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BOOT2C.COM & BOOTPW.COM (VERSION 1.0) Copyright (c) 1993 Douglas Boling ------------------------------------------------------------------------- First Published in PC Magazine March 30, 1993 (Utilities) ------------------------------------------------------------------------- BOOT2C & BOOTPW: BOOT2C lets you force a boot from your hard disk even if you have left a floppy in a disk drive. BOOT2C does this not only for DOS-based PCs, but for machines that run under OS/2 as well. BOOTPW lets you password-protect a floppy or hard disk. Once protected, your disks won't be recognized as valid disks by either DOS or OS/2. How often do you forget that you've left a floppy disk in drive A: when you boot up your PC? Don't you wish that instead of giving you an annoying ``Non-System disk or disk error'' message and making you unlock drive A:, your machine would simply go to drive C: and boot up from there directly? BOOT2C, eliminates the forgotten-disk syndrome. By installing a tiny bit of code in the boot sector of the drive A: floppy disk, BOOT2C automatically forces the machine to load the operating system files from the hard disk. BOOT2C does this not only for DOS-based PCs, but for machines that run under OS/2 as well. Indeed, with a BOOT2C-modified disk in drive A:, your machine will actually boot faster. That's because your PC won't receive and process an error when it attempts to read the A: drive. Normally, a PC retries its initial floppy disk read up to five times before giving up and booting from the hard drive. If you're running under OS/2, BOOT2C can actually redirect its boot-up instruction to another physical disk--drive D:, for example. Unfortunately, current releases of DOS don't support this feature, but in a future DOS version you may not have to be limited to booting from drive A: or C:. The second utility, BOOTPW, a program that will password protect either a floppy or a hard disk. Once protected, the disk won't be recognized as a valid disk by either DOS or OS/2. You won't subsequently need the BOOTPW program to unprotect the disk. Just start the machine with the password-protected floppy disk in the A: drive, and the BOOTPW code will ask for the proper password. Note that once the password is entered, the protection is removed; to reprotect the disk BOOTPW must be reinstalled. USING BOOT2C BOOT2C is a simple program with a simple interface. Its full syntax is BOOT2C d: [n] where d: is the disk drive on which the redirection code is to be installed and the optional n parameter (to be explained below) is the number of the disk that holds the boot-up files (OS/2 only). For the normal case, assuming that the BOOT2C.COM program is on your path, you can install the necessary code on the floppy disk in the A: drive by entering BOOT2C A: You can then leave the floppy disk in drive A:, and you'll never have to see that irritating ``Non-System disk'' error message again. The redirection code written by BOOT2C is not a file you'll find with the DIR command; it resides in the boot sector of the disk. You can write the code to a floppy disk located in drive B: by entering BOOT2C B: Obviously, since a PC can't boot from the B: drive, a modified disk in the B: drive won't force a hard disk boot. Such a command is useful, however, if you want to prepare the disk in your drive B: for use in another machine's drive A:. It might be, for example, that you need to put BOOT2C on a 3.5-inch floppy disk and your 3.5-inch drive is drive B:. By using the n parameter, BOOT2C can also modify a floppy disk so it will boot the second hard disk of the system. By the ``second hard disk'' here I mean a second physical hard disk, not a secondary partition on the first hard disk. Again, this will not work with current DOS, but OS/2 users may wish to try it. To modify a floppy disk in the A: drive to boot the second hard disk, you use the command BOOT2C A: 81 In the above command, 81 represents the physical disk number the BIOS uses to address the disk. According to the BIOS numbering scheme, 0 is the first floppy disk, 1 is the second floppy disk, and so on. Hard disks are numbered starting with 80 hex, so the first hard disk is 80, the second is 81. Thus, to force a floppy disk in the A: drive to boot the disk in the B: drive on systems where the B: drive is bootable, the command is BOOT2C A: 1 BOOT2C must use the BIOS number instead of the drive letter because at the point in the boot-up sequence at which the BOOT2C boot program is executed, the operating system isn't available to tell BOOT2C which physical drives it calls the A:, B:, and C: drives! The drive letters with which we are familiar don't exist at the BIOS level when a PC is booted. Once the new BOOT2C boot record has been installed, the floppy disk will act like any other. Files can be read and written to the disk and programs executed from the disk. The only difference occurs when you boot the system with a BOOT2C-modified floppy disk in the A: drive. In that case the floppy disk displays a message indicating that BOOT2C is present and then continues by booting the files on the hard disk. Removing BOOT2C from a disk requires executing either the DOS SYS or FORMAT commands on the disk. Although this may seem to be a drastic step to take just to remove a program, there really isn't a reason for removing BOOT2C unless you want to make the disk bootable. The SYS command puts the DOS system files onto the disk and replaces the boot record--including any BOOT2C code. FORMAT