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#endif
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callw putn # Print a newline
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/*
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* Start out with a pointer to the 4th byte of the first table entry
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* so that after 4 iterations it's beyond the end of the sector
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* and beyond a 256 byte boundary. We use the latter trick to check for
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* end of the loop without using an extra register (see start.5).
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*/
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movw $(partbl+0x4),%bx # Partition table (+4)
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xorw %dx,%dx # Item number
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/*
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* Loop around on the partition table, printing values until we
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* pass a 256 byte boundary.
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*/
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read_entry: movb %ch,-0x4(%bx) # Zero active flag (ch == 0)
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btw %dx,_FLAGS(%bp) # Entry enabled?
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jnc next_entry # No
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movb (%bx),%al # Load type
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test %al, %al # skip empty partition
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jz next_entry
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/*
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* Scan the table of bootable ids, which starts at %di and has
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* length TLEN. On a match, %di points to the element following the
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* match; the corresponding offset to the description is $(TLEN-1)
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* bytes ahead. We use a count of TLEN+1 so if we don't find a match
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* within the first TLEN entries, we hit the 'unknown' entry.
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*/
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movw $bootable_ids,%di # Lookup tables
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movb $(TLEN+1),%cl # Number of entries
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repne # Locate
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scasb # type
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/*
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* Get the matching element in the next array.
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* The byte at $(TLEN-1)(%di) contains the offset of the description
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* string from %di, so we add the number and print the string.
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*/
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addw $(TLEN-1), %di # Adjust
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movb (%di),%cl # Partition
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addw %cx,%di # description
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callw putx # Display it
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next_entry: incw %dx # Next item
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addb $0x10,%bl # Next entry
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jnc read_entry # Till done
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/*
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* We are past a 256 byte boundary: the partition table is finished.
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* Add one to the drive number and check it is valid.
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* Note that if we started from a floppy, %dl was 0 so we still
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* get an entry for the next drive, which is the first Hard Disk.
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*/
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popw %ax # Drive number
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subb $0x80-0x1,%al # Does next
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cmpb NHRDRV,%al # drive exist? (from BIOS?)
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jb print_drive # Yes
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/*
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* If this is the only drive, don't display it as an option.
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*/
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decw %ax # Already drive 0?
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jz print_prompt # Yes
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/*
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* If it was illegal or we cycled through them, go back to drive 0.
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*/
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xorb %al,%al # Drive 0
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/*
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* Whatever drive we selected, make it an ascii digit and save it
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* back to the "nxtdrv" location in case we want to save it to disk.
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* This digit is also part of the printed drive string, so add 0x80
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* to indicate end of string.
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*/
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print_drive: addb $'0'|0x80,%al # Save next
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movb %al,_NXTDRV(%bp) # drive number
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movw $drive,%di # Display
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callw putx # item
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/*
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* Menu is complete, display a prompt followed by current selection.
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* 'decw %si' makes the register point to the space after 'Boot: '
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* so we do not see an extra CRLF on the screen.
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*/
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print_prompt: movw $prompt,%si # Display
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callw putstr # prompt
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movb _OPT(%bp),%dl # Display
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decw %si # default
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callw putkey # key
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jmp start_input # Skip beep
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/*
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* Here we have the code waiting for user input or a timeout.
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*/
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beep: movb $ASCII_BEL,%al # Input error, print or beep
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callw putchr
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start_input:
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/*
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* Actual Start of input loop. Take note of time
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*/
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xorb %ah,%ah # BIOS: Get
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int $0x1a # system time
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movw %dx,%di # Ticks when
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addw _TICKS(%bp),%di # timeout
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read_key:
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/*
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* Busy loop, looking for keystrokes but keeping one eye on the time.
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*/
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#ifndef SIO
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movb $0x1,%ah # BIOS: Check
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int $0x16 # for keypress
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#else /* SIO */
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movb $0x03,%ah # BIOS: Read COM
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call bioscom
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testb $0x01,%ah # Check line status
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# (bit 1 indicates input)
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#endif /* SIO */
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jnz got_key # Have input
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xorb %ah,%ah # BIOS: int 0x1a, 00
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int $0x1a # get system time
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cmpw %di,%dx # Timeout?
