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140 lines
6.2 KiB
Plaintext
140 lines
6.2 KiB
Plaintext
The "BootProg" Boot Sector
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What is BootProg?
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BootProg is a collection of 512-byte boot sectors (for the x86 PC) capable of
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loading and executing a program from a FAT12-formatted floppy or a FAT16/32-
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formatted hard disk (bootable USB sticks and CDs can also be made with
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BootProg).
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BootProg understands programs in the MS-DOS .COM or .EXE format. This makes
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it possible to use existing 16-bit compilers such as Borland/Turbo C/C++,
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Sybase/Open Watcom C/C++ and Smaller C and a variety of assemblers such as
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NASM, FASM, TASM and MASM among the others.
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BootProg doesn't require that the program occupy a contiguous span of sectors
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or FAT clusters or reside at a specific fixed location on the disk. BootProg
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faithfully parses the root directory and the chain of FAT clusters in order to
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locate the program contents. The only requirement is that the program be named
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"STARTUP.BIN" (without quotes). This makes updating the program easy. You just
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need to update the file and you can reboot and execute it immediately.
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What can BootProg be used for?
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You can make a boot loader for your OS. The program that BootProg loads can be
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your 2nd stage boot loader. Or, if your OS is relatively small, STARTUP.BIN
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could contain the entire OS.
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You can write low-level utilities to work with your PC's hardware and load them
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with BootProg without having to jump through the hoops with your Windows, Linux
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or even DOS.
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You can make cool graphics demos or games that run on bare hardware.
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What can't BootProg be used for?
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Many things. Most importantly, if you make a DOS program that uses any MS-DOS
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service functions (e.g. int 21h) or data structures, it will not work when
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loaded by BootProg. It must use either BIOS services (e.g. int 10h, int 16h,
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int 13h and such) or access hardware directly or both.
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However, it is possible to create universal/hybrid programs that would work
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both in DOS and when loaded by BootProg. BootProg will set registers si, di and
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bp to the values 16381, 32749 and 65521 respectively before transferring control
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to your program. Your program can then check the values in these registers and
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use DOS services in DOS or something else instead on bare hardware. You can also
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choose to make the program run with reduced functionality if not on DOS or
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vice versa.
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How does it work?
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Nothing special. It just finds STARTUP.BIN, loads it, performs any relocations
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necessary for the .EXE type of programs, sets the magic numbers 16381, 32749
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and 65521 in registers si, di and bp respectively and passes control to your
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program.
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If BootProg can't find STARTUP.BIN, it will print "NF" to the screen. If it
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fails to load the file due to a read error, it will print "RE". This is how the
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FAT12 and FAT16 versions of BootProg work. The FAT32 version has much less space
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for these errors and so in both above cases it will simply print "E".
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How do I put BootProg on my disk?
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If you have a 1.44MB 3"5 floppy, just format it regularly with FAT12 in DOS or
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Windows and then write flp144.bin to the very first sector of the floppy with
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whatever tools you find/have for that. After that you can copy STARTUP.BIN to
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the floppy and off you go.
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If you want to create an image of a 1.44MB 3"5 floppy, it might be even easier.
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Compile the mkimg144.c program contained here with your favorite C compiler
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and use it:
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mkimg144 [option(s)] [file(s)]
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Options:
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-bs <file> Specifies the boot sector to use, e.g. "-bs flp144.bin"
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-o <file> Specifies the name of the output file ("floppy.img" is the
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default, if this option isn't specified)
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-us Uses the current time to set the volume ID of the FAT to a unique
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value (the volume ID is used to distinguish between different
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removable disks and detect disk change more accurately)
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E.g: "mkimg144 -bs flp144.bin -o flp144.img -us startup.bin".
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Btw, you can rename the supplied file "demo1.com" to "startup.bin" to try it
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out.
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For all other cases you'll need to become a little more familiar with FAT and
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a little more intimate with disk tools and BootProg's source code.
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You will need to populate the BPB's of boot16.asm and boot32.asm with the
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values appropriate to the type and size of the file system that you already have
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on a disk or that you intend to create on the disk.
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See the source code, these places are marked with question marks, for example:
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bpbBytesPerSector DW ? ; 0x0B
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The best is to format your disk with some standard tools (e.g. FORMAT.COM in
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DOS), extract the BPB values from the FAT-formatted disk, put them into BootProg
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and then write thusly adjusted BootProg over the original boot sector.
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You may find a disk editor handy when manipulating BPB values and/or
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reading/writing boot sectors.
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Limitations and implementation details
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boot12.asm (flp144.asm) and boot16.asm require an i80186/i80188/i80286 or a
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better CPU. boot32.asm naturally requires an i80386 or a better CPU.
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boot12.asm (flp144.asm) was not tested on hard disks (but it might work as the
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boot sector on FAT12 primary partitions (file system ID 1)).
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boot16.asm was written for and tested on primary FAT16 partitions (file system
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IDs 4 and 6). Its expected use is the boot sector of the partition and not the
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MBR. The FAT16 version may allocate up to 128KB of RAM for the entire FAT16,
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leaving less room for STARTUP.BIN. But ~400KB left should still be plenty of
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space for its code, data and stack.
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boot32.asm was written for and tested on primary FAT32 partitions (file system
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IDs 0Bh and 0Ch) and for BIOSes supporting function 42h of int 13h (IOW, for
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systems supporting HDDs larger than 8GB). Its expected use is the boot sector
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of the partition and not the MBR.
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BootProg does not check the size of STARTUP.BIN and reads into memory all of its
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clusters, which means that up to 32767 extra bytes may be read from the disk
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and written to the memory after the last byte of STARTUP.BIN (max cluster size
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is 32KB). It also means that you may append data to your program and it will be
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loaded. You may create oversized .COM-style STARTUP.BIN larger than ~64KB,
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however, note that the stack will naturally overwrite its contents from offset
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65535 of the program segment (offset 65279 of the file) downwards.
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If your PC has the full 640KB of conventional/DOS memory, you should be able to
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load program files of size of up to ~400KB.
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