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According to the AC3 forums, windowed mode is currently not supported. However, often there are hacks/workarounds available to get windowed mode working anyway.

So is there some way to play AC3 in windowed mode? I can't believe that a game released in 2012 still doesn't support this most basic and pretty important thing for everyone who just has a single monitor...

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vm it use the VM window as the window – ratchet freak Nov 22 '12 at 15:32
No thanks. That's overkill and is likely to be pretty slow (I always play games with max details/quality). – ThiefMaster Nov 22 '12 at 17:05

2 Answers 2

First of all, select any in-game resolution (eg. 800x600). Then exit the game. Now delete the file datapcdx11.forge. It will be approximately 490 MB. Start the game again.

Nothing will happen, but a process ac3sp.exe will open in the task manager. Go to the Recycle Bin, and restore the file you deleted earlier. The game will launch automatically in Windowed mode. Works for me, hope it works for you, too.

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You don't have to delete the file. You can rename it to something. As soon as the file is renamed to something else (just add a single character at the end of the name of change the file extension by adding a character) then launch the game. The launch window appears and goes away but the game does not launch. It will still be in the process list as user42336 answered. Then rename the file back to original and it will launch just fine. This does put it in windowed mode. BUT it doesn't really funciton as windowed mode well. You can ALT+TAB out of it and have mouse just fine without a fullscreen blink and all that. But if you go back into it, well, it reverts to fullscreen. There's no actual WINDOWED mode support for this game. There could be some others ways. This was the first google result and it was answered well just not fully nor with enough credit.

One of the things I do is write a batch file with something like this:

@echo off
set bw=blank

::renames DataPC_DX11.forge
ren "C:\Program Files (x86)\Steam\steamapps\common\Assassin's Creed 3\DataPC_DX11.forge" DataPC_DX11_.forge

::Launches AC3SP
"C:\Program Files (x86)\Steam\steamapps\common\Assassin's Creed 3\AC3SP.exe"

::Begins check for Start
for /f "tokens=1 delims=," %%a in ('tasklist /fo csv ^| find /i "AC3SP"') do (set bw=%%a)
if %bw%==blank (goto start)

::Waiting for the process to actually fully launch you may have to increase the -n value if it takes your computer longer to launch the game
ping -w 1 -n 10 > NULL

::renames back to start game once ac3sp starts
ren "C:\Program Files (x86)\Steam\steamapps\common\Assassin's Creed 3\DataPC_DX11_.forge" DataPC_DX11.forge
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Make sure to follow instructions carefully or you might end up messing up screen resolution which is quite tricky to resolve – Kim Minseo Apr 5 at 15:44

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