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MKBOOT: A Floppy Disk Bootstrap Installer

MKBOOT.C is a utility which installs a custom boot loader onto a floppy disk, that loads and executes a named file. It is used to create non-dos disks which are bootable on a PC.
Command syntax: MKBOOT <drive:> [filename [offset [segment]]]

   <drive:>  - Floppy drive (A: or B:) to which boot loader is written
   filename  - Name of file to bootstrap
   offset    - Offset into segment where file loads (Default $0100)
   ;segment   - Memory segment into which file is loaded (Default $0050)
The booted file must be located in the ROOT directory of the floppy, and MUST be CONTIGUOUS on the disk (all sectors in sequential order). The best way to insure that a file is contiguous is to write it IMMEDIATLY after formatting the diskette. You can check a file to see if it is contiguous by specifying its name to the DOS CHKDSK command.

    eg:  CHKDSK A:BOOT.COM
Disk optimizing utilities such as "DEFRAG", "SPEED-DISK", "VOPT" or "ORG" can also be used to re-arrange your disk, and insure that the files are contiguous.
The default offset of $0100 is correct for .COM files. Note that any file (.COM or otherwise) which is booted from a disk MUST NOT contain any DOS calls (INT 21 etc.) since DOS is not loaded, and any calls to it will crash the system. Most high-level languages call DOS at least once in their startup code, making them unsuitable for this type of work. You will have either write your program in assembler, or use development tools intended for stand-alone applications (Such as our DDS MICRO-C Developer Kits).
The memory map at boot time looks like this:

   0:0000-03FF  = Interrupt Vectors
   0:0400-04FF  = BIOS Data area
   0:0500-7BFF  = Unused - small boot programs load here (segment 50)
   0:7C00-7DFF  = BOOT Loader code
   0:7E00-7FFF  = BOOT Loader work buffer
   0:8000-+     = Remainder of memory free (Segment 800 up)
   
Booted programs requiring less than 30464 bytes of memory can be loaded at the default segment $0050, below the BOOT loader code. Larger programs should be loaded at segment $0800, above the boot loader.
Since .COM files normally begin at offset $0100, MKBOOT uses this for its default load offset. This allows .COM program to be run from DOS as well as booted from disk. If you do not wish to do this, you can origin your program to $0000 at reclaim that 256 bytes.
You could also set the boot segment to $0040, and use the $0100 offset, thereby using ALL lower memory, and retaining the ability to run the .COM program from DOS. Be aware however, that doing so will place the BIOS data area within the address space of your program, where it can be altered (on purpose or not).
MKBOOT double-buffers the data as It reads, to insure that DMA will not occur across a 64K boundary (which doesn't work on a PC).
MKBOOT does not format the disk, it simply installs the custom boot loader, and patches it to contain the specified filename, segment and offset.
Examples:

    FORMAT A:                  ; Format a floppy drive

    COPY BOOTS.COM A:          ; Small DOS .COM file
    MKBOOT A: BOOTS.COM        ; Loads at 0050:0100

    COPY BOOTB.SYS A:          ; Big SYSTEM file
    MKBOOT A: BOOTB.SYS 800 0  ; Loads at 0800:0000
    
The WINDEMO.COM program included in this archive was created with the DDS 8086 Developers Kit, and does not call DOS except when it tries to terminate. You can make a "bootable" version of this program with:

    FORMAT A:
    COPY WINDEMO.COM A:
    MKBOOT A: WINDEMO.COM
    
This program will boot and run fine, however your computer may crash if you attempt to exit the program by pressing ESCAPE at the main menu.

See also:


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