.COM to ROM

The usual way to run a .COM file is to run it from CP/M. However, there are .COM files (such as games) that can be run from CP/M, but don't actually need any of the CP/M functionality. These can be made into ROM images using this program.

The program works by making a ROM file which is a loader, followed by the .COM file data. When the ROM loader is invoked, it copies itself to high memory, and then copies the rest of the ROM content into RAM, and jumps to it.

The loader is small, and for small programs, the loader and the .COM file data may fit into 8KB. Larger programs may require > 8KB of ROM, and this is fine also, providing you have a ROM card that supports subpages. Just place successive 8KB chunks in successive subpages. The loader will fetch data from each subpage in turn.

There is also a dual-ROM variant which distributes 8KB pages of data over two ROM slots. Most of the original internal MTX ROM cards had space for 4 8KB EPROMs. So using the dual-ROM loader can allow you to support .COM files beyond 32KB (minus the loader overhead), upto the largest possible in CP/M. First the loader loads from ROM slot N subpage 0, then ROM slot N+1 subpage 0, then ROM slot N subpage 1, then ROM slot N+1, subpage 1, and so on. Care is needed to place each 8KB from the output file into the right 8KB EPROM on the right ROM card.

There are loader variants that have the 8,7,6,5,4,3,2,1 autostart signature. These have names ending with 'A'. If these are used, when the MTX is reset, it will automatically run the ROM without the user having to type ROM x.

Examples

Imagine we want to turn Quasar (5KB) into a single 8KB EPROM, which we'll put in ROM slot 7, and we want it to autostart when the MTX is reset :-

$ ./com2rom LOADER1A.BIN QUASAR.COM QUASAR1A.ROM
$ memu -rom7 QUASAR1A.ROM -s -v

Or, to spread Mission Alphatron (10KB) over two subpages of ROM slot 2, and because this needs the larger ROM card usually installed inside the MTX, we can't quickly remove it, so we won't have it autostart :-

$ ./com2rom LOADER1.BIN ALPHA.COM ALPHA1.ROM
$ memu -n-subpages 2 2 -rom2 ALPHA1.ROM -s -v
ROM 2

Or, to spread Scrolling Maze Game (36KB) over the first few subpages of ROM slots 2 and 3 :-

$ ./com2rom LOADER2.BIN SMG.COM SMG2.ROM
$ memu -n-subpages 2 4 -n-subpages 3 4 -rompair2 SMG2.ROM -s -v
ROM 2

Some of these examples assume the use of a recent MEMU that supports

• User defined number of subpages on all ROM slots
• using -romX option to upload to multiple subpages
• the new -rompair2 option to upload to multiple subpages over two ROM slots

Download

COM2ROM can be downloaded from http://www.nyangau.org/com2rom/com2rom.zip.

Copying of this program is encouraged, as it is fully public domain. Even the source code is included in the package. It was created on the authors time and equipment. Caveat Emptor.

The author of COM2ROM and this documentation is Andy Key (email andy.z.key@googlemail.com).

{{{ Andy