erases all information on the disk and writes a new DOS boot record. USING BOOTPW Using BOOTPW is just as simple as BOOT2C. Just type BOOTPW along with the drive letter of the disk to protect. For example, to protect a floppy disk in the A: drive, you enter the command BOOTPW A: Once started, BOOTPW will ask you for a password. The password, which can be up to eight characters long, is not displayed on the screen as you enter it. BOOTPW then asks for the password again to ensure that you entered the password correctly. The disk should then be removed from the drive. If you now place the floppy disk in the drive again and try a DIR (or any other disk-related) command, DOS will respond with a ``General failure'' message. The password protection can be removed in either of two ways. First, if you run BOOTPW on a drive that is already password-protected, BOOTPW will ask you to enter in the password for the disk. If you enter the proper password, BOOTPW will unprotect the disk, restoring it to its original state. Since you might not always have BOOTPW available, a floppy disk can be unprotected by a second method. Simply place the disk in the A: drive and reboot your machine. Instead of booting the operating system, the disk will ask you for the password for the disk. If you enter it correctly, the password protection will be removed and the disk will continue to boot as if it had never been protected. That's to say, if the disk is not a bootable disk you'll get the standard Non-System disk error message. When this happens, all you need to do is remove the floppy disk and reboot your system. How much actual protection does BOOTPW provide? As with all password schemes, BOOTPW can be broken. Its protection is at the operating system level; the data on the disk is not modified in any way, and an intrepid programmer can remove the password with a few hours' work. So, using BOOTPW won't absolutely prevent people from reading your data, but it will deter them--at least for a while. REVEILLE When a PC first wakes up or is reset, it goes through a built-in series of Power-On Self Tests (POST) designed to initialize the system components and ensure memory integrity. Only after the machine has been tested and initialized does it look to the outside world for a program to run. Normally, this first program is an operating system, such as DOS or OS/2. BOOTPW and BOOT2C are exceptions to the general rule, for they take control of the machine before DOS or OS/2. To load its first program, the PC reads the first sector of the first floppy disk in the system and loads the information there at segment 0, offset 7C00 hex. If the data in that sector contains the word value AA55 hex (known as the signature word) at offset 510 of the block, the sector is assumed to contain valid code and the BIOS jumps to the first byte of the block (address 0:7C00). At this point, all the microprocessor registers are undefined with the exceptions of the following: the code segment register (CS), which contains 0; the instruction pointer (IP), which contains 7C00 hex; and DL, which contains the number of the drive where the boot sector was found. If the first floppy disk sector does not have the correct signature word--because there isn't a floppy disk in the drive or there isn't a drive at all--the BIOS attempts to load the first sector of the first hard disk in the system. The process is the same as when reading the floppy disk: The first sector is placed at address 0:7C00 and the signature word is checked. If a valid signature is not found, the PC assumes that there isn't an external program to load. In this case, an IBM PC will start its ROM BASIC program. Machines that lack ROM BASIC simply wait for the user to place a proper floppy disk in the system. The DOS boot sector has two components that are vital to DOS and OS/2 FAT drives. The first component is the BIOS Parameter Block (BPB), which is located immediately following a Jump opcode and the OEM identification field. The figure below shows the layout of the first 60 bytes of a DOS boot record. The DOS Boot Record Offset Name Size Description --------------------------------------------------------------------------- 0 jump opcode 3 bytes Long jump or short jump followed by a NOP 3 OEM field 8 bytes Identifies the DOS that formatted the disk **11 Bytes per sector Word Sector size in bytes **13 Sectors per Byte Number of sectors per allocation unit cluster **14 Reserved Sectors Word Number of sectors reserved after the boot record **16 Number of FATs Byte Number of file allocation tables on-disk **17 Root entries Word Number of entries in the root directory **19 Total sectors Word Total sectors on disk 0 if number over 65,535 **21 Media descriptor Byte Indicates the type of media **22 Sectors per FAT Word Size in sectors of each FAT **24 Sectors per track Word Number of sectors on each track on the disk **26 Number of heads Word Number of heads on-disk **28 *Hidden Sectors Word Number of hidden sectors on-disk ***32 Huge Sectors DWord Total sectors if over 65,535 ***34 Drive number Byte BIOS drive number ***35 Byte Reserved ***36 Boot signature Byte Must be 29 hex for extended boot record ***37 Volume ID DWord Volume ID for disk ***41 Volume label 11 bytes Copy of volume label ***52 File system label 8 bytes Identifies file system type (FAT12 or FAT16) * In DOS 3.3 this field was extended to a double word. ** BIOS Parameter Block *** Added in DOS 4 Figure: This table shows the layout of the data at the start of the first 60 bytes of a DOS boot