If you are on Linux you can do it with DD utility. There is a version of DD for Microsoft Windows as well.
General DD usage
If you wish to make a zero filled virtual disk image the size of a 720K floppy you can use dd like this:
dd if=/dev/zero of=disk.img bs=1024 count=720
This would create a file called disk.img that is 1024*720 = 737280 bytes in size. A 1.44MB floppy image that is zero filled can be created with:
dd if=/dev/zero of=disk.img bs=1024 count=1440
Writing a binary image to a virtual floppy starting at the beginning of the image can be done like this:
dd if=bootload.bin of=disk.img conv=notrunc
This example take the file bootload.bin and places it at the beginning of the disk image (called disk.img in this case) without truncation (conv=notrunc) If you don’t use conv=notrunc on a virtual disk image it will write bootload.bin and truncate disk image to the size of the bootloader.
DD also has the ability to write to specific parts of a a disk image by jumping to a point other than the beginning of the disk. This is useful if you need to place information (code/data) in a particular sector. This example could be used to place the second stage of a boot loader after the first 512 byte sector of the disk image:
dd if=stage2.bin of=disk.img bs=512 seek=1 conv=notrunc
bs=512 sets the block size to 512 (makes it easier since it is the typical size of most floppy disk sector). seek=1 seeks to the first block (512 bytes) past the beginning of the image and then writes the file stage2.bin . We need conv=notrunc again because we don’t want DD to truncate the disk image at the point where stage2.bin ends.
dd if=stage2.bin of=disk.img bs=512 seek=18 conv=notrunc
This example is similar to the last but it skips over 9216 bytes (512*18) before writing stage2.bin
If you have a floppy attached to a Linux system (and root access) you can write the bootloader with something like
dd if=bootload.bin of=/dev/fd0
where /dev/fd0 is the device for your floppy. /dev/fd0 is generally floppy disk A (if present) and /dev/fd1 is floppy disk B (if present).
DD for Windows
If you are running on Microsoft Windows there is a version of the DD utility available here . The latest download is dd-0.6beta3.zip and is the minimum recommended version. It has some features older ones didn’t. Just open the zip file and extract it to a place on your Windows path.