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jb read_key # No
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/*
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* Timed out or default selection
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*/
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use_default: movb _OPT(%bp),%al # Load default
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orb $NOUPDATE,_FLAGS(%bp) # Disable updates
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jmp check_selection # Join common code
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/*
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* Get the keystroke.
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* ENTER or CR confirm the current selection (same as a timeout).
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* Otherwise convert F1..F6 (or '1'..'6') to 0..5 and check if the
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* selection is valid.
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* The SIO code uses ascii chars, the console code uses scancodes.
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*/
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got_key:
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#ifndef SIO
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xorb %ah,%ah # BIOS: int 0x16, 00
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int $0x16 # get keypress
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movb %ah,%al # move scan code to %al
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cmpb $KEY_ENTER,%al
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#else
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movb $0x02,%ah # BIOS: Receive
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call bioscom
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cmpb $ASCII_CR,%al
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#endif
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je use_default # enter -> default
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/*
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* Check if the key is acceptable, and loop back if not.
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* The console (non-SIO) code looks at scancodes and accepts
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* both F1..F6 and 1..6 (the latter costs 6 bytes of code),
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* relying on the fact that F1..F6 have higher scancodes than 1..6
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* The SIO code only takes 1..6
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*/
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#ifdef SIO /* SIO mode, use ascii values */
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subb $'1',%al # Subtract '1' ascii code
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#else /* console mode -- use scancodes */
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subb $KEY_F1,%al /* Subtract F1 scan code */
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#if !defined(ONLY_F_KEYS)
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cmpb $0x5,%al # F1..F6
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jna 3f # Yes
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subb $(KEY_1 - KEY_F1),%al # Less #1 scan code
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3:
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#endif /* ONLY_F_KEYS */
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#endif /* SIO */
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check_selection:
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cmpb $0x5,%al # F1..F6 or 1..6 ?
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#ifdef PXE /* enable PXE/INT18 using F6 */
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jne 1f;
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int $0x18 # found F6, try INT18
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1:
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#endif /* PXE */
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jae beep # Not in F1..F5, beep
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/*
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* We have a selection. If it's a bad selection go back to complain.
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* The bits in MNUOPT were set when the options were printed.
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* Anything not printed is not an option.
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*/
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cbtw # Extend (%ah=0 used later)
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btw %ax,_MNUOPT(%bp) # Option enabled?
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jnc beep # No
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/*
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* Save the info in the original tables
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* for rewriting to the disk.
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*/
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movb %al,_OPT(%bp) # Save option
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/*
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* Make %si and %bx point to the fake partition at LBA 0 (CHS 0:0:1).
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* Because the correct address is already in %bp, just use it.
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* Set %dl with the drive number saved in byte 0.
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* If we have pressed F5 or 5, then this is a good, fake value
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* to present to the next stage boot code.
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*/
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movw %bp,%si # Partition for write
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movb (%si),%dl # Drive number, saved above
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movw %si,%bx # Partition for read
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cmpb $0x4,%al # F5/#5 pressed?
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pushf # Save results for later
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je 1f # Yes, F5
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/*
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* F1..F4 was pressed, so make %bx point to the currently
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* selected partition, and leave the drive number unchanged.
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*/
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shlb $0x4,%al # Point to
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addw $partbl,%ax # selected
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xchgw %bx,%ax # partition
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movb $0x80,(%bx) # Flag active
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/*
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* If not asked to do a write-back (flags 0x40) don't do one.
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* Around the call, save the partition pointer to %bx and
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* restore to %si which is where the next stage expects it.
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*/
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1: pushw %bx # Save
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testb $NOUPDATE,_FLAGS(%bp) # No updates?
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jnz 2f # skip update
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movw $start,%bx # Data to write
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movb $0x3,%ah # Write sector
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callw intx13 # to disk
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2: popw %si # Restore
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/*
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* If going to next drive, replace drive with selected one.
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* Remember to un-ascii it. Hey 0x80 is already set, cool!
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*/
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popf # Restore %al test results
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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:
|