record. ----------------------------------------------------------------------------- The second important part of a DOS boot record is the boot program itself, which finds and loads the DOS system files. The boot program is a simple routine that uses the data in the BPB to locate the root directory on the disk. The directory is then read into memory and its entries are checked for the two DOS system programs: IO.SYS and MSDOS.SYS (for MS-DOS), or IBMBIO.COM and IBMDOS.COM (for PC-DOS). If these files are not in their proper place on the disk, the boot sector code prints its infamous ``Non-System disk or disk error'' message. If these files are found, the boot routine reads the first few sectors of IO.SYS (or IBMBIO.COM) into memory and then jumps to the newly loaded code. From that point on, the machine is controlled by the DOS system programs. By using a slightly different boot program and loading the file OS2LDR instead, the machine will run OS/2 instead of DOS. In the case of the programs BOOT2C and BOOTPW, the boot record takes a different path still. HOW BOOT2C AND BOOTPW WORK Both BOOT2C and BOOTPW work by modifying the boot record on the target floppy disk. Although both BOOT2C and BOOTPW contain code necessary for installation (and removal, in the case of BOOTPW), the replacement boot records of these programs are of primary interest. When BOOT2C is run, it replaces the original boot record on the floppy disk with the BOOT2C boot record. Before the BOOT2C boot record is written to the disk, the fields in the BIOS Parameter Block are filled in with the information from the original boot record. The boot sector code is quite simple. The first instruction is the required JMP opcode. This is followed by an empty area that is filled in with the BIOS Parameter Block from the original boot record during the installation. After the BPB comes the byte that holds the drive number toward which BOOT2C will redirect the boot. The boot code that follows is relatively short and can be described straightforwardly. First, the stack segment (SS) and stack pointer (SP) must be initialized. Since the POST procedure initializes only the CS, IP, and DL registers, all the other registers must be initialized before they are used. Once the registers are initialized, the boot code must be relocated, for the boot record that BOOT2C is to load must occupy the place that BOOT2C has taken over. BOOT2C therefore moves its own code up by 512 bytes, to address 0:7A00 hex, and then uses a far return to jump to the new location. Once relocated, BOOT2C prints a short message to indicate where it is and then proceeds to read the boot sector on the redirection disk. Since DOS has yet to be loaded, however, BOOT2C must use the BIOS disk services. In order to read a sector via the BIOS, a program must encode the cylinder, head, and sector information in the CX and DX registers. ES:BX points to the buffer for the data and AL is loaded with the number of sectors to be read. Finally, AH is loaded with a 2 to indicate a disk read, and an Interrupt 13h is performed. Specifically to read the boot sector of the disk, the registers are loaded so that BX contains 7C00, CX is equal to 1, DH is 0, and register DL contains the disk number. Following the procedure of the BIOS routine, BOOT2C checks to see whether the sector read has the proper AA55 boot flag located at offset 510. If not, or if an error occurred during the read, BOOT2C displays an error message and halts the machine. If the boot record is valid, BOOT2C loads DL with the new boot drive number and jumps to the new boot code. The boot code for BOOTPW performs a very similar litany, except extra code is added to query the user for the password. The tasks of the BOOTPW boot record are to ensure that the user enters the proper password, then to find and read the original boot record, write the original boot record back to its proper place, and finally to reboot the machine. Once the original boot record code has been rewritten to the boot sector, the reboot will cause the machine to continue the boot process as if BOOTPW had never existed. Of course, if there are no system files on the floppy disk, the reboot of the original boot record will display the ``Non-System disk'' message. The protection provided to the disk is actually quite simple. The BOOTPW boot record does not conform to the DOS requirement for a BIOS Parameter Block located at the start of the disk. Thus, when DOS attempts to read the protected disk, it does not see a proper BPB and consequently reports a General Error back to the user. Until the correct boot sector is replaced, DOS is not able to access the disk. BOOTPW can also unprotect a disk without requiring a machine reboot. To do this the BOOTPW program reads and writes the protected disk with DOS interrupt 25 (Absolute Sector Read) and DOS interrupt 26 (Absolute Sector Write). These functions can read the boot record of a protected disk even with a BOOTPW boot record installed. Both BOOTPW and BOOT2C show what can be done with no more than the limited 512 bytes of a boot record. By modifying the boot record structure, BOOTPW confuses DOS so that the disk becomes inaccessible. On the other hand, BOOT2C's almost trivial code frees us from the frustration of that silly ``Non-System or disk error'' message. If you use a lot of floppy disks, you'll rejoice that the full use of your A: drive has been returned to you. ------------------------------------------------------------------------ DOUGLAS BOLING IS A CONTRIBUTING EDITOR TO PC MAGAZINE.