![]() ![]() Using the web disassembler to examine the first few bytes of the file, we’ll get output like this: As with the hex editor, there are many options for 68K disassemblers, including this slick web-based disassembler. In this case, we know that the bytes are 68000 machine code, so we can feed them through a 68000 disassembler to create a more human-readable version. To do anything useful with this, we’ll need to understand how these bytes are structured and what they mean, so that we’ll have some idea how to modify them. In the example above, the value at offset 32 is 6E, which is equivalent to the ASCII character ‘n’. That’s not especially useful in this example, but it’s handy when examining ROM code that contains embedded string constants. In the right panel we see those same 16 bytes again, but displayed as printable characters instead of as hex values. In the center panel, we see the actual bytes from the file, displayed in hex, 16 bytes per line. ![]() These are the same as the addresses within the ROM code. What’s all this? Running down the left side are the file offsets in hex. If we open the ROM-inator II file in xvi32, we’ll see this: Check out Synalyze It and Hex Fiend for some other examples. I’ll be using a Windows hex editor called xvi32, but there are many other options for Windows, Mac, and Linux. To modify or patch the file, we’ll need a tool called a hex editor. Only the first 512K of the file is ROM code, and the rest is data for the ROM disk. The latest ROM-inator II contents are available on the product’s web page, and at the time of writing it’s this file. The ROM-inator II’s contents are also based on the IIsi ROM, with many modifications and additions. The original IIsi ROM is just a 512K chunk of raw data, which you can find here. It’s a universal ROM, meaning that even though it was designed for the IIsi, it also works in many other Mac models. This is an advanced tutorial for major gear-heads, so hang on to your hat! But why stop there? For the truly adventurous, this tutorial will demonstrate how to patch the ROM code to alter the machine’s low-level behaviors. With the optional ROM SIMM Programmer, you can edit the ROM’s contents, altering the ROM disk or writing a different stock ROM image. The ROM-inator II replacement flash ROM for the Mac II series and SE/30 comes pre-programmed with nifty new features for your vintage Macintosh. Gregg Lemke on Yellowstone Universal Disk Controller for Apple II.Hightray on Floppy Emu Disk Emulator for Apple II, Macintosh, and Lisa.Steve on Floppy Emu Disk Emulator for Apple II, Macintosh, and Lisa.John Payson on NMOS 6502 Phantom Reads, Odd Yellowstone Bugs.Jeffg on The Amazing Disk II Controller Card.Yellowstone Universal Disk Controller for Apple II Floppy Emu Disk Emulator for vintage Apple II, Macintosh, and Lisa
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