Get a hex editor program.... Such as hexedit32.

And yes, it's probably not going to work. You can't just throw stuff in a dll and expect it to work. You can change things (keeping in the constraints of the unchanged file) but adding is mostly not going to work.

Compiled code can not be edited. While you can change the file using a hex editor, it won't allow much, as you can not add or remove stuff, only overwrite bytes that are already there. If you add or remove even a single byte, the code will break and crash. You need to decompile it, make the changes, and compile a new DLL. That's actually not that hard, but probably undoable if you have no experience with coding.

I feel the advice above is ... basically true, but an over-simplification. Here's the long version:

First, the .DLL file is going to be some mixture of any or all of the following: Compiled code (ergo, machine code - presumably opcodes that the CPU will execute verbatime); Headers and other metadata that tell Windows how to load and use the .DLL; Resources like images, data blobs, icons, sounds, etc.; Strings of text -- either as inline data that is used in the code, or as tables of data that is referenced by the code.

The advice to the effect that "you can't modify a DLL" is true in that the file will have bits and pieces that are expected to be in a given place. If you move them (by inserting or deleting any bytes before that block of code), then pointers to that code will be "off" and end up pointing to whatever happens to be at that location in the file. Windows may still try to execute those incorrect regions as code, and it is very unlikely to end well. This is what people mean when they say you can edit stuff or change it, but not add or delete anything. In a visual sense, you have to remove a blue marble and replace it with a red marble. If you take a marble out, or put one in, then you've broken everything.

That still gives you some leeway, though. Often it's used to change text or other resources, but you can actually change code. It's not trivial, since you have to essentially look at the code as CPU op-codes and figure out what it's doing and how to modify it to do what you want. You can even replace whole sections, so long as any entrances and exits to/from that code still work afterward. Not for a beginner, for sure.

But, what you CAN do, even without being an x86 ASM guru, is replace values. Often these are single bytes, or 16- or 32-bit words, that contain either a number or a series of flag bits that will be stored in CPU registers, and/or passed to other functions. Sometimes that's all you need to do, and so long as the number or flag you need to express is the same size (# of bytes) as the one you need to replace, you're good to go. If the comparison you looked at between those two files was a couple bytes, you may be in luck. And, as long as you have a backup of the original files, there's no harm in trying. Worst that will happen is it locks up the computer and you have to reboot and undo your change.

A hex editor is the tool for the job. There are many. I used XVI32 in the past, but you have your pick of dozens. It simply lists the file as a linear block of bytes, typically with the left half shown as hexidecimal values between 00 and FF, and the right half as a textual representation of the same data. The right half just helps you see patterns, namely readable text which is nearly inscrutable when displayed as ASCII numeric values on the left. The editor will let you type over any value with another, without changing the total number of bytes in the file, and thus without breaking the expected offsets of all the code regions and other resources.

Thanks to all, and notably SirNickity for all the details! I do, in fact, just need to replace some values, not add or subtract data, so this at least gives me enough incentive to give it a shot! I’ll take a whack at it sometime soon and see what comes of it. 😀

SirNickity wrote on 2020-02-25, 19:09:

A hex editor is the tool for the job. There are many. I used XVI32 in the past, but you have your pick of dozens.

I used that, too. It has an unusual feature - an option to fix the 200MHz speed bug in Turbo Pascal 7 (?) programs, as far as I remember.
However, it doesn't work so well at splitting/merging binary files into odd/even parts.
A feature that is sometimes required in order to use dumps to physically re-create a device's firmware chips.
WinHex is better suited for this, I guess. 😀

Oh yeah, I think I remember seeing that TP fix option. Sign of the times in which it was written, I guess! Otherwise it is a little bare-bones, but lightweight and IIRC doesn't even need an installer, which is nice.

Most of the time now I use Okteta for KDE, or Norton Disk Edit in DOS. I don't have quite as much use for hex editors in Windows these days. 😀

I have used the win32 software ResEdit before for simple changes to dll files. It helped to sort blocks of data and made it easy to export, edit, then replace stuff.

Ultra Edit? One of my favorites editors.

Any hex editor will do the work, but for more complex stuff than just cahnge some bites, , x32dbg is great. It works really good for debug and trace