PCjs Machines

Home of the original IBM PC emulator for browsers.


PCjs FileImage Utility

FileImage.js is a Node command-line application that reads/writes PCjs file images, producing JSON-encoded versions of binary files (eg, ROM images). It supersedes the older PCjs FileDump utility.

PCjs ROM Image Files

Some early (“v0”) examples of PCjs ROM image files include the Ohio Scientific System ROM, which were little more than text files with 2-digit hex byte values separated by whitespace.

However, ROM image files were soon changed to a JSON-like format (“v1”). Examples include the updated Ohio Scientific System ROM and PCx86 ROMs like the IBM PC ROM BIOS.

Strictly speaking, those files weren’t – and still aren’t – JSON-compliant, which is why they now use .json5 file extensions. PCjs parses them using the JavaScript eval() function inside a try/catch block, since JSON.parse() complains about hexadecimal constants, comments, and other perfectly acceptable JavaScript constructs. The JSON specification was unnecessarily strict, so it’s good to see the world finally embracing JSON5.

PCjs v2 ROM image files will be more formalized. At a minimum, they should contain a width value (eg, 8, 16, 32) and a values array. Other optional properties include endianness (littleEndian defaults to true), load address (addr), the origin of the ROM (file name and/or source URL), and so on. Here’s an example from the TI-57 emulation:

  "addr": 0,
  "size": 2048,
  "width": 13,
  "littleEndian": false,
  "file": "ti57be.bin",
  "source": "http://seanriddle.com/ti57.bin",
  "values": [

In the above example, the ROM definition is actually included inside the ti57.json machine configuration, rather than as a separate file. The approach will vary with the machine, but the principles are the same. ROM image files should be JSON-compliant by default, unless you use non-default options, such as octal constants, comments, etc.

To Be Continued…

More information about using FileImage and the “v2” ROM image file format will be added here as the utility and the file format specification evolves. In the meantime, check out the ‘Usage” comments for the main() function in FileImage.js.