The PC Booter term refers to games or software that used to start without relying on an already installed
operating system, usually directly from a floppy at power on.
This technique was popular during the 80's. For PCs without a hard drive, this was somewhat convenient
as it saved some steps: No need to boot dos first and the change the disk to start the game. On the
other hand, it also made the game a bit more difficult to copy backup (it was a form of copy protection)
and prevented hard drive installation...
Many games from this era are now available on abandonware sites, but most have been converted
to .COM and .EXE formats for convenience.
I acquired a Tandy 1000 EX with a single floppy drive and no hard drive. I thought it would be fun to relive
the experience of booting the games directly, as I used to do years ago.
Of course nothing would have been easier than creating a bootable DOS floppy (with format /s for instance)
with an autoexec.bat to start the .COM or .EXE version of the game. But that would not have been very interesting.
Instead, I chose the difficult (and very instructive) path of creating my own loader and tool to convert .COM and .EXE file to a floppy disk image.
How booterify works
The PC BIOS loads the first floppy sector (512 bytes) to a known memory location and executes it. This
small piece of code often loads a larger program to do the real work, but for booterify, 512 bytes
is more than enough to load and run the game.
The loader calls the BIOS interrupt 13h, function 2 in a loop to read floppy sectors to memory. Since
sector 0 holds the loader, the game program is divided in blocks of 512 bytes and simply stored from
Sector 1 to N according to its total size (maximum 64kB at the moment).
Some memory locations in the loader program are automatically patched by the floppy image creation tool:
Number of sectors to copy
Number of sectors per track (depends on the type of floppy [eg: For a 360k floppy, there are 9 sectors per track])
Program entry point offset (initial IP) for the program (ex: For a .COM, it is always at 100h)
Initial start pointer (SP) offset (For a .COM, points at the end of the segment)
Destination code segment (CS)
Data and stack segments (DS, SS) (All equal to CS for .COM executables)
.COM executables are easy. Only the first two parameters (size and sectors per track) will vary. The other
parameters make .EXE support possible, provided the load module size does not exceed 64Kb. The floppy
image creation tool will detect the file format (.COM or .EXE) and take care of complications specific
to .EXE files, such as segment relocation.
Beware: Be it a .EXE or a .COM, the game or utility will work fine as long as it does not
make calls to DOS services. The image creation utility will try to detect byte sequences that
look like DOS interrupt calls (CD xx sequences) and will emit a warning and a hex dump around the call.
Disk creation will not be aborted however. Said byte sequences may appear anywhere, not only in code (eg: In data).
It is up to you do disassemble your executable or the hex dump to investigate. You can of course just try booting
the thing too. As a rule of thumb, if the game has data files, it certainly will not work. If the
file is a single executable, maybe it will work. (And if the game was listed somewhere as a PC
booter, it is a good sign.)
 The loader remains in memory and implements in fact a few DOS services: INT 21,AH=09h, INT 21h,AH=25h and INT 21h,AH=35h. The
other int 21h services are not implemented and control is immediately returned to the caller via RETI.
Creating a disk image
Here is help:
$ ./booterify -h
Usage: ./booterify [options] bootsector.bin executable output.dsk
- bootsector.bin is the bootloader,
- executable is the input file (.exe or .com),
- output.dsk is the output file.
-h Prints help
-c Force input file format as .com (default: auto-detect)
-e Force input file format as .exe (default: auto-detect)
-s Sectors per track
-t Target disk size (padded with zeros). If 0, no padding.
-B file Generate list of breakpoints commands for bochs for all
instances of DOS int calls
Common disk parameters:
360k floppy: -s 9 -t 368640
1.44MB floppy: -s 18 -t 1474560
Note: Padding with -t as above is not required when writing to
a physical floppy, but it can help for emulators or virtual
machines that use the file size to determine the floppy type.
Here is a use example creating a bootable 360kB disk for Alley Cat:
If you only own one drive able to write the target floppy format and it happens
to be the only drive in the host machine which do not have a hard drive, instead
of temporarily moving the drive to another computer, consider using PPUSBComm.
With PPUSBComm a disk image can be sent using USB and the target machine's parallel port and
the image is written directly to floppy sectors so no temporary storage (hard drive or extra floppy drive) is required.
Since version 1.6, booterify includes a loader (pcjrloader.asm) for loading from a PCjr ROM cartridge instead of
a floppy. A companion tool called jrromchk is also introduced to compute the CRC, set the size in the ROM header
and convert between raw binary and .JRC rom formats.
Have a look to my PCjr cartridge project for
a good example on how to use this to make an Alley Cat cartridge.
Alley Cat on a PCjr cartridge!
Here is the source code. To compile the loader (bootsector.bin) you will need nasm. And to compile
the disk image creation tool, the usual Linux development tools are required (gcc, make, etc.).
For the moment, I only distribute the project in source form. But if there is some interest,
I'm willing to prepare a Windows build. (But it will still be a command-line tool)
New, June 2018: For a precompiled Windows version working in command-line, download the .zip file below.
I cannot be held responsible for any damages that could occur to you
or your equipment while following the procedures present on this page.
Also, I GIVE ABSOLUTELY NO WARRANTY on the correctness and usability
of the informations on this page. Please note, however, that the procedures
above have worked in my case without any damages or problems.